Question

Calculate the weight of $0.885$ moles of $Mg{\left( {N{O_3}} \right)_2}$.
(A) $13.1g$
(B) $131g$
(C) $130g$
(D) $88.5g$

Answer 1

Amount of substance is measured in moles. One mole is equal to $6.022 \times {10^{23}}$ particles i.e. Avogadro’s number. The mass of substance for a given number of moles can be calculated if molecular mass is known. Molecular mass in turn can be calculated by knowing structure or chemical formula is known.

Complete step by step answer:
The formula for calculating number of mass is:
${\text{Number of moles}}\left( n \right) = \dfrac{{{\text{Given mass}}\left( m \right)}}{{{\text{Molecular mass}}\left( M \right)}}$
Or $n = \dfrac{m}{M}$
In the given question we need to find the mass for given numbers of moles i.e. $m$. Therefore, we can rearrange the formula as
$m = n \times M$
Number of moles of $Mg{\left( {N{O_3}} \right)_2} = 0.885$
And molecular mass of $Mg{\left( {N{O_3}} \right)_2}$
$= 24 + {\left( {14 + \left( {16 \times 3} \right)} \right)_2}$
$= 24 + {\left( {62} \right)_2}$
$= 24 + 124$
$= 148g$
Hence, the mass of $0.885$ moles of $Mg{\left( {N{O_3}} \right)_2}$ is $131g$.

Additional information: In the US, October $23$ is known as Mole day. It is an informal holiday in honour of units among scientists. It starts from $6.02$ am because Avogadro number is $6.022 \times {10^{23}}$.
Number of moles have an important role in the calculation of mole fraction, molar concentration, molar mass, etc.

Note:
The amount of substance i.e. moles have as many elementary particles as there are atoms in 12g of C-12 isotope of carbon. The terms like gram – molecule and gram atom were used before the introduction of mole.