# Equation Stoichiometry Chemical Equation – indicates the reactants and products in a rxn; it also tells you the relative amounts of reactants and products.

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Equation Stoichiometry Chemical Equation – indicates the reactants and products in a rxn; it also tells you the relative amounts of reactants and products involved in the rxn. EX: 2 Mg + O 2  2 MgO reactantsproducts Stoichiometry – the study of the mathematical relationships involving chemical formulas and equations.

Chemical equations reflect the law of conservation of mass – matter is neither created nor destroyed in an ordinary chemical rxn. This is represented in an equation by having the same number of each atom on both sides of the equation. Coefficients are used to balance equations. The mass of the products equals the mass of the reactants that reacted.

(Review) Coefficients can stand for: 1) The relative number of atoms, molecules, or formula units 2) The relative number of moles 3) The relative volume of gases EX: 2 H 2 + O 2  2 H 2 O 2 molecules H 2 + 1 molecule O 2  2 molecules H 2 O 2 mol H 2 + 1 mol O 2  2 mol H 2 O 2 volumes H 2 + 1 volume O 2  2 volumes H 2 O

***Coefficients tell you the relative number of moles of each reactant and product involved in a rxn.*** EX: CH 4 + 2 O 2  CO 2 + 2 H 2 O Example mole ratios: What are the possible the mole ratios for: 3 Mg + 2 P  Mg 3 P 2 mole ratio (coefficient ratio) – a conversion factor that allows you to convert from moles of one substance to moles of another substance in a rxn.

I. Mole – Mole Problems You are given moles of one substance in a rxn and must find the moles of another substance. Use the mole ratio from the equation to solve Mole-mole problems: Moles of given substance coefficient of substance sought coefficient of given substance = Moles of substance sought

Example mole-mole problems 1. For the rxn N 2 + 3 H 2  2 NH 3 a) How many moles of N 2 will react with 8.0 mol of H 2 ? b) How many moles of NH 3 can be made from 12.0 mol of H 2 ? 2. For the rxn C 5 H 12 O + 6 O 2  5 CO 2 + 3 H 2 O a) If 127 mol of CO 2 are produced, how many moles of H 2 O are produced? b)If 15 mol of O 2 react, how many moles of CO 2 are produced?

II. Mass-Mole, Mole-Mass, and Mass-Mass Problems used as a conversion factor to convert A. Review: Molar Mass - used as a conversion factor to convert between grams (mass) of a substance and # of moles. 1) to convert grams to moles, use the following set up: 1 mol given grams X molar mass = moles 2) to convert moles to grams, use the following set up: molar mass given moles X 1 mol = grams

B. Mass-Mole Problem (2 steps) – you are given the mass(grams) of one substance in a rxn, and are asked to find the number of moles of another substance. EX: For the rxn N 2 + 3 H 2  2 NH 3 1) How many moles of NH 3 can you make from 28g of N 2 ? 2) How many moles of H 2 are needed to make 100. g of NH 3 ?

C. Mole-Mass Problem (2 steps) – you are given the moles of one substance in a rxn, and are asked to find the number of grams (mass) of another substance. EX: For the rxn CH 4 + 2 O 2  CO 2 + 2 H 2 O 1)How many grams of H 2 O are produced by combusting 2.25 x 10 12 moles of CH 4 ? 2)If 0.42 moles of O 2 react, how many grams of CH 4 react?

GIVEN GRAMS OF “A” (mass) GIVEN MOLES OF “A” 1 mol A. X molar mass of A MOLES OF “B” SOUGHT coefficient of sought (B) X coefficient of given (A) GRAMS OF “B” SOUGHT (mass) molar mass X of B. 1 mol B Suppose you have two substances, “A” and “B”, in a rxn. The following flow chart shows the stoichiometric relationship between them:

III. Mass-Mass Problems (3 steps) You are given the mass (grams) of one substance in a rxn, and are asked to find the number of grams (mass) of another substance. EX: For the rxn N 2 + 3 H 2  2 NH 3 1) How many grams of H 2 will react with 237g of N 2 ? 2) How many grams of N 2 do you need to produce 82.8 g NH 3 ?

2) What mass of oxygen reacted if 3.5 x 10 4 g of H 2 O was produced? EX2: Decane, C 10 H 22, is a component of gasoline. 1) How many grams of CO 2 will be produced by the complete combustion of 345g of C 10 H 22 ?

EX3: Aluminum metal can be produced by the electrolytic decomposition of aluminum oxide. 1)How many kilograms of Al can be produced by decomposing one metric ton (1.0 x 10 3 kg) of aluminum oxide? 2) How many grams of oxygen would be consumed in the production of 3.5 x 10 6 g of Aluminum?

