[Rutherford]

Sample of a volatile liquid (m=0.115g), after evaporating in an apparatus, squeezed out 22cm³ of air, whose temperature above water is 300K, and the level of water in the gas burette was 210mm higher than the level of water in a pneumatic tub. A pressure of 1bar was measured. What is the molar mass of the liquid?

Not sure where to start from, and how to use the 210mm?

[Borek]

Somewhere and somehow you have to account for the pressure of 210 mmH₂O.

[Rutherford]

How to convert that to pascals?
pV=nRT I have p, T and V (those 220ml or the squeezed 22ml)?

[Borek]

http://en.wikipedia.org/wiki/Pressure#Liquid_pressure

[Rutherford]

p=2.06Pa, I use it to calculate the number of moles of the gas, but what then?

[gritch]

p=2.06Pa, I use it to calculate the number of moles of the gas, but what then?

Remember molar mass is defined as Mass/Moles. You have the amount of moles and the question provided the total mass of the sample (m=0.115g). From these you can easily determine the molar mass

[Rutherford]

The pressure is 2.06Pa and the volume is 22ml. With these low numbers I can't get a reasonable answer (the correct one is 138.2).

[curiouscat]

ρ_H2O=996.512 kg/m3 @27 C

P_ambient-P_burette=ρgh
P_ambient-P_burette=2052.9 (Pa)

P_ambient=1bar=10⁵ Pa

P_burette=97.947E3 Pa

n=PV/RT
n=8.639E-4 gmol

M=m/n=133.1 gm/gmol

I still can't figure out where my small disagreement with your reported answer is..

[Rutherford]

It is probably the density. Thanks for the answer, only I don't understand how is the volume of air (V=22ml) related with the number of moles of the liquid. Why is n of air equal to n of the liquid?

[Borek]

http://en.wikipedia.org/wiki/Avogadro's_principle

[Rutherford]

I knew the law, but I didn't know how to use here. I have an apparatus filled with the liquid at the bottom and the apparatus is open (so air gets in). When the liquid evaporates it squeezes 22ml of air from the apparatus, meaning that 22ml of a gas was produced. How is the volume of squeezed air measured?

And I don't understand what curiouscat wrote about the pressures. Precisely the ambient pressure.
The gas is taken into a gas burette, so the increase in pressure increases the height of water and the pressure in the burette is the sum of the water pressure and the gas pressure. Shouldn't it be p_burette=p_gas+p_water?

[Borek]

When the liquid was evaporated is squeezed out 22 mL of the gas. Why does it matter how it was measured? You know liquid produced 22 mL of the gas, which is all you need to calculate number of moles.

That is, after you will get this pressure thing right.

[Rutherford]

I was just curious, but I guess all in good time.