Hi Justine,
Because STP is defined as 0 Celsius and 1 atm, it is a bit far off as an approximation for this question, as the answers are pretty close together mathematically (C is 3.36 x 10^-2 and D is 1.68 x 10^-2).
However, I would say that this answer provided in the Altius is pretty unnecessarily complicated. I would say it's definitely more complicated than what I would expect for the MCAT
They try to get the atm down to 0.95 atm, what I would have done instead on a back of the envelope calculation is:
P = 1 atm (overestimate)
V = 0.4 L (underestimate)
T = 25 C = 298 K = 300 K (overestimate)
R = 0.1 L*atm/mol*K (overestimate)
PV = nRT ;
n = PV/RT = (1 atm)(0.4 L)/(300 K)*(0.08 L*atm/mol*K)
= (0.4)/30 mol --round 0.4 down to 0.3
= 3/300 mol
= 1 x 10^-2 mol
Only D matches my answer, and it makes sense that it is slightly smaller because I rounded down in my answer - a lot of my estimates cancel out otherwise.
I hope this helps, please let me know if you need clarification!
Katt