- multiply atomic mass of each element by number of atoms of that element in the formula (shown by the subscript)

- find the sum of all the atomic masses --this is formula mass (unit is a.m.u.)

- express formula mass in grams (unit is g/mol). This is the
**Molar Mass.**

**II. Calculating % Composition (from formula)**

- calculate formula mass

- divide the total atomic mass of each element by the formula mass and multiply by 100

**III. Calculating % Composition (from masses of each element)**

- divide the mass of each element by the total mass of the compound and multiply by 100

- convert % of each element to grams based on 100 grams of the compound

- multiply grams of each element by 1/molar mass that element

- compare ratio of moles of each element and divide each by the smallest

- if result in step 3 gives a ratio with decimal equivalent to 1/4, 1/3, 1/2, 2.3, 3/4 instead of whole numbers, convert to the fraction and multiply all ratios by the denominator or the fraction

**V. Calculating Empirical Formula (from experimentally determined masses)**

- multiply the mass of each element (in grams) by 1/molar mass of that element

- continue with steps 3 & 4 from IV above.

Example

**VI. Finding Molecular Formulas (when molar mass is known)**

- calculate the empirical formula
- use the equation : (empirical formula mass)x = molar mass

- find value for x: x = molar mass/empirical formula mass

- multiply each subscript in empirical formula by value for x

Example

Gwen Sibert at the

Roanoke Valley Governor's School

gsibert@rvgs.k12.va.us