Stoichiometry Tutorials: Limiting Reagents (from a complete OLI stoichiometry course) We are now ready to pull everything we know about reaction stoichiometry Limiting Reactant and Percent Yield Practice. Reading assignment on limiting reactant. For the example in the previous paragraph, complete reaction of the hydrogen would yield. You can find what reactant is used up first and stops the reactions. Mass to Mass Wks. 10 M NaOH? 2. Stoichiometry Review. Predict the products and leftovers after reaction, based on the quantities of reactants and ratios of molecules in the balanced chemical equation. Limiting Reactant and Percent Yield Problems: A certain chemical reaction was predicted to produce a theoretical yield of 42. In this lab we observed the double replacement reaction between sodium carbonate and calcium chloride. 2SO2 + O2 ---> 2SO3 In a certain experiment, 90g of SO2 (MW=64) reacts with 100g of O2. When lead (II) sulfide is burned in air, lead (II) oxide and sulfur dioxide are produced. In these calculations, the limiting reactant is the limiting factor for the theoretical yields of all products. Theoretical Yield. Identify the limiting reactant in a chemical reaction. Which reactant is the limiting reactant? (the one that produces the fewest g of copper) What is your actual yield? (This is the mass of the dry Cu left in the cup) What is your percent yield for the reaction? Questions and Conclusions. All comparisons of reactants must be done in units of moles. This smallest yield of product is called the theoretical yield. Copper Iron Stoichiometry Lab Report Essay 1808 Words | 8 Pages. Use molar ratios from the balance chemical equation to determine which reactant is limiting, and which reactant is in excess. 2 SO 4 (aq) 2 H 2 O (l) + Na 2 SO 4 (aq)How many grams of sodium sulfate will be formed if you start with 200 grams of sodium hydroxide and you have an excess of sulfuric acid? 2)Using the following balance equation: Pb(NO. After doing this we determined percent yield of the reaction. In a synthesis reaction, 7. of a reaction is the ratio. A summary of Limiting Reagent in 's Stoichiometry: Real World Reactions. 461 Limiting Reactant And Percent Yield Worksheet. Mountain View High School Limiting Reagent Notes. It seems like a fairly complex name for a very simple concept. 50 g Ca₃(PO₄)₂ to check if the other reactants are limiting. 5 M CaCl2 was added to the supernatant liquid in test tube 1, and a precipitate formed. Mole to Mole Practice problems - Answers: 11. Demonstrate the. Identify the limiting reactant in a chemical reaction. The reactant you run out of is called the limiting reagent; the other reactant or reactants are considered to be in excess. 0 & Answer Key Worksheet - Limiting Reactant Practice 2. The reactant that will be depleted first is called the limiting reagent. Note on nitration of naphthalene. The reactant that gets used up first, and therefore limits the amount of product that can be made, is called the limiting reactant. Objectives: 1. Intro to Limting Reactants Lab (lab handout, lab data tables) HW: Limiting Reactants Practice (answers only key), prep for limiting reactants lab (bring a hair tie!) Wednesday, 9/4. Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (oxygen) actually did react with the limiting reactant (ammonia). Solve problems in chemistry using dimensional analysis. - Stoichiometry Practice 2. By determining which reactant is the limiting reagent, the expected amount of product can be calculated. Conclusion: Perform with students a sample calculation, if needed. Introduction: By observing the amounts of gas generated, I will be able to identify the limiting and excess reactant in each situation. Worksheet for Basic Stoichiometry. Choose from a variety of reactions. The amount recovered divided by the theoretical yield gives a percent yield (% yield) or actual yield. Calculate mole ratio of the reactants. The activity takes approximately 2 hours of class time to complete. How many grams of hydrogen peroxide are needed to produce 6. State two reasons why you may not recover the theoretical amount of copper in this experiment. The maximum amount of product is going to be determined by the limiting reactant, i. Vinegar and Baking Soda Stoichiometry Lab Purpose: To predict the amount of Carbon Dioxide gas that should be produced in a chemical reaction; then calculate the amount of CO2 released, the percent yield. 2 grams N2 are actually formed during the experiment, what is the percent yield? 