# Henry's law Numerical

I got very basic doubt. Henry law's Give relation between concentration vs Partial pressure. So here is the Numerical Of my textbook

**Now My question is, why concentration is Considered only in Grams? shouldn't it be in Molarity, Mole fraction or Strength(gm/Vol) etc? **

1. In Other textbook, They simply used formula to solve it. mass = Henry's Constant * pressure.
• Frankly I think you're right. It is a sloppy problem statement. The extra "$\mathrm{L}^{-1}$" should be there. !@#$%^& the formula for the answer uses c for concentration not m for mass of solute. For the saturated solution the volume of the solution doesn't make any difference. However for the unsaturated solution if the total volume is 1 liter or 10 liters it does make a difference. – MaxW Apr 15 '16 at 20:50 • Thank you for reply. I didnt get you. How Volume is linked upto saturation? – aman Apr 16 '16 at 8:29 • The concentrations of the gases in solution should be$\text{ g L}^{-1}$not just$\text{ g}$. – MaxW Apr 16 '16 at 14:54 ## 3 Answers They are implicitly assuming that the number of moles of ethane in solution is very small (in both cases) compared to the number of moles of the solvent. So, in both cases, the concentration of ethane and its mole fraction are essentially proportional to the mass of ethane in the solution and the amount of solvent is the same. • Hi i have added the Screenshot of other Solution as well. How they came up with that relation? – aman Apr 14 '16 at 13:46 • That formula works but is only useful for this special case with a constant, unknown volume, and the calculation should never be done this way, calculating an unnecessary quantity like "kH" in between. It's not even a Henry's constant, that should of be concentration per pressure. I'd try to get rid of that book. – Karl Apr 14 '16 at 15:26 • I agree totally with @Karl. They basically assumed what I indicated in my answer, but with no explanation of what they were doing, or their assumptions. And, as Karl said, that$K_H\$ is not even close to what we conventionally define as the Henry's law constant. Apr 14 '16 at 15:33
• What if we take Concentration in GRAMS/VOLUME and perform the experiment in same Flask (same Volume). Wouldn't in this case mass will be proportional to partial pressure? ( without any assumption)
– aman
Apr 14 '16 at 21:21
• Henry's law is usually expressed using molar concentrations, which makes more sense, because the partial pressure is proportional to activity of the solute, strictly speaking. But as long as you stay at low concentations, you could use any measure, vol-%, mol/kg, g/l, etc., Henry's law will still work acurately.
– Karl
Apr 14 '16 at 23:04

For this example, the volume of the solution doesn't matter. It's a thought experiment, doesn't say what sovent is used, and you could replace ethane by helium or miraculane.

As long as you talk about the same compound, mass is proportional to molar quantity. Plus the units cancel out in the calculation anyway.

Think of the hypothetic experimentator as a guy who only has one glass beaker, without graduation. ;)

• SO it is assumed that (Volume of) container is same in both cases? But most of the text are simply using formula ( mass= Kh*P)
– aman
Apr 14 '16 at 13:08
• Have you understood the principle behind Henry's law? You sound like you just want to learn this by heart for an exam. ;) That formula you cite is constructed for this special case, and the way they give it in your second textbook is totally inadequate.
– Karl
Apr 14 '16 at 15:02
• Yes I understand the principle. Personally I think , if I put concentration as mass/Volume. Volume will cut out each other as they are same in both cases, but i checked 4-5 textbooks, All of them are using formula (m=k.p) which according to me is blunder!! .... it is actually not for exam. I am just curious guy who cant proceed Till the point I am 100% satisfied with the basic concept.
– aman
Apr 14 '16 at 21:15
• What textbooks are those?
– Karl
Apr 14 '16 at 22:53
• Most of the Indian Author's relate henry law with mass per unit volume. I dont know the reason.
– aman
Apr 16 '16 at 8:30

If in the calculation, both concentration are multiplied with the relative molecular mass of ethane, which is as good as multiplying by 1. The quantities to use on the concentration variables will be masses.