Question

The mass of a litre of oxygen at standard conditions of temperature and pressure is 1.43 g and that of a litre of ${SO}_2$ is 2.857 g.

(i) How many molecules of each gas are there in this volume?

(ii) What is the mass in grams of a single molecule of each gas?

(iii) What are the molecular masses of ${SO}_2$ and $O_2$ respectively?

Answer 1

1)

At STP , 1 mol of gas = 22.4L
You have 1.0L of each gas . Therefore you have $\frac{1}{22.4}$ mol of each gas.
1 mol of anything contains $6.022\times {10}^{23}$ molecules
Therefore you have $\frac{6.022\times {10}^{23}}{22.4}$molecules of $O_2$ and $S{O}_2$

2)

Oxygen: $2.69\times {10}^{22}$molecules = 1.43g
1 molecule $=\frac{1.43}{(2.69\times {10}^{22})}=5.32\times {10}^{-23}g$
Therefore,
$S{O}_2$ 1 molecule $=\frac{2.857}{(2.69\times {10}^{22})}=1.06\times {10}^{-22}g$
3)
$1mol=22.4L$ at STP
Oxygen ,$1.0L=1.43g,22.4L=1.43\times 22.4=32.0g$
$S{O}_2=1.0L=2.857g,22.4L=2.857\times 22.4=64.0g$