77% yield. To identify the limiting reactant, calculate the number of moles of each reactant present and compare this ratio to the mole ratio of the reactants in the balanced chemical equation. The reactant that will be depleted first is called the limiting reagent. The other reactants are partially consumed where the remaining amount is considered "in excess". A limiting reagent is a chemical reactant that limits the amount of product that is formed. In this limiting reagents problem, students are given random volumes and concentrations of DNA solutions and are asked to predict what will remain after a reaction has occurred. Lab 35 – day 3 procedure. - Smore Lab Introduction to Stoichiometry Limiting Reactants/Percent Yield Answer Key. com › Gizmos Stoichiometry. For time and speed reasons, the reaction mixtures in lab will usually have a limiting and an excess reactant. We now need to calculate how much H 2 is left over after the reaction is complete and in order to do it, we will first calculate how much of it is consumed in the reaction based on the moles of the limiting reactant. The same word meteorologists use when talking about falling rain and snow, a precipitate in a scientific sense is the solid formed during a reaction between two aqueous compounds. The Stoichiometry of Alka-Seltzer This lab will count as your Honors Project. Answer the following stoichiometry-related questions: 12) Write the balanced equation for the reaction of acetic acid with aluminum hydroxide to form water and aluminum acetate: 13) Using the equation from problem #12, determine the mass of aluminum acetate that can be made if I do this reaction with 125 grams of acetic acid. In a similar lab, Limiting Reactant Candy, students use candy to help them understand what is meant by the term, "limiting reactant" and identify it in a non-chemistry situation. It can be deduced or worked out using masses found by experiment. 288-294) C. How many grams of precipitate are formed in this reaction? d. Im asking for help on how to calculate how much heat energy will be released when 3. com Limiting reactant × (mole ratio of PRODUCT / limiting. As you were able to see in the PhET activity, when you run out of one reactant, the reaction can no longer proceed. Announcements: So you don't have Power Point, but you need to print the notes?. Review - What is a limiting reactant? 2. This experiment is designed to illustrate the relationship between quantities of reactants and the amount of products produced by a chemical reaction. •Virtual Chem Labs, Lab 14 378 Chapter 12 12CHAPTER Study Guide Key Concepts 12. The NaHCO3 is the limiting reactant and the HCl is the excess reactant in this experiment. limiting reactant. Limiting Reactant: Recap You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT. Basic Stoichiometry PhET Lab Let's make some sandviches! Introduction: When we bake/cook something, we use a specific amount of each ingredient. Once you have a value of ΔH for one mole of limiting reactant, you will need to convert it to represent the number of moles of limiting reactant in the balanced equation. 2 grams N2 are actually formed during the experiment, what is the percent yield? 77% yield. This quiz will cover some basic limiting reactant problems. The principles of stoichiometry and limiting reagents will be used to predict the amount of product that should be produced when mixing two solutions to produce an insoluble product. Objectives. calculate the moles of each to determine which is the limiting reactant. __ Cl 2 (aq) + __ H. To find out how much of the excess reactant is left over, 1. 0mL of calcium chloride (0. 2 M K2CrO4, but nothing happened. The limiting reactant for the formation of solid calcium monohydrate is determined from two precipitation tests of the final mixture from Part A. 25 g of NH 3 are allowed to react with 3. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers. Honors Chemistry Lab 13: Limiting Reactant Exploration Introduction: As you have learned, the limiting reactant of a reaction is the reactant that would run out first if all the reactants were to be reacted together. Week News from Science Links After School Help Fun Stuff Teachers' Lounge Bloomington South E-mail Mr. Nitrogen dioxide reacts with water to form nitric acid and nitrogen monoxide. 5M) is added to 25. • Limiting reactant – the reactant present in a reaction in the least amount, based on its reaction coefficients and molecular weight. Limiting Reactant and Percent Yield Problems: A certain chemical reaction was predicted to produce a theoretical yield of 42. The moles of each reagent are changed in each flask in order to demonstrate the limiting reagent concept. change all the given measurement to mole 2. find limiting reactant. you get less of the product with the 6 mol Cl2, so that's your limiting reactant, and all you're going to get of the product (because that reactant runs out, so the no more product is made) so the answer to the first set of reactants is 3 mol TiCl4. In flasks 1 and 2, a small amount of Mg is used and therefore the metal is the limiting reagent. When the amount of product that will be formed is important, the stoichiometry of the reaction and the actual experimental procedure should be examined to determine which material is the limiting reactant. Stoichiometry Example Problem 2 - Khan Academy. Other Results for Chapter 12 Stoichiometry Answer Key: Chapter 12 Assessment Stoichiometry Answers - zombiedoc. To find the amount of excess, subtract the amount used from the given amount. Limiting Reactions. Discuss rocket lab 2. 