Question

Molecular Mass of $Ca{{(OH)}_{2}}$ is:
(a)- 61
(b)- 57
(c)- 74
(d)- None of above

Answer 1

Hint: The given compound in the question is $Ca{{(OH)}_{2}}$ and this compound is called calcium hydroxide, which there are 5 atoms involved in the formation of this molecule, i.e., one calcium atom, two hydrogen atoms, and two oxygen atoms.

Complete answer:
Molecular mass is the mass of the molecule that is formed with similar or different types of atoms. Each atom in the periodic table has an atomic number and atomic mass. This atomic mass is used to calculate the molecular mass of the molecule.
The given compound in the question is $Ca{{(OH)}_{2}}$ and this compound is called calcium hydroxide, which there are 5 atoms involved in the formation of this molecule, i.e., one calcium atom, two hydrogen atoms, and two oxygen atoms.
So, by adding the atomic mass of these atoms we can find the molecular mass. The atomic mass of hydrogen is 1, the atomic mass of oxygen is 16, and the atomic mass of calcium is 40.
We have to add this mass in the number of atoms ratio present in the molecule.
Molecular mass = atomic mass of calcium + 2 (atomic mass of hydrogen + atomic mass of oxygen)
Molecular mass = 40 + 2 (1 + 16)
Molecular mass = 40 + 34
Molecular mass = 74
So, the molecular mass of $Ca{{(OH)}_{2}}$ (calcium hydroxide) is 74.

Therefore, the correct answer is an option (c)- 74 .


Note:
Don’t forget to multiply the number of each atom present in the molecule. If you want to take the exact mass of the atoms, then the atomic mass of hydrogen will be 1.000784, the atomic of oxygen will be 15.999, and the atomic mass of calcium will be 40.078.