Chemistry 112010-2011

Percentage Composition

• There are some compounds that we know have elements in fixed mass proportions.

• WaterH2O 2 g : 16 g

• Carbon dioxide

CO2 12 g : 32 g

Percentage Composition

The elements in a given chemical compound are always present in the same proportions by mass.

Law of Definite Proportions

Mass Percent (Percentage Mass)

The mass of an element in a compound, expressed as a percent of the total mass of the compound.

Note: The Law of Definite proportions does not imply that elements in compounds are always present in the same relative amount.

% = Part__ WholeWhat percent of 92 is 42?

In other words, the Part “42” of the whole “92” is what percent?

42 x 100 = 45.7 %

92

Percentage Mass

% Mass of Element = Mass of Element x 100% Mass of CompoundExample 1: CO2

C = 12.00 g = 12.00 gO = 2 x 16.00 g = + 32.00 g_ 44.00 g = Molar Mass

% Mass of C = 12.00 g_ x 100 = 27.3% 44.00 g

% Mass of O = 32.00 g_ x 100 = 72.7% 44.00 g

Finding % Mass

% Mass of Element = Mass of Element x 100% Mass of Compound

Example 2: C6H12O6

C = 6 x 12.00 g = 72.00 gH = 12 x 1.01 g = 12.12 gO = 6 x 16.00 g = + 96.00 g_ 180.12 g = Molar Mass

Finding % Mass

% Mass of Element = Mass of Element x 100% Mass of Compound

Ex. C6H12O6

% Mass of C = 72.00 g_ x 100 = 40.0%

180.12 g

% Mass of H = 12.12 g_ x 100 = 6.7%

180.12 g

% Mass of O = 96.00 g_ x 100 = 53.3%

180.12 g

Finding % Mass

Example 3:

A sample of a compound has a mass of 48.72 g. The sample is found to contain 32.69 g of zinc and 16.03 g of sulfur. What is the percentage composition of the compound?

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Page 82Question’s 1, 2, 3, & 4.

Questions

1: In a homogeneous sample, the size of the sample does NOT matter.

2: Assume one mole of compound. Therefore, you can calculate % composition using the molar mass and chemical formula.

Using a Chemical Formula to Compute % Composition

Find the % composition of ZnO.

Example

Page 85Question’s 5, 6, 7, & 8.

Questions

• In chemistry, empirical means “by experiment”.

• Thus, an empirical formula is a formula devised from experimental data (rather than theory).

Empirical Formula

• When an unknown compound is synthesized in the laboratory, an elemental analysis is performed to determine its percent composition. This gives the percent (by mass) of each element of the compound. From this data, the empirical formula of the compound can be determined.

Empirical Formula(Cont’d)

A compound is 82.6% carbon and 17.4% hydrogen by mass. What is the empirical formula?

Step 1: Assume a 100 gram sample

82.6% of 100 g = 82.6 g

17.4% of 100 g = 17.4 g

Sample Problem

Step 2: Convert grams to moles

82.6 g C x 1 mol = 6.883 mol C 12.0g

17.4 g H x 1 mol = 17.4 mol H 1.01g

C6.883H17.4

Although this is a mole ratio of C to H, we want a WHOLE NUMBER Mole Ratio.

Empirical Formula

Step 3: Get the WHOLE NUMBER mole ratio by dividing each value by the smallest

6.883 mol = 16.883 mol

17.4 mol = 2.528 = 2.56.883 mol

This is still not a whole number ratio, so we need to determine the least common multiple (LCM)

Empirical Formula

Step 4: Find the least common multiple

6.883 mol = 1 x 2 = 26.883 mol

17.4 mol = 2.528 = 2.5 x 2 = 56.883 mol

Since this is the lowest possible whole number ratio between carbon and hydrogen, the empirical formula of the compound is C2H5.

Empirical Formula

Calculate the empirical formula of a compound that is 85.6% carbon and 14.4% hydrogen.

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