Question

What is the atomic mass of calcium sulphate?

Answer 1

Hint: A molecule or a compound is an assembly of atoms whose identities are given in their chemical formula and thus we can calculate the atomic mass of any molecule or compound from its composition by adding together the masses of the constituent atoms. The chemical formula of calcium sulphate is $\text{CaS}{\text{O}}_4$.

Complete answer:
The atomic mass of a substance is the sum of the masses of the atoms present in one molecule of a substance. It can be calculated by adding together the atomic masses of the elements in the substance, each multiplied by the number written as their subscript (if no subscript is present, then multiply by 1) in the molecular formula.
The step-by-step procedure for calculating the atomic mass of calcium sulphate is illustrated below:
1 – The molecular formula of calcium sulphate can be written as: $\text{CaS}{\text{O}}_4$. It shows that calcium sulphate has one calcium atom, 1 sulphur atom, and 4 oxygen atoms.
2 – Taking the values of atomic masses of respective elements from the periodic table, we can write:
$$\begin{array}{rl}& 1\times \text{atomic mass of calcium}=1\times \left(40.078\text{ amu}\right)=40.078\text{ amu}\\ & 1\times \text{atomic mass of sulphur}=1\times \left(32.065\text{ amu}\right)=32.065\text{ amu}\\ & 4\times \text{atomic mass of oxygen}=4\times \left(15.999\text{ amu}\right)=63.996\text{ amu}\end{array}$$
3 – Now, add all the masses together to obtain the atomic mass of $\text{CaS}{\text{O}}_4$.
$$\begin{array}{rl}& \text{Atomic mass of CaS}{\text{O}}_4=(40.078+32.065+63.996)\text{ amu}\\ & \therefore \text{Atomic mass of CaS}{\text{O}}_4=136.139\text{ amu}\end{array}$$
Hence, the atomic mass of calcium sulphate is 136.139 atomic mass unit (amu).

Note:
Calcium sulphate is found as a pentahydrate compound in nature $\left(\text{CaS}{\text{O}}_4\text{.5}{\text{H}}_2\text{O}\right)$ and in its anhydrous form, it is used as a desiccant. It is one of the constituents that make the water hard. Its hemihydrate form $\left(\text{CaS}{\text{O}}_4\text{.}{\displaystyle \frac{1}{2}}{\text{H}}_2\text{O}\right)$ is popularly known as Plaster of Paris that is used in setting fractured bones.