Question

What of the following is the correct Molecular mass of $Ca{(OH)_2}$?
A. $61$
B. $57$
C. $74$
D. None of the above

Answer 1

Hint: As we have learnt that molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each of the elements by the number of its atoms and finally adding them together. It is measured in unified mass written as “u” or Dalton.

Complete step by step answer:
As we have learnt that molecular mass is the sum of atomic masses of the elements present in a molecule. Different molecules contain different isotopes of an element, therefore every molecule possesses different molecular mass.
-Molecular mass is obtained by multiplying the atomic mass of each of the elements by the number of its atoms and finally adding them together. It is measured in unified mass written as “u” or Dalton.
So we can calculate the molecular mass of Calcium hydroxide as:

Atomic mass of calcium + atomic mass of hydrogen multiplied by two + atomic mass of oxygen multiplied by two.

The atomic mass of an element informs us about how many times an atom of the element is heavier than one-twelfth of an atom of carbon. The atomic mass of Calcium is $40$, oxygen is $16$ and hydrogen is $1$.

${M_{Ca{{(OH)}_2}}} = 40 + 2 \times 1 + 2 \times 16$
$M = 42 + 2 + 32$
$M = 74\;u$

Therefore the correct answer is (C).

Note: In case of solids, the formula of the compound does not represent its molecules, but only represents the ratio of different ions in the compounds which is commonly called the formula unit of the compound. Therefore we do not use the molecular mass in case of solid compounds. Instead we use formula mass of the compound obtained by adding atomic masses of the atoms in the compound.