05g of sodium bicarbonate and 1. Stoichiometry Review Worksheet. Stoichiometry Due! Lab: Day 3 Cu and AgNO3 *determine mass of beaker and silver *dispose of silver into the collection beaker and clean beaker before returning it (throw away tape) *work on lab questions *lab is due next class and is graded on accuracy (careful of sig figs!) pHET simulation: Limiting Reactants Notes: Limiting Reactant Steps. Convert grams of reactants to moles 2. Determine the percent yield of the experiment above actually produced 250. Not every single time, but every now and then remind the students, 'Remember, you still have to think about stoichiometry and limiting reagents. In real-life chemical reactions, not all of the reactants present convert into product. 0 g of NaCl. In some stoichiometry problems the limiting reactant must first be identified through a preliminary calculation. ChemCollective: Stoichiometry. (? = coefficient in the balanced equation). This was a lab i did. The reactant used up first is known as the limiting reactant. STOICHIOMETRY CALCULATORS and PROBLEM SOLVERS ONLINE. Thus, reactant A is considered in excess, and reactant B is considered limiting. Video Tutorial by Ms. 1 The Mole and Molar Mass 3. All comparisons of reactants must be done in units of moles. Stoichiometry and Limiting Reactant Introduction This experiment is designed to illustrate the relationship between quantities of reactants and the amount of product produced by a chemical reaction. Collect handouts and review answers to post-laboratory and discussion questions. b) Calculate the theoretical yield of CO 2 in grams. This experiment is designed to illustrate the relationship between quantities of reactants and the amount of products produced by a chemical reaction. What is the limiting reactant. In other words, there is essentially an unlimited supply of acetic acid in the vinegar bottle, and the. The concept of limiting reactant is very important in the study of the stoichiometry of chemical reactions. Introduction In this lab, you will be investigating reaction stoichiometry by doing a series of mixing experiments using acids and bases in different amounts. Once the limiting reactant or ingredient runs out, the process must stop, regardless of how much of the other ingredients is left. The data i have calculated thus far is. Nitrogen dioxide reacts with water to form nitric acid and nitrogen monoxide. The reactant used up first is known as the limiting reactant. Stoichiometry Example Problem 2 - Khan Academy. All stoichiometry calculations must begin with the amount of the limiting reactant — not the amount of the excess reactant. 50 g of O 2. Calculate the theoretical yield of MgO for trial 1 and 2. The rest of my lab requires to find Mass/Mole of the limiting reactant, the mass of the excess reactant, and the mass percents of each. pdf FREE PDF DOWNLOAD Lesson Info: Stoichiometry Gizmo | ExploreLearning www. And I said before, the word reagent and reactant are used interchangeably. Determine the moles of lead II iodide produced. Stoichiometry. This page provides exercises in using the limiting reagent to determine the quantity of a product that should be produced. Reaction Stoichiometry and Limiting Reagents. Percentage yield yield percentage yield practice. Our online stoichiometry trivia quizzes can be adapted to suit your requirements for taking some of the top stoichiometry quizzes. Obtain and record the mass of 100 mL beaker. Comment: when I was in the classroom, teaching the technique for determining the limiting reagent, I would warn against using the results of the division, in this case the 19 for the NaOH, in the next step of the calculation. NH 4 NO 3 N 2 O + H 2 O e. The activity takes approximately 2 hours of class time to complete. The purpose is to find the ratio of moles of a reactant to the moles of. of the limiting reactant is consumed. • Limiting reactant (or limiting reagent)—the reactant that is completely consumed in a chemical reaction. • Possible ions in solution are Ba2+ or PO 4 3-• Excess Ba2+ is tested by the addition of a phosphate ion, if a precipitate forms, excess Ba2+ ions are present, no precipitate means no Ba2+, therefore