Question

I'm doing a chemistry lab, where I have to determine the molar mass of an unknown after finding the vapor density. So basically, I have to put 3 test-tubes in a hot water bath, after measuring their volume, and mass the test tube + condensed unknown. So, if, hypothetically, there was some unknown liquid left in the test tube when we were massing the test tube after we heated them, how would the experimental molar mass change? Would it increase or decrease? Or would it stay the same? I think it would stay the same, because, after all, it's the unknown liquid itself. But, then I'm worried about the fact that the gas doesn't fill up the volume of the entire test tube because that unknown liquid takes up part of the test tube...

Any help would be greatly appreciated! Thanks in advance!

Answer 1

The volume occupied by the gas will be less than it should occupu.

so it may cause

1) The molecular weight reported will be small, as the volume occupied by the gas will be more as expected as the test tube already contains some liquid. so the volume term will be high in the gas law expression

PV = nRT

PV = mass R T / Molecular weight

So molecular weight is inversely proportional to volume at it will be less due to high volume

2) The molar mass may be very high, as the mass of vapour of liquid will be high as the unvapourixed liquid is denser than the vapour alone.