STOICHIOMETRY TUTORIAL Revised PPT presentation by Paul Gilletti

Instructions: This is a work along tutorial Instructions: This is a work along tutorial. Each time you click the mouse or touch the space bar on your computer, one step of the problem solving occurs. Pressing the PAGE UP key will backup the steps. Get a pencil and paper, a periodic table and a calculator, and let’s get to work.

(1-2-3) General Approach For Problem Solving: 1. Clearly identify the Unknown (the goal, or what you are trying to find) and the UNITS involved. 2. Determine what is Given and the UNITS. 3. Use conversion factors (which are really ratios) and their UNITS to CONVERT what is given into what is desired.

Table of Contents: Click on each tab to view problem types. View Complete Slide Show Sample problem 1 Sample problem 2 Converting grams to moles Mole to Mole Conversions Gram-Mole and Gram-Gram Problems Limiting/Excess/ Reactant and Theoretical Yield Problems :

Sample problem for general problem solving. Sam has entered into a 10 mile marathon. Use ALL of the following conversions (ratios) to determine how many inches there are in the race. 5280 ft = 1 mile; 12 inches = 1 ft 1. What is the unknown and what units are needed? Goal = ______ inches 2. What is given and its units? 10 miles Units match 3. Convert using factors (ratios). 10 miles = inches 633600 Goal Given Convert Menu

Sample problem #2 on problem solving. A car is traveling at a speed of 45 miles per hr (45 miles/hr). Determine its speed in kilometers per second using the following conversion factors (ratios). 1 mile = 5280 ft; 1 ft = 12 in; 1 inch = 2.54 cm; 1 km = 1 x 103 m; 1 cm = 1 x 10-2 m; 1 hr =60 min; 1 min = 60 s Given Unknown = km s 0.020 Units Match!

Use the molar mass to convert grams to moles. Converting grams to moles. Determine how many moles there are in 5.17 grams of Fe(C5H5)2. Given Unknown units match 5.17 g Fe(C5H5)2 = moles Fe(C5H5)2 0.0278 Use the molar mass to convert grams to moles. Fe(C5H5)2 2 x 5 x 1.001 = 10.01 2 x 5 x 12.011 = 120.11 1 x 55.85 = 55.85

coefficients give MOLE RATIOS Stoichiometry (more working with ratios) Ratios are found within a chemical equation. 2HCl + Ba(OH)2  2H2O + BaCl2 1 1 coefficients give MOLE RATIOS 2 moles of HCl react with 1 mole of Ba(OH)2 to form 2 moles of H2O and 1 mole of BaCl2

Mole – Mole Conversions When N2O5 is heated, it decomposes: 2N2O5(g)  4NO2(g) + O2(g) a. How many moles of NO2 can be produced from 4.3 moles of N2O5? 2N2O5(g)  4NO2(g) + O2(g) 4.3 mol ? mol Units match 4.3 mol N2O5 = moles NO2 8.6 b. How many moles of O2 can be produced from 4.3 moles of N2O5? 2N2O5(g)  4NO2(g) + O2(g) 4.3 mol ? mol 4.3 mol N2O5 = mole O2 2.2

gram ↔ mole and gram ↔ gram conversions When N2O5 is heated, it decomposes: 2N2O5(g)  4NO2(g) + O2(g) a. How many moles of N2O5 were used if 210g of NO2 were produced? 2N2O5(g)  4NO2(g) + O2(g) ? moles 210g Units match 210 g NO2 = moles N2O5 2.28 b. How many grams of N2O5 are needed to produce 75.0 grams of O2? 2N2O5(g)  4NO2(g) + O2(g) ? grams 75.0 g 75.0 g O2 = grams N2O5 506

First write a balanced equation. Gram to Gram Conversions Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid? Al(s) + HCl(aq)  AlCl3(aq) + H2(g) 2 6 2 3 First write a balanced equation.

Now let’s get organized. Write the information below the substances. Gram to Gram Conversions Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid? Al(s) + HCl(aq)  AlCl3(aq) + H2(g) 2 6 2 3 3.45 g ? grams Now let’s get organized. Write the information below the substances.

gram to gram conversions Aluminum is an active metal that when placed in hydrochloric acid produces hydrogen gas and aluminum chloride. How many grams of aluminum chloride can be produced when 3.45 grams of aluminum are reacted with an excess of hydrochloric acid? Al(s) + HCl(aq)  AlCl3(aq) + H2(g) 2 6 2 3 3.45 g ? grams Units match 3.45 g Al = g AlCl3 17.0 Let’s work the problem. We must always convert to moles. Now use the molar ratio. Now use the molar mass to convert to grams.

Limiting Reagents/ Reactants When compounds react, one compound will disappear before the other compound. To determine which reactant will disappear first (this is called the limiting reagent) you will need to convert the reactants to mole or grams of one of the products (it must be the same product) When you convert, the answer that is the smallest is from the limiting reagent.

