Question

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

Answer 1

Hint: The relative formula mass of a compound is derived from the total mass of elements comprising the compound. To calculate the relative formula mass, we would need the relative mass number of the comprising elements.

Complete step by step answer:
Let us see the basics and solve the given illustration;
We have the compound as calcium carbonate i.e. $CaC{{O}_{3}}$ . we need to have a brief idea about the number of atoms, calcium carbonate is made up of. This can be done just by looking at the molecular formula.
 $CaC{{O}_{3}}$ has one calcium, one carbon and three oxygen.
- Now, we need to have information about the relative mass numbers of all those elements. This information can be derived from the modern periodic table. Mass number is the sum total of the number of protons and neutrons in the nucleus of an atom.
Mass number of Calcium = 40
Mass number of Carbon = 12
Mass number of Oxygen = 16
Thus, the relative formula mass can be stated as;
$\begin{align}
 & Ca\left( 1\times 40 \right)+C\left( 1\times 12 \right)+O\left( 3\times 16 \right) \\
 & = 40 + 12 + 48 \\
 & = 100g/mol \\
\end{align}$
This tells us that one mole of $CaC{{O}_{3}}$ will have the mass of 100 g.

Note: Here, we have used a relative mass number of the elements rather than the atomic masses. This is because the mass number is specific for the element i.e. it would be the sum total of neutrons and protons for that specific element whereas, atomic mass will be the average mass of all the isotopes of that element.