Question

I was wondering which method would be the best to finding density in a STP question.
volume for one mole of gas is 22.4L, I think

1.) d = molarmass/V - I saw this being used on youtube

2.) d = P*MM/RT

One mole of which gas has the greatest density at STP?
A) Ar
B) N2
C) CO
D) All are the same

I got Ar as my answer through MM/V as 1.78 g/L but I don't know if it is right.

Answer 1

d = 1*39.948/0.08205*273.15 = 1.782 g/L by method 1.
d - 39.948/22.414 = 1.782 g/L by method 2.
So it makes no difference which method you use.
In addition, the problem doesn't ask for the density; i.e., only which is greatest of the three. Therefore, method 1 is mass/k and method 2 is mass/k so since Ar is close to 40 and the other two are 28/k you choose Ar.