# How do you calculate relative formula mass?

Dec 30, 2017

Refer to the explanation.

#### Explanation:

Relative formula mass, ${\text{M}}_{r}$, is determined by multiplying the subscript of each element in the formula by its relative atomic mass, ${\text{A}}_{r}$ on the periodic table. The results for each element are added together to calculate the relative formula mass. The unified mass unit or dalton, u or Da, is applied to the relative formula and atomic masses, though they actually have no unit.

Example: What is the relative formula mass for sodium sulfate, ${\text{Na"_2"SO}}_{4}$ using whole numbers for the ${\text{A}}_{r}$.

$\text{M"_r " Na"_2"SO"_4}$=(2xx"23 u Na")+(1xx"32 u S")+(4xx"16 u O")="142 u Na"_2"SO"_4

When working with moles, the relative atomic and formula masses are numerically equal the molar mass. Molar mass is the mass in grams of one mole of a substance. So the mass of one mole of sodium, $\text{23 u}$, would become $\text{23 g/mol sodium}$, and the mass of one mole of sodium sulfate, $\text{142 u}$, would be $\text{142 g/mol}$.