Question

The mole in 16 g calcium is 0.4. What is the molar mass of Ca atoms?
a.) 4g/mole
b.) 6.4 g/mole
c.) 0.04 g/mole
d.) 40 g/mole

Answer 1

Hint: Mole is a convenient unit to use because of the great number of atoms, molecules, or others in any substance. Mole is defined as the $6.02214076 \times 10^{23}$ of some chemical unit, whether its atoms, molecules, ions, or others.

Complete step by step solution:
The molecular mass of one molecule is its mass expressed in atomic mass units or 1/12 the mass of one atom of the isotope carbon-12 its atomic unit is Dalton or Da. The relative molecular mass is frequently indicated as molecular weight of a molecule.
To calculate the number of moles, divide the mass of the substance in grams by its molecular weight. This will give you the number of moles of that substance that are in the specified mass.
$Number\,of\,moles\,=\,\dfrac{Given\,mass}{Molecular\,mass}$
For example, there is 12 g of water, and the molecular mass of water can be calculated by:
(Molecular mass of Hydrogen) × 2 + molecular mass of Oxygen
= (1) × 2 + 16
=18 g/mol
Number of moles = $\dfrac{12g}{18g/mol}$ = 0.666 moles.
When dividing by two units that both use grams in their measurement the grams cancel each other out, leaving only the moles.
Now, as given here in the question,
Number of moles of calcium given = 0.4
Given mass of calcium =16g
From the formula given above, we can write.
$0.4=\dfrac{16g}{Molecular\,mass}$
Molecular mass = 40g/mole

Therefore, from the above given statements we can conclude that the molecular mass of calcium is 40g/mole. So, the correct option is (d).

Note: The molecular mass of a compound can be calculated by adding the standard atomic masses of the constituent atoms as we have done in our problem for calculating the molecular mass of water.