OK - I am going to claim the mark scheme is wrong on this one - it's a practice paper, but I'd really like if someone could either agree or disagree with me on this one.
My logis is this - 1 bar of pressure (e.g. water in a central heating system) is about 100kPa - the question talks about a gas at 120kPa., so we are talking about similar pressure. They give an answer of 1,340 m/s, which means that any molecule would be travelling at almost 1 mile per second - just in my head that seems way too fast. This means in a pipe of diameter of 10cm, a molecule could go back and forth across that pipe 13,400 times in 1 second based upon the mark scheme answer.
MASS= n x MOLAR MASS
so that means that Mass=N/NA x Molar mass
Using 0.5 M Crms^2=3/2 R/NA T.
You will have to rearrange the equation to get NA x Mass. So the NA will cancel out. N=1 so you just have to divide by Molar mass. I think