# How do you calculate formula mass of a compound?

Jun 6, 2017

See explanation

#### Explanation:

The easiest way to do this, in my opinion, is to look up the mass of each element that make up the compound via the Periodic Table and add them up.

Example: Find the formula Mass of ${\text{CaCl}}_{2}$

We start by finding the atomic mass of each element: (Note: Different Periodic Tables like to give the mass in two, three, four, five digits of rounding but they should all be within the same range)

The atomic mass of $\text{Ca}$ is $40.078 \text{ *amu}$

The atomic mass of $\text{Cl}$ is $35.453 \text{ *amu}$ but if you notice, there is a subscript of $2$ in our compound which means there are two atoms of Chlorine and thus we need to account for that. Therefore, the atomic mass of ${\text{Cl}}_{2}$ is $\left(35.453 \cdot 2\right) = 70.906 \text{ *amu}$

The formula mass is then the sum of each element that make up the compound:

$\therefore 40.078 + 70.906 = 110.984 \text{ *amu}$

Sometimes, instead of writing amu, we often would express the formula mass in "g"/("mol") so writing 110.984 "g"/("mol") would also be correct