Mass percent composition of a molecule shows the amount each element in a molecule contributes to the total molecular mass. Each element's contribution is expressed as a percentage of the whole. This step by step tutorial will show the method to determine the mass percent composition of a molecule.

### An Example With Potassium Ferricyanide

Calculate the mass percent composition of each element in a potassium ferricyanide, K_{3}Fe(CN)_{6} molecule.

### The Solution

**Step 1: Find the atomic mass of each element in the molecule.**

The first step to finding mass percent is to find the atomic mass of each element in the molecule.

K_{3}Fe(CN)_{6} is made up of potassium (K), iron (Fe), carbon (C) and nitrogen (N).

Using the periodic table:

Atomic mass of K: 39.10 g/molAtomic mass of Fe: 55.85 g/molAtomic mass of C: 12.01 g/molAtomic mass of N: 14.01 g/mol

**Step 2: Find the mass combination of each element.**

The second step is to determine the total mass combination of each element. Each molecule of KFe(CN)6 contains 3 K, 1 Fe, 6 C and 6 N atoms. Multiply these numbers by the atomic mass to get each element's mass contribution.Mass contribution of K = 3 x 39.10 = 117.30 g/molMass contribution of Fe = 1 x 55.85 = 55.85 g/molMass contribution of C = 6 x 12.01 = 72.06 g/molMass contribution of N = 6 x 14.01 = 84.06 g/mol

**Step 3: Find the total molecular mass of the molecule.**

The molecular mass is the sum of the mass contributions of each element. Simply add each mass contribution together to find the total.

Molecular mass of K_{3}Fe(CN)_{6} = 117.30 g/mol + 55.85 g/mol + 72.06 g/mol + 84.06 g/mol

Molecular mass of K_{3}Fe(CN)_{6} = 329.27 g/mol

**Step 4: Find the mass percent composition of each element.**

To find the mass percent composition of an element, divide the mass contribution of the element by the total molecular mass. This number must then be multiplied by 100% to be expressed as a percent.

Mass percent composition of K = mass contribution of K/molecular mass of K_{3}Fe(CN)_{6} x 100%

Mass percent composition of K = 117.30 g/mol/329.27 g/mol x 100%Mass percent composition of K = 0.3562 x 100%Mass percent composition of K = 35.62%Mass percent composition of Fe = mass contribution of Fe/molecular mass of K_{3}Fe(CN)_{6} x 100%

Mass percent composition of Fe = 55.85 g/mol/329.27 g/mol x 100%Mass percent composition of Fe = 0.1696 x 100%Mass percent composition of Fe = 16.96%Mass percent composition of C = mass contribution of C/molecular mass of K_{3}Fe(CN)_{6} x 100%

Mass percent composition of C = 72.06 g/mol/329.27 g/mol x 100%Mass percent composition of C = 0.2188 x 100%

Mass percent composition of C = 21.88%Mass percent composition of N = mass contribution of N/molecular mass of K_{3}Fe(CN)_{6} x 100%

Mass percent composition of N = 84.06 g/mol/329.27 g/mol x 100%Mass percent composition of N = 0.2553 x 100%Mass percent composition of N = 25.53%

### The Answer

K_{3}Fe(CN)_{6} is 35.62% potassium, 16.96% iron, 21.88% carbon and 25.53% nitrogen.

It is always a good idea to check your work. If you add up all the mass percent compositions, you should get 100%.35.62% + 16.96% + 21.88% + 25.53% = 99.99%Where is the other .01%? This example illustrates the effects of significant figures and rounding errors. This example used two significant figures past the decimal point. This allows for an error on the order of ±0.01. This example's answer is within these tolerances.