Ba2+ is the Limiting Reactant • Likewise, PO43-excess is tested by addition of. ib answers. Step 3: Calculate the moles of "desired" substance from your answer in Step 2 using the coefficients from the balanced chemical equation. Obtain and record the mass of 100 mL beaker. High school. A limiting reactant is the reagent that is completely consumed during a chemical reaction. Observing a limiting reactant lab answers are a great way to get information. Otherwise, the second reactant is the limiting reactant. You will probably want to review all three of these modules before trying the quiz. In this example the four slices of bread is. That reactant is said to be in excess (there is too much). The reactant that produces the least amount of product must be the limiting reagent. Stoichiometry Lab: Leftover Aluminum Wire What is a limiting reactant. __ Cl 2 (aq) + __ H. Before doing this lab experiment, students should be able to do basic stoichiometry problems dealing with finding the limiting reagent and excess reagent. Mixture is tested for an excess of oxalate ion with limited calcium ion 2. 6 g O that means the 15. Assignments Directory. Stoichiometry / ˌ s t ɔɪ k i ˈ ɒ m ɪ t r i / is the calculation of reactants and products in chemical reactions. Based on your answer to #2 above, how many graham crackers would I need to make that many S’mores? 4. Do two stoichiometry calculations. Abuzar Kabir Purpose/Abstract The purpose of this lab was to determine the limiting reactant in a mixture of to soluble salts and the percent composition of each substance in a salt. (c) If you wanted 2 CoSO 4 's, how many CoCl 2 's would you need? _____ (d) How many grams of CoCl 2 would you need to make 2. (ANSWER 386. It can also handle equations that contains fractions and decimals. One of your tasks will be to determine which of your reactants is limiting and which is in excess. Double check your stoichiometry homework with these problem solvers for chemistry! Find number of moles for a given mass of substance, grams for a given number of moles, moles or grams of product given moles or grams of reactant. Its main function is to absorb excess stomach acid (HCl). 3g of LiNO3) 4) Using the following equation: Fe2O3 + 3 H2 -----> 2 Fe + 3 H2O. Limiting Reactant Lab. Our objectives were to determine which reactant is the limiting reactant and which is the excess reactant, determine the theoretical mass of the precipitate, and to calculate the percent yield. STOICHIOMETRY HONORS CHEMISTRY The theoretical yield is dependent on the limiting reactant. Stoichiometry, Percent Yield, Micro Mole Rockets Lab Since KI is the limiting reactant, Copper and Silver Nitrate lab Data; Stoichiometry Continued: Limiting. Limiting Reactants Lab. In this example the four slices of bread is. Abstract: The purpose of the lab, Stoichiometry of a Precipitation Reaction, is to be able to calculate the amount of a second reactant we need to react with the reactant one. 2 HCl + Zn --> ZnCl2 + H2 When you use 25 ml. which can be produced. At high temperatures, sulfur combines with iron to form the brown-black iron (II) sulfide: Fe (s) + S (l) FeS (s) In one experiment, 7. Stoichiometry: Mixed Problems (KEY) 1) N2 + 3H2 → 2NH3 What volume of NH3 at STP is produced if 25. The reactant you run out of is called the limiting reagent; the other reactant or reactants are considered to be in excess. 00mol Fe* 1molS/1molFe= 2. To find out how much of the excess reactant is left over, 1. If you start with 25. S 29/32 Learning Summary The limiting reactant is the reactant th t runs t t that out first and causes the reaction to stop. Start studying Experiment 4: Limiting Reactant. Read the background, update your table of contents, add title, write purpose (In your own words 3-5 sentences), Write procedures, data table. Stoichiometry is the study of the relationships or ratios between two or more substances undergoing a physical or chemical change (Helmenstine). In this lab, you will determine the reaction for mixing two reactants together. The reaction proceeds until the limiting reactant is completely used up. Groupwork – Limiting Reactants and Team Learning Worksheet 9. Purpose: To determine the limiting reactant in the synthesis of S’mores To determine the amount of excess reactant in the synthesis of S’mores Materials: Per Group: 1. Which reactant is in excess and by how many grams? d. How to Solve a Limiting Reactant Problem: 1. A comprehensive reaction stoichiometry calculator that can solve problems of all situations. What is the limiting reactant. Keep this answer for Step 4. The maximum theoretical yield of a chemical reaction is dependent upon. It is very simple you just find the moles of both reactants, and convert them both to moles of one of the products, but remember it has to be the same product. Mole to Mole Practice problems - Answers: 11. g of copper. 