Stoichiometry with Chemical Reactions

Stoichiometry Unit Objectives

Solve reaction stoichiometry problems:

Mole- MoleMass – MoleMass-MassMass Volume

Introduction In any stoichiometry problem we start with a

given quantity of a reactant or product. We use conversion factors and unit analysis to determine the quantities of the unknowns that react or form.

Conversion factors: 1. Molar Mass : ________________ 2. Mole Ratio : _____________ 3. Molar Volume of a Gas: ______________

The GENERAL PROCESS

Start or Stop where needed to solve for desired unit from the given information.

Grams or Liters of known substance

Moles of known substance

Moles of unknown substance

Grams or Liters of unknown substance

Mole to Mole

Mole Ratio:

Mole ratio comes from: ___________________ Example: Find all mole ratios for the reaction of

the combustion of ethane. ___C2H6 + ___O2 ___CO2 + ___H2O

Mole to Mole - Problem Solving Process: Use a mole ratio to convert from moles of known to moles of unknown.

Mole-Mole Example:

How many moles of CO2 will be produced from the combustion of 0.575mol of C2H6?

Mole to Mass Problems: Start with a given number of moles, convert to unknown number of grams.

Problem Solving Process:

Mole to Mass Example In photosynthesis, plants use the sun’s energy

to produce glucose and oxygen from water and carbon dioxide. How many grams of glucose are produced when 5.25 moles of Carbon Dioxide react with excess water?

Mole to Mass Example (cont.)

Mole to Mass (cont.)

What mass of water is needed to react with the above carbon dioxide?

Mass – Mass Problems

Start with a given number of moles, convert to unknown number of grams.

Mass to Mass - Problem Solving Process

Mass to Mass Industrial solvent Carbon disulfide is produced

in the following unbalanced reaction:

C + SO2 CS2 + CO

How many grams of CS2 and CO are formed if 12.5g of SO2 react?

Volume Problems

Recall the standard molar volume of a gas:22.4 L/mol This is true only at STP: 1 ATM and 0 degree C Avogadro’s Law: Since a mole of any gas occupies the same volume

as any other gas, a volume ratio is the same as a mole ratio.

Ex: __1_ N2 + _3__ H2 __2_ NH3

Mole Ratios: 1 mol N2: 3 mole H2: 2 mol NH3

Volume Ratios: 1L N2: 3 L H2: 2 L NH3

Mass to Volume - Problem Solving Process

Mass to Volume

A piece of magnesium metal with a mass of 2.76 g is added to a solution of Hydrochloric acid. What is the volume of Hydrogen gas produced at STP?

Limiting Reactants Stoichiometric Proportions:

Limiting Reactant:

Excess Reactant:

LR – How ToTo find the Limiting reactant in a chemical reaction:1. Pick a reactant and assume it is the LR. 2. Calculate the amount of the other reactant needed to use

up all of the LR.3. Compare the amount needed to the amount available, for

the second reactant. (given in the problem.) If you have enough of it (more available than needed) then it is

the XS reactant, and you guessed right: The first reactant is the LR

If you don’t have enough of it (more needed than you are given) then you guessed wrong: the second reactant is the LR and the first is in XS.

2 H2 + O2 2 H2O

Determine the LR of the reaction when given 1.22g O2 and 1.05g H2.

H2 + Cl2 2HCl

Determine the LR of the reaction when given 2.92g Cl2 and 3.65g H2.

% Yield in a Chemical Reaction % Yield =

Actual Yield =

Expected Yield =

Actual Yield is always less than expected yield and there are several reasons for this:

Actual Yield is always less than expected yield. There are several reasons for this:

1.

2.

3.

4.

% Yield Example Problem

When 4.9g of Magnesium are burned in excess oxygen, 6.5g of Magnesium oxide are formed. What is the % yield of the chemical reaction?