How to Calculate Mass Percent

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, K₃Fe(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. K₃Fe(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/mol
• Atomic mass of Fe: 55.85 g/mol
• Atomic mass of C: 12.01 g/mo
• lAtomic 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/mol
• Mass contribution of Fe = 1 x 55.85 = 55.85 g/mol
• Mass contribution of C = 6 x 12.01 = 72.06 g/mol
• Mass 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 K₃Fe(CN)₆ = 117.30 g/mol + 55.85 g/mol + 72.06 g/mol + 84.06 g/mol
Molecular mass of K₃Fe(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.

For K:

• Mass percent composition of K = mass contribution of K/molecular mass of K₃Fe(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%

For Fe:

• Mass percent composition of Fe = mass contribution of Fe/molecular mass of K₃Fe(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%

For C:

• Mass percent composition of C = mass contribution of C/molecular mass of K₃Fe(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%

For N:

• Mass percent composition of N = mass contribution of N/molecular mass of K₃Fe(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

K₃Fe(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.