Equation Stoichiometry

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

EX: 2 Mg + O2 2 MgO

reactants products

Stoichiometry – the study of the mathematical relationshipsinvolving chemical formulas and equations.

Chemical equations reflect the law of conservationof mass – matter is neither created nor destroyedin 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 massof the reactants that reacted.

(Review) Coefficients can stand for:

1) The relative number of atoms, molecules, or formula units2) The relative number of moles

3) The relative volume of gases

EX: 2 H2 + O2 2 H2O

2 molecules H2 + 1 molecule O2 2 molecules H2O

2 mol H2 + 1 mol O2 2 mol H2O

2 volumes H2 + 1 volume O2 2 volumes H2O

***Coefficients tell you the relative number of molesof each reactant and product involved in a rxn.***

EX: CH4 + 2 O2 CO2 + 2 H2O

Example mole ratios:

What are the possible the mole ratios for:

3 Mg + 2 P Mg3P2

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 andmust 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 N2 + 3 H2 2 NH3

a) How many moles of N2 will react with 8.0 mol of H2?

b) How many moles of NH3 can be made from 12.0 mol of H2?

2. For the rxn C5H12O + 6 O2 5 CO2 + 3 H2O

a) If 127 mol of CO2 are produced, how many moles of H2O are produced?

b) If 15 mol of O2 react, how many moles of CO2

are produced?

II. Mass-Mole, Mole-Mass, and Mass-Mass Problems

A. Review: Molar Mass - used as a conversion factor to convert used as a conversion factor to convert between grams (mass) of a substance and # of moles.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 =

moles2) 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 N2 + 3 H2 2 NH3 1) How many moles of NH3 can you make from 28g of N2?

2) How many moles of H2 are needed to make 100. g of NH3?

C. Mole-Mass Problem (2 steps) – you are given the molesof one substance in a rxn, and are asked to find the number of grams (mass) of another substance.

EX: For the rxn CH4 + 2 O2 CO2 + 2 H2O1) How many grams of H2O are produced by combusting

2.25 x 1012 moles of CH4?

2) If 0.42 moles of O2 react, how many grams of CH4 react?

GIVENGRAMS OF“A” (mass)

GIVENMOLES 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 relationshipbetween 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 N2 + 3 H2 2 NH3 1) How many grams of H2 will react with 237g of N2?

2) How many grams of N2 do you need to produce 82.8 g NH3?

The Mole Road

2) What mass of oxygen reacted if 3.5 x 104 gof H2O was produced?

EX2: Decane, C10H22 , is a component of gasoline.

1) How many grams of CO2 will be produced by the complete combustion of 345g of C10H22 ?

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 103 kg) of aluminum oxide?

2) How many grams of oxygen would be consumed in theproduction of 3.5 x 106 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 upin a rxn; you still have some of it left after the rxn stops.

In this example, 10 molecules of H2 react with 7 molecules of O2, according to the equation: 2 H2 + O2 2 H2O

The limiting reactant would be H2.

The excess reactant would be O2.

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 1st 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 Br2 and 75.0g Li are reacted together according to the equation: 2 Li + Br2 2 LiBrWhat 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 CH4 is burned in a closed container that holds 50.0g of O2. a) What is the limiting reactant? b) How manygrams of CO2 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.

GIVENGRAMS OF“A” (mass)

GIVENMOLES 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 relationshipbetween them:

EX: 50.0g of Na is reacted with 50.0g of Cl2 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: N2H4 + 2 H2O2 N2 + 4 H2O

65.0g of N2H4 reacts with 75.0g of H2O2

a) what is the limiting reactant?b) how much N2 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 productthat 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 YieldTheoretical Yield

Percent Yield = x 100

Percent Yield Problems

1. In an experiment, 100. g of Na is reacted with excess Cl2. 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 O2 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 soluteliters of solution

Molarity is used to convert between volume of solutionand moles of dissolved solute.

1. How many moles of NaCl are in 3.5 L of 2.0 M NaCl?

2. How many moles AgNO3 are in 125 mL 3.00 M AgNO3?

GIVENGRAMS OF“A” (mass)

GIVENMOLES 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 AgNO3 andthe following rxn occurs: 2 AgNO3 + Cu 2 Ag + Cu(NO3)2

What mass of Ag can be produced?

2) 125 mL of 2.00 M Pb(NO3)2 is mixed with excess NaCl andthe following rxn occurs: 2 NaCl + Pb(NO3)2 PbCl2 + 2 NaNO3

What mass of PbCl2 precipitates?