Cake Recipe To make one cake you need: 3 eggs, 4 cups flour, 2 qts of milk, and 1 cup of sugar. You have: 12 eggs, 12 cups flour, 10 qts of milk, and 6 cups of sugar. How many cakes can you make?

Limiting Reactants

In most reactions that occur, you don’t use “exact” amounts of each reactant. Usually one reactant gets used up before the other. limiting reactant – the reactant that gets totally used up in a rxn; it’s the reactant you run out of. (It “limits” the rxn.) excess reactant – any reactant that is not totally used up in a rxn; you still have some of it left after the rxn stops.

In this example, 10 molecules of H 2 react with 7 molecules of O 2, according to the equation: 2 H 2 + O 2  2 H 2 O The limiting reactant would be H 2. The excess reactant would be O 2.

Limiting Reactant Problems You are given the initial amounts of the reactants in a rxn, and must determine: a) which reactant is the limiting reactant. You also should be able to determine: b) how much product is produced, and c) how much of the excess reactant is left over. **a) To determine which reactant is the limiting reactant:** Find out how much product can be made from the 1 st reactant Find out how much product can be made from the 2nd reactant Which ever reactant produces the LEAST product is the limiting reactant. (In other words: You need to do two mass-mass problems to figure out the limiting reactant.)

EX: 125g of Br 2 and 75.0g Li are reacted together according to the equation: 2 Li + Br 2  2 LiBr What is the limiting reactant?

**b) The amount of product actually produced is the amount produced by the limiting reactant.** In the example above, 136g of LiBr would actually be produced. EX: 24.0g of CH 4 is burned in a closed container that holds 50.0g of O 2. a) What is the limiting reactant? b) How many grams of CO 2 is produced? **c) To find out the amount of excess reactant left over:** calculate the mass of excess reactant that reacted with the limiting reactant. (mass-mass problem) subtract the amount of excess reactant that reacted (from step 1) from the original mass of excess reactant; this tells you how much was left over.

GIVEN GRAMS OF “A” (mass) GIVEN MOLES OF “A” 1 mol A. X molar mass of A MOLES OF “B” SOUGHT coefficient of sought (B) X coefficient of given (A) GRAMS OF “B” SOUGHT (mass) molar mass X of B. 1 mol B Suppose you have two substances, “A” and “B”, in a rxn. The following flow chart shows the stoichiometric relationship between them:

EX: 50.0g of Na is reacted with 50.0g of Cl 2 in synthesis reaction. a) what is the limiting reactant? b) how much NaCl is produced? c) what is the excess reactant, and how much of it remains after the rxn? Practice Limiting Reactant Problems

EX: N 2 H 4 + 2 H 2 O 2  N 2 + 4 H 2 O 65.0g of N 2 H 4 reacts with 75.0g of H 2 O 2 a) what is the limiting reactant? b) how much N 2 is produced? c) what is the excess reactant, and how much of it remains after the rxn?

(The amount that “theoretically” could have been produced.) Theoretical yield is determined by using the rxn’s equation and doing a mass-mass problem. Theoretical and Percent Yield Theoretical Yield – the maximum amount of product that can be produced in a rxn. Actual yield – the measured amount of product collected in an experiment. (The amount “actually” produced.)

Percent Yield A comparison of the amount actually obtained to the amount it was possible to make Actual Yield Theoretical Yield Percent Yield =x 100

Percent Yield Problems 1. In an experiment, 100. g of Na is reacted with excess Cl 2. 239 g of NaCl is collected. a) What is the theoretical yield of NaCl? b) What is the percent yield of NaCl? 2. 150. g of O 2 is reacted with excess Mg. 300. g of MgO is collected. a) What is the theoretical yield of MgO? b) What is the percent yield of MgO?

Solution Stoichiometry Problems

Review: Molarity Molarity (M) = moles of solute liters of solution Molarity is used to convert between volume of solution and moles of dissolved solute. 1. How many moles of NaCl are in 3.5 L of 2.0 M NaCl? 2. How many moles AgNO 3 are in 125 mL 3.00 M AgNO 3 ?

GIVEN GRAMS OF “A” (mass) GIVEN MOLES OF “A” 1 mol A. X molar mass of A MOLES OF “B” SOUGHT coefficient of sought (B) X coefficient of given (A) GRAMS OF “B” SOUGHT (mass) molar mass X of B. 1 mol B VOLUME OF “A” GIVEN

Example Solution Stoich Problems 1) Excess Cu is placed in 250 mL of 0.50 M AgNO 3 and the following rxn occurs: 2 AgNO 3 + Cu  2 Ag + Cu(NO 3 ) 2 What mass of Ag can be produced? 2) 125 mL of 2.00 M Pb(NO 3 ) 2 is mixed with excess NaCl and the following rxn occurs: 2 NaCl + Pb(NO 3 ) 2  PbCl 2 + 2 NaNO 3 What mass of PbCl 2 precipitates?

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