2/12 W Activity – Smores. Do two stoichiometry calculations. Stoichiometry Balloon Races Limiting and Excess Reactants Introduction If a little is good, more is better, right? Increasing the mass of a reactant in a chemical reaction may not increase the amount of product that can be formed—the yield may be "limited" based on the mole ratio of reactants. 0 of N2 is reacted with an excess of H2? 3 3 3 2 3 2 2 2 40. Limiting Reactants Available Ingredients 4 slices of bread 1 jar of peanut butter 1/2 jar of jelly Limiting Reactant bread Excess Reactants peanut butter and jelly C. But when people talk about the limiting ones, they tend to call it the. Day 12 – Discussion and examples of limiting reactant. chemistry-12-3. AP-Chemistry: Stoichiometry Practice Problems with Answers. Labs in Chapter 9 include a study of stoichiometry as practiced in the lab, where students will compare their actual yields to the theoretical yield of a product of a chemical reaction. Limiting Reagent Worksheet W 324 Everett Community College Student Support Services Program 1) Write the balanced equation for the reaction that occurs when iron (II) chloride is mixed with sodium phosphate forming iron (II) phosphate and sodium chloride. Any amount of the other reactants beyond that needed to meet the requirements of the chemical equation will remain in the products. Step 3: Calculate the moles of "desired" substance from your answer in Step 2 using the coefficients from the balanced chemical equation. com › Gizmos Stoichiometry. 0 g of HCl and 0. 0 & Answer Key Worksheet - Limiting Reactant Practice 2. Thus, Carbon is the limiting reactant since we have a fewer amount of it than we require. The color stayed the same. Lab #7 Results 1. You will dissolve, mix, and react them to make products. You will need to calculate the limiting reactant, and the theoretical yield, from your measured amount of each reactant. 1)Using the following balanced equation: 2 NaOH (aq) + H. Do NOT use commas or scientific notation when entering large numbers. That leaves 1. do the same thing for the other reactants, your answers should be:. ib answers. If we assume that one reactant is completely consumed, we can calculate how much of the second reactant is needed in the reaction. Each reactant amount is used to separately calculate the amount of product that would be formed per the reaction's stoichiometry. Write and balance the chemical equation. This determines how much product can be made. reasons, the reaction mixtures in lab will usually have a limiting and an excess reactant. 2g of water. Mountain View High School Limiting Reagent Notes. This could also be used to find out how much of a reactant is needed based on how much of a different reaction you have. Collect handouts and review answers to post-laboratory and discussion questions. What is the limiting reagent, and what is the reactant in excess? b. This change in limiting reactant is due to the molar ratio from the balanced equation. Introduction. Stoichiometry II. Limiting Reactants Lab. In this limiting reagents problem, students are given random volumes and concentrations of DNA solutions and are asked to predict what will remain after a reaction has occurred. They are provided to assist your review of some topics covered in Chp 3 of the Zumdahl textbook. Have them explain how this problem will affect the final product. Obtain and record the mass of 100 mL beaker. At high temperatures, sulfur combines with iron to form the brown-black iron (II) sulfide: Fe (s) + S (l) FeS (s) In one experiment, 7. Limiting Reactant and Percent Yield Problems: A certain chemical reaction was predicted to produce a theoretical yield of 42. But, in case you don't believe me, let's check whether or not the Ca₃(PO₄)₂ is limiting. Calculate the number of moles of each species that remains after the reaction. In some stoichiometry problems the limiting reactant must first be identified through a preliminary calculation. Circle limiting in your chemical equation. Correctly phrased, the answer is 57 formula units. In this activity, students will use candy to investigate stoichiometry and mole-gram relationships in chemical equations. So that would have a 5 mols to begin with. What is the theoretical yield of NaNH2?. Students should have learned how to do stoichiometry problems, be experienced in using a balance, and be able to follow basic directions. Stoichiometry - Limiting and Excess Reactant Introduction to Limiting Reactant and Excess Reactant The limiting reactant or limiting reagent is the first reactant to get used up in a chemical reaction. What safety precautions are cited in this experiment? 