# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: hilsenna on July 31, 2020, 06:34:07 AM

Title: Gas Solubility Question
Post by: hilsenna on July 31, 2020, 06:34:07 AM
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?

Title: Re: Gas Solubility Question
Post by: Borek on July 31, 2020, 08:35:47 AM
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".
Title: Re: Gas Solubility Question
Post by: MNIO on July 31, 2020, 11:50:40 AM
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?
Title: Re: Gas Solubility Question
Post by: Borek on July 31, 2020, 12:17:39 PM
why do you think the answer should be 28mL?