Limiting Reagents/Reactant What is the limiting reactant when 1 mol of hydrogen reacts with 1 mol of oxygen to produce water? First write the balanced chemical equation. Convert 1 mole of each to moles of water. Will be shown on board. To determine how many moles of water are produced, you use the answer that is the smallest. In this case, hydrogen will run out before oxygen. When hydrogen runs out the reaction stops only producing that amount of water.

4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) Limiting/Excess/ Reactant and Theoretical Yield Problems : Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) a. How many moles of O2 can be produced from 0.15 mol KO2 and 0.10 mol H2O? b. Determine the limiting reactant. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) First copy down the the BALANCED equation! Now place numerical the information below the compounds.

Two starting amounts? Where do we start? Limiting/Excess/ Reactant and Theoretical Yield Problems : Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) a. How many moles of O2 can be produced from 0.15 mol KO2 and 0.10 mol H2O? b. Determine the limiting reactant. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) 0.15 mol 0.10 mol ? moles Hide one Two starting amounts? Where do we start?

4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) Limiting/Excess/ Reactant and Theoretical Yield Problems : Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) a. How many moles of O2 can be produced from 0.15 mol KO2 and 0.10 mol H2O? b. Determine the limiting reactant. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) 0.15 mol 0.10 mol Hide ? moles Based on: KO2 0.15 mol KO2 = mol O2 0.1125

4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) Limiting/Excess/ Reactant and Theoretical Yield Problems : Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) a. How many moles of O2 can be produced from 0.15 mol KO2 and 0.10 mol H2O? b. Determine the limiting reactant. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) 0.15 mol Hide 0.10 mol ? moles Based on: KO2 0.15 mol KO2 = mol O2 0.1125 Based on: H2O 0.10 mol H2O = mol O2 0.150

4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) Limiting/Excess/ Reactant and Theoretical Yield Problems : Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) a. How many moles of O2 can be produced from 0.15 mol KO2 and 0.10 mol H2O? Determine the limiting reactant. 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) 0.15 mol 0.10 mol ? moles Based on: KO2 0.15 mol KO2 = mol O2 0.1125 It was limited by the amount of KO2. Based on: H2O 0.10 mol H2O = mol O2 0.150 H2O = excess (XS) reactant! What is the theoretical yield? Hint: Which is the smallest amount? What is the limiting reactant?

Percent Yield Percent yield tells gives us a comparison of how much product you produce in lab compared to a maximum amount of product that could be produced. % yield = experimental value / calculated value x 100% You use stoichiometry to determine the calculated value.

Percent Yield In lab you react 2.00 g of hydrogen gas with 2.00 g of oxygen gas. You obtain 2.1 g of water vapor. What is your % yield? 1st you need to compute a limiting reagent problem to see how much water should be produced. 2nd you calculate % yield

Theoretical yield vs. Actual yield Suppose the theoretical yield for an experiment was calculated to be 19.5 grams, and the experiment was performed, but only 12.3 grams of product were recovered. Determine the % yield. Theoretical yield = 19.5 g based on limiting reactant Actual yield = 12.3 g experimentally recovered

Limiting/Excess Reactant Problem with % Yield 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) If a reaction vessel contains 120.0 g of KO2 and 47.0 g of H2O, how many grams of O2 can be produced? 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) ? g 120.0 g Hide one 47.0 g Based on: KO2 120.0 g KO2 = g O2 40.51

Limiting/Excess Reactant Problem with % Yield 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) If a reaction vessel contains 120.0 g of KO2 and 47.0 g of H2O, how many grams of O2 can be produced? 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) ? g 120.0 g 47.0 g Hide Based on: KO2 120.0 g KO2 = g O2 40.51 47.0 g H2O Based on: H2O = g O2 125.3 Question if only 35.2 g of O2 were recovered, what was the percent yield?

4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) ? g 120.0 g 47.0 g If a reaction vessel contains 120.0 g of KO2 and 47.0 g of H2O, how many grams of O2 can be produced? 4KO2(s) + 2H2O(l)  4KOH(s) + 3O2(g) ? g 120.0 g 47.0 g Based on: KO2 120.0 g KO2 = g O2 40.51 47.0 g H2O Based on: H2O = g O2 125.3 Determine how many grams of Water were left over. The Difference between the above amounts is directly RELATED to the excess H2O. 125.3 - 40.51 = 84.79 g of O2 that could have been formed from the excess water. 84.79 g O2 = g XS H2O 31.83

Application of topics Stoichiometry is used to determine maximum yield when performing any type of reaction. You will use Stoichiometry, Limiting Reagents, Percent Yield, and Percent Error when performing labs. It is necessary to do these calculations to determine your accuracy in lab. I will be looking at your accuracy when I grade labs.