Calculating Percent Composition Calculating Percent Composition From FormulaFrom Formula

% Composition% Composition: % by mass of each : % by mass of each element in compoundelement in compound

molarmolar mass element mass element x 100% x 100%

total molar mass compoundtotal molar mass compound

Calc. % Composition ContinuedCalc. % Composition Continued

1)1) Find total molar mass of elementFind total molar mass of element

2)2) Divide by molar mass of compoundDivide by molar mass of compound3) 3) Multiply by 100Multiply by 100Hint: The percentage of all elements should add up to = Hint: The percentage of all elements should add up to =

100%.100%.

Calculating % CompositionCalculating % Composition

Find % composition for each element in Find % composition for each element in CaCOCaCO33

Ca: Ca: 40.08g Ca__ 40.08g Ca__ x 100% = 40.04% x 100% = 40.04%

100.09g CaCO100.09g CaCO3 3

C: C: 12.01g C12.01g C x 100 %= 12.00% x 100 %= 12.00%

100.09g CaCO100.09g CaCO33

O: O: 48.00g O 48.00g O x 100%= 47.96% x 100%= 47.96%

100.09g CaCO100.09g CaCO33

Calculating % Composition From DataCalculating % Composition From Data

mass element in cmpd. mass element in cmpd. x 100% x 100% molar mass compoundmolar mass compound

Ex: Ex: A 3.60g sample of a compound contains A 3.60g sample of a compound contains 1.70g of oxygen. What is the percent 1.70g of oxygen. What is the percent compostion of oxygen in the compound?compostion of oxygen in the compound?__________1.70g O______ 1.70g O______ x 100 = 47.2 %O x 100 = 47.2 %O 3.60g compound3.60g compound

What is the percent nitrogen in a 11.2g What is the percent nitrogen in a 11.2g sample that contains 9.48g N?sample that contains 9.48g N?

Empirical FormulaEmpirical Formula- lowest whole number - lowest whole number ratio of elements in a compoundratio of elements in a compound

Molecular formulaMolecular formula- actual# of elements in - actual# of elements in a compound as it appears in naturea compound as it appears in nature

Molecular and empirical formulas may or Molecular and empirical formulas may or may not be the samemay not be the same

Examples Examples Empirical/Molecular Empirical/Molecular

FormulasFormulas EmpiricalEmpirical

COCO22

HOHO CC22HH44OO

MolecularMolecular

COCO22

HH22OO22

CC44HH88OO22

To find empirical formula:To find empirical formula: 1. Change % comp. to g1. Change % comp. to g 2. Change g to mol2. Change g to mol 3. Divide by lowest number of moles3. Divide by lowest number of moles 4. Round or Multiply to get whole numbers4. Round or Multiply to get whole numbers 5. #’s become subscripts5. #’s become subscriptsTips for #4: Tips for #4: .2 or lower, round down .2 or lower, round down .8 or above round up.8 or above round up.5 X by 2 .5 X by 2

.3 X 3.3 X 3

.25 , .75 X 4.25 , .75 X 4

What is the empirical formula of a What is the empirical formula of a compound that is 25.9 % N & 74.1%O?compound that is 25.9 % N & 74.1%O?

25.9g N x 25.9g N x 14.01g14.01g= 1.85 mol ÷ 1.85= 1 x 2=2= 1.85 mol ÷ 1.85= 1 x 2=2

1 mol1 mol

74.1g O x 74.1g O x 16.00g16.00g= 4.63 mol ÷ 1.85= 2.5x 2=5= 4.63 mol ÷ 1.85= 2.5x 2=5

1 mol1 mol

NN22OO55

To find Molecular FormulaTo find Molecular Formula

1.Calculate mass of empirical 1.Calculate mass of empirical formula formula

2. Divide molecular 2. Divide molecular mass/empirical massmass/empirical mass

3. Multiply subscripts in 3. Multiply subscripts in empirical formula by number empirical formula by number calculated in step 2calculated in step 2

Calculate molecular formula of compound Calculate molecular formula of compound whose molar mass is 216.03g/mol with an whose molar mass is 216.03g/mol with an empirical formula of Nempirical formula of N22OO55..

A compound is composed of 40.68% C, A compound is composed of 40.68% C, 5.08% H & 54.24%O. It has a molecular 5.08% H & 54.24%O. It has a molecular mass of 118.1g/mol. Find the empirical & mass of 118.1g/mol. Find the empirical & molecular formula.molecular formula.