Ideal gas law, concentrations, mole fractions

[SaRaH...]

I haven't been able to try all parts of this question yet as I've been running into problems fairly near the start. I've asked for help with it but have just been told 'use the ideal gas law' which is what I tried to do but can't seem to get it right. We have been given answers to parts a, b and c but as of yet I haven't managed to work out the solutions to them. Any guidance would be a great help.

1. Problem statement

a) In an industrial estate, the median indoor airborne concentration of chloroform (CHCL_3) was found to be 0.4 micrograms/m^3. Convert this concentration to a mole fraction in parts per billion (ppb) assuming a temperature of 20 degrees C and atmospheric pressure.

b) The same study estimated that the measured mean concentration of chloroform in drinking water was 42 micrograms/m^3. Convert this to a mass fraction in ppb.

c) A typical adult might inhale about 20m^3 of air and ingests about 2 litres of water per day. On the basis of the measured concentrations, compare the exposure to chloroform via inhalation (micrograms/day) with the exposure via ingestion (i.e. drinking).

d) For tetrachloroethylene (C_2Cl_4), the study reported a mean concentration in indoor air of 2.1 micrograms/m^3. Determine the relative exposure for this compound as in part c.

e) Comment on the relative significance of each of the exposure pathways for the different compounds.

Homework Equations

PV = nRT

The Attempt at a Solution

This is for part a)

The mole fraction, as far as I'm aware, is V/n so I rearranged the equation as:
V/n = RT/P

Substituting the given values in, I got:
V/n = 0.08205(20 + 293.15)/P

What exactly the P is I wasn't sure. Using P = 1atm didn't give the required answer. I thought that maybe we should use a partial pressure (ppb = P_i/P_total) but had too many unknowns. I looked up ways that a concentration could be converted to a mole fraction but none on these involved a temperature or pressure and didn't give the right answer either. Is there something really obvious that I'm missing? Or am I going about this completely the wrong way?

Any help would be fantastic,
Sarah

 

[Borek]

The mole fraction, as far as I'm aware, is V/n

No, it is n_substance/n_total. Take 1 cubic meter of air, calculate number of moles of all gases present (hint: you don't have to treat them separately, Avogadro's law), calculate number of moles of chloroform (hint: what is mass of chloroform in this cubic meter of air), divide, express as ppb.