Calculation of % composition

There are situations when it is required to determine the percent of an element in a known chemical formula. This section will show how to make these calculations.

There are concerns that there is too much sodium in our diets. There are many compounds used in food products that contain sodium. Sodium chloride (NaCl, table salt) is the most commonly used compound  to add salt to foods.

What is the percent sodium in the compound NaCl?
Step 1: Calculate the gram formula weight for NaCl.

The gram atomic weight (GAW) for Na is 22.99 g / mol
The gram atomic weight (GAW for Cl is 35.45 g / mol
The gram formula weight (GFW) for NaCl is 58.44 g / mol

Step 2 Calculate the % Na in NaCl:
From the previous Section  Determination of Chemical Formulas you should know the calculation is:

%Na in NaCl

An Example:
A biscuit recipe has salt (NaCl) and baking powder that contains baking soda (NaHCO3), sodium aluminum sulfate (NaAl(SO4)2 ). Which compound has the highest percent Na? From the calculation above, we know NaCl is 39.34 %.

Percent Na in NaHCO3 :
GFW Bicarb

%Na in Bicarb

Percent Na in NaAl(SO4)2

% Na in AlSO4

Based on the preceding calculations, NaCl has the highest % of Na

Knowing the percent composition of compounds is used to calculate the grams of an element in a given mass of the compound(s). An example of this can be shown using a biscuit recipe.

Baking powder is a mixture containing 30 % baking soda and 24 % sodium aluminum sulfate.

A biscuit recipe calls for 3g of NaCl and 9 g baking powder (NaHCO3 & NaAl(SO4)2)

Since baking powder is 30 % NaHCO3 and 24 % NaAl(SO4)2 , the first calculation would be to multiply 9 g of baking soda by the percent of each chemical to determine the grams of each chemical, NaHCO3 and NaAl(SO4)2.

Grams of NaHCO3 = 0.30 x 9 g baking powder = 2.7 g NaHCO3

Grams of NaAl(SO4)2 = 0.24 x 9 g baking powder = 2.16 g NaAl(SO4)2

Grams total from ingredients: = grams from NaCl + grams from NaHCO3 + g from NaAl(SO4)2.

The calculation is: (using the percents calculated above)

Total grams = (0.3934 x 3 g NaCl ) + (0.2738 g x 2.7 g NaHCO3 ) + ( 0.095 x 2.16 g NaAl(SO4)2 )   = 2.125 g Na

The student will encounter calculations such as these in a regular general chemistry class.