Question

The weight of $100$ molecules of each of the following substances is:
(a) Chlorine gas
(b) Calcium oxide

• A. (a) $6.022\times {10}^{-23}$ g (b) $9.29\times {10}^{-21}$ g
• B. (a) $11.78\times {10}^{-23}$ g (b) $9.29\times {10}^{-23}$ g
• C. (a) $23.96\times {10}^{-21}$ g (b) $9.29\times {10}^{-23}$ g
• D. (a) $11.78\times {10}^{-21}$ g (b) $9.29\times {10}^{-21}$ g

Answer 1

The correct option is D (a) $11.78\times {10}^{-21}$ g (b) $9.29\times {10}^{-21}$ g

Number of molecules $=N_m$ $=100$

Weight of 100 molecules $=m$

We know, $N_m=\frac{m}{MW}\times 6.023\times {10}^{23}$

$\Rightarrow m=\frac{N_m\times MW}{6.023\times {10}^{23}}$ .....(1)

Let us apply the equation (1) to each of the given cases -

(a) Chlorine gas (MW= $71$ g/mol)

$m=\frac{100\times 71}{6.023\times {10}^{23}}$

$=11.78\times {10}^{-21}$ g

(b) Calcium oxide (MW= $56$ g/mol)

$m=\frac{100\times 56}{6.023\times {10}^{23}}$

$=9.29\times {10}^{-21}$ g
Option $D$ is correct.