# How do you calculate the relative formula mass (M) of calcium carbonate? Relative atomic masses C = 12, O = 16, Ca = 40?

Jul 1, 2018

100

#### Explanation:

To calculate the relative formula mass of calcium carbonate, we first need to write out its formula. It's an ionic compound formed between calcium ion $\left(C {a}^{\text{2+}}\right)$ and carbonate ion $\left(C {O}_{3}^{\text{2-}}\right)$.

There are several ways to figure out the formula, one of the easier way is using the cross-over method. You may watch it here:
How to Write formula for Ionic Compound

After crossing over the number of the charges in $C {a}^{\text{2+}}$ and carbonate ion $C {O}_{3}^{\text{2-}}$, we'll get $C {a}_{2} {\left(C {O}_{3}\right)}_{2}$, which can be further simplified into $C a C {O}_{3}$.

To calculate the relative formula mass, we'll need to count the number of atoms that are present in $C a C {O}_{3}$.
How to Count Atoms

In $C a C {O}_{3}$, we have
1 Ca
1 C
3 O

And then, we multiply with its atomic mass and add them all up, like this,

Relative formula mass $C a C {O}_{3}$ = (1 x 40) + (1 x 12) + (3 x 16) = 100