[Rutherford]

A mixture of H₂ and O₂ (volume ratio is 2:3) is above water on 100kPa and 273K.One dm³ of water apsorbs 0.001935g of hydrogen and 0.07g of oxygen in standard conditions. Calculate the mass and the volume of O₂ and H₂ that were apsorbed in 1dm³ of water in the given conditions.
What to do here? Don't know where to start from.

[fledarmus]

What is the definition of "standard conditions"? How are the concentrations of H₂ and O₂ in your problem different from those standard conditions?

[Rutherford]

p=101.3kPa, T=273K. Don't understand.

[fledarmus]

Close, but not quite - when you are talking about the absorption of O₂ at standard conditions, that would mean a pressure of 101.3 kPa of pure O₂.

Does this help?

[Rutherford]

I came to this idea:
pV=mRT/M from here I can calculate separately the volume of H₂ and O₂ that are apsorbed in water by s.c. From p₁V₁=p₂V₂ I could calulate the volume of the apsorbed gasses on 100kPa and 273K. And with pV=nRT I can calulate the mass of both H₂ and O₂ that are apsorbed on 100kPa. But, I didn't use the volume ratio that is given. Where have I mistaken?

[fledarmus]

Do you know Henry's law?

[Rutherford]

I know. So, from the info in the beggining of the problem, I can calculate the partial pressures of the gasses ( p(H₂)=60kPa, p(O₂)=40kPa). From Henry's law I get the mass of hydrogen: 60kPa/101.3kPa=m/0.001935g and similar for oxygen. To calculate the volume I have to use pV=nRT. Is this correct?

[Rutherford]

I know. So, from the info in the beggining of the problem, I can calculate the partial pressures of the gasses ( p(H₂)=60kPa, p(O₂)=40kPa). From Henry's law I get the mass of hydrogen: 60kPa/101.3kPa=m/0.001935g and similar for oxygen. To calculate the volume I have to use pV=nRT. Is this correct?

Made a mistake, the partial pressures of the gasses are: p(H₂)=40kPa, p(O₂)=60kPa. At the end I get the same result as in my book. Thanks for the help.

[Rutherford]

Need just one more.
1dm³ of water apsorbs 171.3cm³ of CO₂ under standard conditions. Calculate the mass of CO₂ that is apsorbed in 250cm³ of water ( T=273K, p=150kPa).
I need the mass of CO₂ that is apsorbed under s.c. so I can use Henry's law then, but I can't get the right result (2.79g). The 250cm³ of water confuse me.

[fledarmus]

Could you show your calculations? That would make it easier to see what you are getting wrong. What results are you getting?

[Rutherford]

From pV=mRT/M  (V=171.3cm³) I can calculate the mass of CO₂ that is apsorbed under s.c. I used Henry's law now, but I couldn't get the right result (got a very small result-0.000257g). I suppose that I am wrong because I calculate the mass apsorbed in 1dm³ of water but later I need to calculate the mass  of CO₂ apsorbed under different conditions and in 250cm3 of water.

[fledarmus]

What are your units for R? I suspect it is a unit conversion that is giving you your initial very low number. What numbers (with units) did you plug in for P, T, R, and M?

Once you know how much material dissolves in 1dm³ of water, you just need to find the same ration to get the amount dissolved in 250cm³  -  (x grams)/(1dm³) = (y grams/250 cm³). You will need to do another unit conversion to get your dm³ and cm³ to match.

[Borek]

Please - change this atrocious spelling. My eyes hurt from reading it

It is absorption.

[Rutherford]

Please - change this atrocious spelling. My eyes hurt from reading it

It is absorption.

Oh, sorry .

What are your units for R? I suspect it is a unit conversion that is giving you your initial very low number. What numbers (with units) did you plug in for P, T, R, and M?

Once you know how much material dissolves in 1dm³ of water, you just need to find the same ration to get the amount dissolved in 250cm³  -  (x grams)/(1dm³) = (y grams/250 cm³). You will need to do another unit conversion to get your dm³ and cm³ to match.

Didn't calculate well. From pV=mRT/M I get that m=0.336g. Using Henry's law I get: p₁/p₂=m₁/m₂ m₂=0.498g in 1dm³ of water. In 250cm³ m=0.124g wich is lower than the right result (2.79g).

[fledarmus]

Hmmm - I'm getting essentially the same numbers you are. Possible problem with either the question or the answer?