Chapter 10II. Formula Calculations

Topic 9 – The Mole

A. Percentage Composition

• the percentage by mass of each element in a compound

100mass total

element of massncompositio %

%Fe =

%O =

• Ex 1: Find the percentage composition of a sample that is 28 g Fe and 8.0 g O.

%Cu =

%S =

• Ex 2: Find the % composition of Cu2S.

• Ex 3: How many grams of copper are in a 38.0-g sample of Cu2S? (from example 2 , we know the % Cu in Cu2S).

Cu2S is 79.852% Cu (from the last slide)

• We solved example 3 in two steps. It can be done in one longer step. Let’s look at that option.

• Ex 4: How many grams of Calcium are in a 50.0-g sample of Calcium Chloride?

%H2O =

• Ex 5: Find the mass percentage of water in calcium chloride dihydrate, CaCl2•2H2O?

B. Empirical Formula

C2H6

CH3

reduce subscripts

• Smallest whole number ratio of atoms in a compound

• If you found a molecular compound to be

Ca6(PO3)4 , what would the empirical formula for that compound be?

B. Empirical Formula Calculation1. Find mass (or %) of each element.

2. Find moles of each element.

3. Divide moles by the smallest # to find subscripts.

4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

Percent to massMass to mole

Divide by smallMultiply ‘til whole

Commit this to memory and you will rock Empirical formulas!

B. Empirical Formula• Find the empirical formula for a

sample of 25.9% N and 74.1% O.

25.9 g 1 mol

14.01 g = 1.85 mol N

74.1 g 1 mol

16.00 g = 4.63 mol O

1.85 mol

1.85 mol

= 1 N

= 2.5 O

B. Empirical Formula

N1O2.5

Need to make the subscripts whole numbers multiply by 2

N2O5

• Ex 2: A compound was found to contain 39.12% C, 8.76% H, and 52.12% O. Determine the empirical formula of the substance.

Percent to massMass to moleDivide by smallMultiply ‘til whole

C. Molecular Formula• “True Formula” - the actual number

of atoms in a compound

CH3

C2H6

empiricalformula

molecularformula

?

• Here is an analogy to use to describe the difference.• If you are at a birthday party and there are 28 friends, 7

of which are men, what is the ratio of women compared to men?

• This is like the empirical formula

• If someone asked you to tell the actual number of women compared to men, what would you tell them?

• This is like the molecular formula.

C. Molecular Formula Calculation1. Find the empirical formula.2. Calculate the empirical formula molar mass.3. Divide the molecular molar mass by the empirical molar mass.4. Multiply each subscript by the answer from step 3.

nmass EF

mass MF nEF

C. Molecular Formula• The empirical formula for ethylene is CH2. Find the molecular

formula if the molecular mass is 28.06 g/mol?

28.06 g/mol

14.03 g/mol = 2.00

empirical mass = 14.03 g/mol

(CH2)2 C2H4

• Example 4: A compound was found to contain 49.98 g of Carbon and 10.47 g of hydrogen. The molar mass of the compound was 58.12 g/mol. Determine the molecular formula.

• Example 5: A gas sample is found to have 2.34 g of nitrogen and 5.34 g of oxygen present. The molecular mass is 92.00 g/mol. What is the molecular formula for this gas?