Mass Composition

The mass composition of a compound refers to the mass percentage of each of its elements. It essentially tells you how much of the compound’s total mass comes from each element.

Calculating Mass Composition

If you want to calculate the mass percentage of a certain element, you should do

\(\text{Mass %} = \frac{\text{Mass of element in 1 Mole of the compound}}{\text{Molar mass of the compound}}\)

For example, let’s do this for \(\ce{CO2}\). Calculating the mass percentage of C, for every mole of \(\ce{CO2}\), there is 1 mole of carbon, since the molar mass of carbon is 12g/mol, there is 12 g of carbon per mole of \(\ce{CO2}\). Dividing that by the molar mass of \(\ce{CO2}\) (44 g/mol) gives.

Now, calculating the mass percentage of oxygen, we that there are 2 moles of oxygen per mole of \(\ce{CO2}\), so the total mass of oxygen is \(2 \times 16 = 32 \: g\), diving that by 44 gives

One way of checking if the results you got makes sense is adding up all the percentages. If the sum is 100%, then your results are consistent, otherwise, some mistake might have been made or you forgot to calculate the composition for some of the elements.

Determine the mass percentage of each element on the following compounds.

\(\ce{CO2}\)
\(\ce{XeF6}\)
\(\ce{PCl5}\)
\(\ce{KCl}\)

Solution:

\(\text{MM} = 12.01 \times 1 + 16.00 \times 2 = 44.01 \:\text{g/mol}\)

\(\text{M}_\text{C} = 12.01 \times 1 = 12.01 \: \text{g/mol}\)

\(\text{M}_\text{O} = 16.00 \times 2 = 32.00 \: \text{g/mol}\)

\(\% \, \text{C} = \frac{\text{M}_\text{C}}{\text{MM}} = \) \(27.29\: \% \)

\(\% \, \text{O} = \frac{\text{M}_\text{O}}{\text{MM}} = \) \(72.71\: \% \)

\(\text{MM} = 131.29 \times 1 + 18.998 \times 6 = 245.28 \: \text{g/mol}\)

\(\text{M}_\text{xe} = 131.29 \times 1 = 131.29 \: \text{g/mol}\)

\(\text{M}_\text{F} = 18.998 \times 6 = 113.988 \: \text{g/mol}\)

\(\% \: \text{Xe} = \frac{\text{M}_\text{Xe}}{\text{MM}} = 53.53\: \%\)

\(\% \: \text{F} = \frac{\text{M}_\text{F}}{\text{MM}} = 46.47\: \% \)

\(\text{MM} = 30.97 \times 1 + 35.45 \times 5 = 208.22 \: \text{g/mol}\)

\(\text{M}_\text{P} = 30.97 \times 1 = 30.97 \: \text{g/mol}\)

\(\text{M}_\text{Cl} = 35.45 \times 5 = 177.25 \: \text{g/mol}\)

\(\% \: \text{P} = \frac{\text{M}_\text{P}}/\text{MM} = 14.87 \: \% \)

\(\% \: \text{Cl} = \frac{\text{M}_\text{Cl}}{\text{MM}} = 85.13 \: \%\)

\(\text{MM} = 39.10 \times 1 + 35.45 \times 1 = 74.55 \: \text{g/mol}\)

\(\text{M}_\text{K} = 39.10 \times 1 = 39.10 \: \text{g/mol}\)

\(\text{M}_\text{Cl} = 35.45 \times 1 = 35.45 \: \text{g/mol}\)

\(\% \: \text{K} = \frac{\text{M}_\text{K}}{\text{MM}} = 52.45 \: \% \)

\(\% \: \text{Cl} = \frac{\text{M}_\text{Cl}}{\text{MM}} = 47.55 \: \% \)

Mass Composition

Introduction

Calculating Mass Composition