2. For the example in the previous paragraph, complete reaction of the hydrogen would yield. LIMITING REACTANT WORKSHEET ANSWERS. Assignment 1: Mole Relationships in Reactions. is limiting and then calculate for each reactant the amount of product that could form. The color stayed the same. 3g of LiNO3) 4) Using the following equation: Fe2O3 + 3 H2 -----> 2 Fe + 3 H2O. pdf FREE PDF DOWNLOAD NOW!!! Source #2: limiting reactant and percent yield practice worksheet answers. What is the limiting reactant? c. In the lab, how can you tell that the equivalence point has been reached? 5. In addition to stretching our ability to calculate product and reactant amounts using reaction stoichiometry, we will also be exploring the concept of the limiting and excess reagents. Pupils are introduced to stoichiometric relationships in chemical equations with a Trail Mix activity before performing a lab to reinforce stoichiometry. It shows you how to perform stoichiometric calculations and how to calculate percent yield. 00 grams of C, which reactant is limiting? Which reagent is in excess?. Tomorrow were doing a lab relating moles to coefficients of chem eq. com Limiting Reagent Worksheet #1 Limiting Reagent Worksheet Answers - resultinfos Worksheets For All Download And Share Worksheets Free On. 35 moles of glucose (C 6 H 12 O 6 )?[Answer = 2. Stoichiometry Problems With a Twist Worksheet. To show the effects of limiting reagents, the relative amounts of the reactants is. Mole Calculations & Stoichiometry. 3AgNO 3 + FeCl 3! 3AgCl + Fe(NO 3) 3 b) Which reactant is the limiting reactant? AgNO 3 c) What is the maximum number of moles of AgCl that could be. I will do a solution assuming KO 2 is the limiting reagent, then I will do a solution assuming CO 2 is the limiting reagent. If you start with 25. 2: Limiting Reactant Problems Sample Study Sheet 10. Decomposition of hydrogen peroxide is exothermic with (change in enthalpy= 192kj). Stoichiometry. Percentage yield yield percentage yield practice. As a result, the limiting reactant determines the amount of product that can be formed. Application of limiting reagents and acid-base stoichiometry. (? = coefficient in the balanced equation). How many grams of sodium hydroxide are produced? What is the limiting reactant? RXN: **8. In a similar lab, Limiting Reactant Candy, students use candy to help them understand what is meant by the term, "limiting reactant" and identify it in a non-chemistry situation. So use the mole ratio. Good luck!! Group: Chemistry Chemistry Quizzes : Topic: Stoichiometry. In this laboratory, student should run the reaction, measure the amount of the product, calculate the amount of reactant consumed according to the mole-mass relationship, plot the experimental data, and finally, correlate the graph obtained to the behavior of limiting reactant in the reaction. By Chuck Roser, Retired Chemistry Instructor North Carolina School of Science and Mathematics Updated October 2019 Next Generation Science Standards (NGSS) This activity is appropriate for high school students and addresses the following Next Generation Science Standards* (NGSS) performance expectation:. 2 grams N2 are actually formed during the experiment, what is the percent yield? 77% yield. Comment: when I was in the classroom, teaching the technique for determining the limiting reagent, I would warn against using the results of the division, in this case the 19 for the NaOH, in the next step of the calculation. In calculating the theoretical yield, first find out what your limiting reactant is. Solution Stoichiometry Flow Chart. Limiting Reactant (Reagent) Problems always involve 2 steps: 1. Introduction. Abuzar Kabir Purpose/Abstract The purpose of this lab was to determine the limiting reactant in a mixture of to soluble salts and the percent composition of each substance in a salt. I have plenty of ammonia, but I don't have enough oxygen to react with it. 00 2 Limiting Reactant: _____ Theoretical Yield: _____ 6. What is the percent yield of the following reaction if 145g of P 2 O 5 reacts with 40 grams of water, but you only collected 112 grams of phosphoric acid? 3 H 2 O + P 2 O 5 -> 2H 3 PO 4. General Stoichiometry Notes UNIT 9 - STOICHIOMETRY STOICHIOMETRY: tells relative amts of reactants & products in a chemical reaction • • • • • • • Given an amount of a substance involved in a chemical reaction, we can figure out the amount of the other substances are needed or produced Always compare the number of MOLES 4 Fe + 3 O2 2 Fe2O3 If I have 4 moles of Fe, I need 3 moles. It will count as a formal grade worth 100 points. It is called the limiting reagent. Stoichiometry. Point out that the Stoichiometry Lab Test on the previous day is an example of the presence of a limiting reactant.