[hilsenna]

Hello,

I am stuck with a question. I know we should use Pa=k*C for this problem. Answer is 28 ml but shouldnt it be 112 ml because the pressure is 4.0 atm??

Q: The solubility of oxygen in 1.0 l water is 28 ml at 25 °C and 1.0 atm. How much oxygen can be dissolved in 1.0 l of water at 25°C and 4.0 atm?

[Borek]

Question is poorly worded. Apparently it expects us to by "how much" understand "what volume at whatever pressure is given". If your answer would be "112 mL measured at 25°C and 1.0 atm" it would be exactly as good as "28 mL measured at 25°C and 4.0 atm".

[MNIO]

so first of all, Henry's law states the relationship between vapor pressure over a solution and solubility of a gas in solution is directly proportional...
  P = k * C
where
  P = vapor pressure of the gas over the liquid solution
  k = a constant
  C = concentration of gas dissolved in the liquid phase

That equation doesn't specify what the units of P and C are.  The constants we use are where those units are specified.  Example if we say our system is CO2 in H2O @ 25°C and lookup the Henry's law constant for that system, we'll find
  KH = 29 atm/M
so that if we use that constant, the units will be atm for P and molarity for C.

however, we might have found the constant in other units.  let's say we found
  KH = 66.8 kPa / (g/L)
in which case the units of P would be kPa and C would be g/L

The relationship doesn't require specific units!

************
let's go back to your problem.. we know this
   P1 = KH * C1
   P2 = KH * C2
dividing to get rid of KH
   P1/P2 = C1/C2
rearranging
   C2 = C1 * (P2/P1)
solving
   C2 = 28mL * (4.0atm / 1.0 atm) = 112mL

why do you think the answer should be 28mL?

[Borek]

why do you think the answer should be 28mL?

Have you read my post?

One of ways of stating Henry's law is "at constant temperature volume of the gas that dissolves in a given volume of liquid is constant and doesn't depend on pressure".

It is all matter of the pressure at which we measure the gas volume, question doesn't clarify that.