Percentage CompositionPercentage Composition

PercentPercent

PartPart

WholeWholeX 100%X 100%

Mass of elementMass of elementMass of compoundMass of compound

X 100%X 100%

Percent by mass =Percent by mass =

Percent CompositionPercent Composition

Can be determined by analysis.Can be determined by analysis.

Or, if you know the formula:Or, if you know the formula:

Just assume you have 1 moleJust assume you have 1 mole

Divide the mass of each element by Divide the mass of each element by the formula mass of the compoundthe formula mass of the compound

Percent Composition of HPercent Composition of H22OO

2 X H = 2 X 1.0 = 2.0 2 X H = 2 X 1.0 = 2.0 1 X O = 1 X 16.0 = 16.01 X O = 1 X 16.0 = 16.0

Molar mass = 2.0 + 16.0 = 18.0Molar mass = 2.0 + 16.0 = 18.0

%H = (2.0/18.0) X 100% = 11.2%%H = (2.0/18.0) X 100% = 11.2%

%O = (16.0/18.0) X 100% = 88.8%%O = (16.0/18.0) X 100% = 88.8%

Percent Composition of CPercent Composition of C66HH1212OO66

6 X C = 6 X 12.0 = 72.06 X C = 6 X 12.0 = 72.0

12 X H = 12 X 1.0 = 12.012 X H = 12 X 1.0 = 12.0

6 X O = 6 X 16.0 = 96.06 X O = 6 X 16.0 = 96.0

Molar mass = 180.0 gramsMolar mass = 180.0 grams

% C =% C =

% H = % H =

% O =% O =

(72.0/180.0) X 100% = 40.0%(72.0/180.0) X 100% = 40.0%

(12.0/180.0) X 100% = 6.67%(12.0/180.0) X 100% = 6.67%

(96.0/180.0) X 100% = 53.3%(96.0/180.0) X 100% = 53.3%

Percent Composition of NaClPercent Composition of NaCl

Molar mass of NaCl = 58.5 g/moleMolar mass of NaCl = 58.5 g/mole

% Na = 23.0/58.5 X 100% = 39.3%% Na = 23.0/58.5 X 100% = 39.3%

% Cl = 35.5/58.5 X 100% = 60.7%% Cl = 35.5/58.5 X 100% = 60.7%

(Or, 100% - 39.3% = 60.7%)(Or, 100% - 39.3% = 60.7%)

Percentage CompositionPercentage Composition

They won’t remind you that you They won’t remind you that you have to calculate the formula have to calculate the formula mass!mass!