How do you work out the percentage mass and empirical formula?

Percentage mass:

The % by mass of EACH element in the formula.

Ex: % Element A = grams A / total grams compound * 100

Ex: Determine % composition of Sodium Nitrate:

NaNO3

1 Na:   1(22.99g)= 22.99g
1 N:     1(14.01g)= 14.01g
3 O:     3(16.00)=   48.00g

Add 22.99+14.01+48.00= 85.00g

% Na =22.99g/85.00g *100= 27.05%
% N  = 14.01g/85.00g *100= 16.48%
% O  =  48.00/85.00g  *100= 56.47%

Empirical Formula:

Determination of formula:

1. Empirical formula- it's the simplest formula for a compound. It shows the ratios in which the elements are combined.

2. Molecular formula- This is the true formula. It shows the actual number of atoms present.

Determination of Empirical Formula:

Given % composition:

1. Assume 100g sample
2. %of an element is the same as mass of the element in grams.
3. Convert grams to moles.
4. Find mole ratio. (Divide by smallest mole)

Ex: Determine the empirical formulas of the following compounds with the following compositions by mass: 21.7% C, 9.6% O, and 68.7% F.


Element:                    grams                      mole

   C                            21.7                          21.7g C / 12.01g C = 1.81 moles C

   O                              9.6                          9.6g O / 16.00g O = 0.60 moles O

   F                             68.7                          68.7g F / 19.00g F =  3.62 moles F

Take smallest mole: 0.60 moles O

C: 1.81 / 0.60 = 3
O:  0.60/ 0.60 = 1
F:  3.62 / 0.60 = 6

Empirical Formula: C3OF6