Question

The mass of a litre of oxygen at STP was 1.43g and that of a litre of sodium hydroxide is 2.857g.

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

II) What is the mass in grams for a single molecule of each gas?

Answer 1

1)At STP , 1 mol of gas = 22.4L
You have 1.0L of each gas . Therefore you have 1/22.4 mol of each gas.
1 mol of anything contains 6.022*10^23 molecules
Therefore you have (6.022*10^23)/22.4 = 2.69*10^22 molecules of O2 and SO2

2) Oxygen: 2.69*10^22 molecules = 1.43g
1 molecules = 1.43/(2.69*10^22) = 5.32*10^-23g

SO2 1 molecule = 2.857 / (2.69*10^22) = 1.06*10^-22 g