Section 3.2 Stoichiometry and Compound

Formulas

Compound Stoichiometry

In this section…

a. Element composition

b.Percent composition from formulas

c. Determining empirical formulas

d.Determining molecular formulas

e.Hydrated compounds

Element Composition:

You have a sample of a compound with knownmass.

What are the amounts of each element present?

Element Composition:You have 2.50-mol sample of CH3CO2H.

What are the amounts of each element present?

Percent Composition:

A compound is formed from two or more elementsin a set ratio of atoms.

The ratio of atoms gives the chemical formula.

The ratio of masses of those atoms gives the percent composition.

Determining Percent Composition:

Determining Percent Composition:

What is the percent composition of each element in N2O5?

*** If given just the formula, assume you have 1 mol of the compound

formula %composition

Determining Empirical Formulas from Percent Composition:

Process:

1.for each element, mass present = % comp in g2.convert g to mol for each element3.compare mol of each element to get formula

*** If given just the percent compositions, assume you have 100 g of the compound

%composition formula

Determining Empirical Formulas from Percent Composition:

Use these % composition values to determine the empirical formula:

Determining Empirical Formulas from Percent Composition:

Some useful fraction-to-formula ratios:

1.2 1 + 1/5 6/5 A5B6

1.25 1 + ¼ 5/4 A4B5

1.33 1 + 1/3 4/3 A3B4

1.4 1 + 2/5 7/5 A5B7

1.5 1 + ½ 3/2 A2B3

1.6 1 + 3/5 8/5 A5B8

1.67 1 + 2/3 5/3 A3B5

1.75 1 + ¾ 7/4 A4B7

1.8 1 + 4/5 9/5 A5B9

Empirical vs. Molecular Formulas

Empirical Formula:

Molecular Formula:

Example: Molar mass of empirical formula (CH2) = 14.03 g/molMolar mass of compound measured by experiment to be 56.11.

If you know the empirical formula, how do you find the molecular formula?

answer: molar masses of the two

Molecular Formula =

Determining Molecular Formulas from Percent Composition and Molar mass

Process:

1.for each element, mass present = % comp in g2.convert g to mol for each element3.compare mol of each element to

get empirical formula4.compare empirical formula molar mass to compound molar mass

%composition empirical formula molecular formula

molar mass

Determining Molecular FormulaA compound has molar mass of 110.11 g/mol and contains:

65.45% C

5.493% H

29.06% O

Empirical Formula =

Empirical Formula (C3H3O)molar mass = 55.06 g/mol

Molecular formula =

Hydrated Compounds

Hydrated Compounds

How do you determine the number of waters of hydration?Answer: heat off the water and see how much mass decreases

mass loss = mass H2O moles H2O = # of waters offinal mass = mass anhydrous compound moles hydration anhydrous compound