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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Helly on February 06, 2020, 02:05:43 AM

the solubility of oxygen in 1 lt water 25 c 1 atm is 28 ml. How much Lt O2 can be dissolve in 1 lt water 25 c 4 atm?
I though using PV=nRT which gives me PV=P2V2, V2 = 7ml.
I dont understand Henry Law...

You can't use some randomly selected law to solve a problem that is described by another one. It won't ever work. But you are right mentioning Henry's Law, that's the one that should be used here.
Can you write the Henry's law? What does it say?

Theres so many equations https://en.m.wikipedia.org/wiki/Henry%27s_law

Sadly, wikipedia article is poorly written, no wonder you feel lost.
See if this formulation (https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Solutions_and_Mixtures/Ideal_Solutions/Dissolving_Gases_In_Liquids%2C_Henry%27s_Law) won't be better.

C = k.P
C = Molarity
k = Henrys Law
P = Pressure
So, how i should find C?

the solubility of oxygen in 1 lt water 25 c 1 atm is 28 ml
You know how much oxygen dissolved and in what volume.

Ok. So, the molarity is n/volume
While the mole of oxygen using pv/RT=n ? = 28/(0.082 * 298) = n ?

Close, but watch your units.

0.028/(0.082 * 298) = mole
Then i get C
should i find k? Or P ?

Pressure is given, find k, then calculating amount of oxygen dissolved at any other pressure will be trivial.

Ok thanks

Note: there are many ways to skin that cat. Actually once you understand the Henry's law and direct proportionality the answer is immediately obvious. I preferred to show you the most sure way, which may be not the fastest, but is guaranteed to yield a correct result.

What is that?

One of possible formulations of Henry's law is that no matter what the pressure is, the volume of the gas that dissolves in a given volume of liquid is always the same.

the solubility of oxygen in 1 lt water 25 c 1 atm is 28 ml. How much Lt O2 can be dissolve in 1 lt water 25 c 4 atm?
I should like to recommend writing the units properly, too, including capital letters.
Litre is L
Degree celsius is °C
and so on and so forth. It's not only easier to read if you follow the habits, it does also make a difference. Readers understand K as kelvin and k as thousand, M as mega and m as milli. Even without readers, you'll fool yourself if you mix up the symbols.

note sure if you have this solved yet, but how about I through in a couple of hints
this is henry's law.
Pa = k * Ca
where
Pa = partial pressure of component "a" over a liquid
k = a constant (provided the temp is constant)
Ca = concentration of component "a" in the liquid phase
you have t conditions of Pa and Ca... so what if we do this?
Pa1 = k * Ca1
Pa2 = k * Ca2
divide
Pa1 / Pa2 = (k / k) * (Ca1 / Ca2)
simplify and rearrange
Ca2 = Ca1 * (Pa2 / Pa1)
now.. that's all in partial pressures. We don't have partial pressures, we have total pressures. But let's make the assumption that
Pa2 / Pa1 = Ptotal 2 / Ptotal 1
which will be close in this case
so that
Ca2 = Ca1 * (Pa2 / Pa1)
you get to finish

I am confused. The mole of O2 is
n = PV/RT
C = n/ V water = PV/RT Vwater
C = k . P
k = C/P = n / P = V/RT V water
C2 = n2/V water = P2 V2/RT V water
C2 = k . P2 = (V/RT V water)(P2)
P2V2/RT V water = P2V/RT V water
V2 = V
Whats the poinf of henry formula? If the Volume is the same?

Ca2 = Ca1 * (Pa2 / Pa1)
n2 = n1 (P2/P1)
P2V2/RT = P1V1/RT (P2/P1)
V2 = V1
C = k P
So, this formula only for different pressure?

One of possible formulations of Henry's law is that no matter what the pressure is, the volume of the gas that dissolves in a given volume of liquid is always the same.
how about beverage drink? As it opened the pressure decrease and the gas come out(the sollubility changes?)

how about beverage drink? As it opened the pressure decrease and the gas come out(the sollubility changes?)
Henry's law is universal, doesn't matter if is a beverage or something else.
When you open the can or bottle pressure inside goes down, so some of the dissolved gas escapes and expands.
Note: the formulation of which I said earlier doesn't mean it is always the same amount of gas. The higher the pressure, the more gas is compressed in the same volume. It happens that number of moles in a given volume is directly proportional to the pressure, just like amount of the gas dissolved in a given volume of liquid.