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Formula Calculations

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Ch. 7 – The Mole

Formula Calculations

EMPIRICAL and MOLECULAR FORMULAS

- the percentage by mass of each element in a compound

127.10 g Cu

159.17 g Cu2S

32.07 g S

159.17 g Cu2S

- Find the % composition of Cu2S.

100 =

%Cu =

79.852% Cu

%S =

100 =

20.15% S

28 g

36 g

8.0 g

36 g

- Find the percentage composition of a sample that is 28 g Fe and 8.0 g O.

100 =

78% Fe

%Fe =

100 =

22% O

%O =

- How many grams of copper are in a 38.0-gram sample of Cu2S?

Cu2S is 79.852% Cu

(38.0 g Cu2S)(0.79852) = 30.3 g Cu

36.04 g H2O

147.02 g H2O

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

24.51%

H2O

%H2O =

100 =

CH3

- Smallest whole number ratio of atoms in a compound

C2H6

reduce subscripts

1. 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.

- There is a poem to help
- Percent to Mass
- Mass to Mole
- Divide by Small
- Multiply ‘til Whole
- Sometimes the mole values are still not whole numbers so multiply by the smallest factor that will make them whole numbers

1.85 mol N

1.85 mol

- Find the empirical formula for a sample of 25.9% N and 74.1% O.

25.9 g N

1 mol N

14.01 g N

= 1.85 mol N

= 1 N

74.1 g

1 mol

16.00 g

= 4.63 mol O

= 2.5 O

N2O5

N1O2.5

Need to make the subscripts whole numbers

multiply by 2

- Ex. What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% Oxygen?

Calculate the empirical formula for a compound that is 94.1% O and 5.90%H

Calculate the empirical formula for a compound that is 79.8% C and 20.2% H

C2H6

- “True Formula” - the actual number of atoms in a compound

CH3

empirical

formula

?

molecular

formula

1. Find the empirical formula.

2. Find the empirical formula mass.

3. Divide the molecular mass by the empirical mass.

4. Multiply each subscript by the answer from step 3.

28.1 g/mol

14.03 g/mol

- The empirical formula for ethylene is CH2. Find the molecular formula if the molecular mass is 28.1 g/mol?

empirical mass = 14.03 g/mol

= 2.00

2(CH2) C2H4

Calculate the molecular formula of the compound whose Molar mass is 60.0g and empirical formula is CH4N.