# How do I work out a formula or molecular mass?

Apr 4, 2016

For a start, you need a Periodic Table; then you need a formula to calculate; I'll use $U {F}_{6}$.

#### Explanation:

The Periodic Table tells me that $\text{Avogadro's number}$ of uranium atoms has a mass of $238.03 \cdot g$, and that such a quantity of $F$ atoms has a mass of $19.00 \cdot g$. In every exam of Chemistry and Physics you ever sit, you will and must be supplied with a Periodic Table.

What is $\text{Avogadro's number}$? It is simply $6.022140857 \times {10}^{23}$ individual items of stuff, here atomic nuclei. So to get the mass of a mole, $\text{Avogadro's number}$ of $U {F}_{6}$, I simply make the calculation $\left(238.03 + 6 \times 19.00\right) \cdot g \cdot m o {l}^{-} 1$. Formulae such as $N a C l$, $C a C {O}_{3}$, $C {O}_{2}$, $F e {\left(N {O}_{3}\right)}_{2}$, are calculated in precisely the same way. Get the atomic molar masses (from the Periodic Table, no-one expects you to remember them!), and sum the atomic masses correctly and appropriately.

This video will show you how to calculate the molar masses of two compounds, $S i {O}_{2}$ and $M g {\left(O H\right)}_{2}$.

Hope this helps!