Stoichiometry

Chemical Equations

Short hand way to represent chemical reactions

H2 + Cl

2 → HCl

Symbols

+ = reacts with

→ = produces, yields

Δ = adding energy (usually heat)

↔ = reversible

aq - aqueous

s - solid

l - liquid

g - gas

Balancing Reactions

Law of Conservation of mass Change coefficients not subscripts why? Start balancing with elements other than H and

O Next balance H and then O last Trial and Error

Balancing Practice

__Al +__HCl → __AlCl3 + __H

2

__H3PO

4 +__ HCl → __PCl

5 + __ H

2O

__ CO(g) + __H2(g) → __C

8H

18(l) + __H

2O

__C2H

6 + __O

2 → __H

2O + __CO

2

__(NH4)

3PO

4 + __Pb(NO

3)

4 → __Pb

3(PO

4)

4 +

__NH4NO

3

Types of Reactions

Combination – A + B → C

N2(g) + 3H2(g) → 2NH3(g)

Decomposition – C → A + B

2KClO3(s) → 2KCl(s) + 3O2(g)

Combustion – oxidation reaction that is exothermic

Most often involves O2

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Single Replacement – AB + C → A + BC

Use activity series

2HCl + F2 → 2HF + Cl2 Double Replacement – AB + CD → AD + CB

3 Driving Factors

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Formula Weights

Definition – sum of the atomic weights of each atom

Molecular weight – formula weight of a molecule

Calculate the formula weight of: Ca(MnO

4)

2

KH2PO

4

Percent Compostion

Definition - percent of mass contributed by each element

% Comp = # of a particular atom x atomic mass

Total mass of the compound Calculate the percentage of phosphorus in

P4O

10.

Mole

Avogadro's # - 1 mole = 6.02x1023 objects Representative Particles

Formula Units Molecules Atoms

Calculate the number of oxygen atoms in 1.5 moles of sodium carbonate.

Mole

Molar mass – mass of one mole of a compound Check if you are working with an isotope Find the sum of the masses of the atoms

Unit is g/mol

Grams ← use g/mol → Moles ← use Avogadro's # → Representative particles

Mole Practice

What is the mass, in grams of 6.33 mol of NaHCO

3?

How many nitric acid molecules are in 4.20 grams of HNO

3? How many oxygen atoms?

Empirical and Molecular Formula

Finding Empirical Formula Assume 100 g Convert from grams to moles Divide by smallest # of moles Use results as subscripts

Convert Empirical to Molecular Divide molar mass by empirical mass Multiply empirical formula by result

Molecular Formula Example

What is the molecular formula of a compound that is 71.65% Cl, 24.27% C, 4.07% H and has a molar mass of 98.96g/mol?

Molecular Formula Practice

Caffeine contains 49.48% C, 5.15% H, 28.27% N, 16.49% by mass and has a molar mass of 194.2 g/mol. Determine the molecular formula of caffeine.

Molecular Formula Example

Isopropyl alcohol is made up of C,H, and O. Combustion of 0.255g of this alcohol produces 0.521g CO

2 and 0.306g H

2O. What is the

molecular formula if the molar mass is 60 g/mol.

Molecular Formula Practice

Caproic acid, which is responsible for the foul odor of dirty socks, is composed of C, H, and O atoms. Combustion of a 0.255g sample of this compound produces 0.512 g CO

2 and 0.209g

H2O. What is the molecular formula is the molar

mass is 116 g/mol?

Calculations from Balances Equations

First ensure that equation is balanced. Use mole ratios Grams A → Moles A → Moles B → Grams B When potassium chlorate is heated it

decomposes in potassium chloride and oxygen. How many grams of oxygen can be prepared from 4.50g of potassium chlorate.

Calculations practice

Milk of magnesia (Mg(OH)2) is often used as an

antacid. It neutralizes excess hydrochloric acid secreted by the stomach. How many grams of milk of magnesia are needed to remove 150 mg of hydrochloric acid?

Limiting Reagents

Limiting Reagents – reactant that runs out Excess Reagents – reactant that is left over Limiting reagent gives you the least amount of

product

Limiting Reagent Example

A strip of zinc metal weighing 2.00g is placed in an aqueous solution containing 2.50g of silver nitrate. Silver and zinc nitrate are produced. Which reactant is limiting? How many grams of each product will form? How grams of excess reagent will be left over?

Limiting Reagent Practice

Nitrogen gas can prepared by passing gaseous ammonia over solid copper (II) oxide at high temperatures. The other products of the reaction are solid coper and water vapor. If a sample containing 18.1g NH

3 is reacted with

90.4g CuO, which is the limiting reagent? How many grams of N

2 are made? How much

excess reagent is left?

Yield

Theoretical Yield – amount of product formed when all of the LR is used up

Actual Yield – amount of product formed during an experiment

Percent Yield – actual yield x 100%

theorectical yield

Yield Example

When iron (III) oxide react with carbon monoxide to form iron and carbon dioxide. If you start with 150g of iron (III) oxide as the limiting reagent, what is the theoretical yield? If the actual yield of iron was 87.9g, what was the percent yield?

Yield Practice

Methanol (CH3OH) is used as a fuel in race cars. It can be manufactured by a combination of gaseous carbon monoxide and hydrogen. If 68.5 kg of CO is reacted with 8.60 kg H2. Calculate the theoretical yield of methanol. If 3.57x104g of methanol is produced, what is the percent yield of methanol?

Homework

6, 8, 18, 30, 46, 48, 56, 58, 60, 70, 74, 76, 78

Homework

6, 8, 18, 30, 46, 48, 56, 58, 60, 74, 76, 78