Answer: The molar concentration of oxygen gas in water is
.
Explanation:
Partial pressure of the
gas = 685 torr = 0.8905 bar
1 torr = 0.0013 bar
According Henry's law:

Value of Henry's constant of oxygen gas at 20 °C in water = 34860 bar


Let the number of moles of
gas in 1 liter water be n.
1 Liter water = 1000 g of water
Moles of water in 1 L 




Molar concentration of oxygen gas in 1 L of water:

The molar concentration of oxygen gas in water is
.
The volume becomes two. You have to use the equation P1 x V1 = P2 x V2
P is pressure and V is volume.
P1 = 50 P2 = 125
V1 = 5 V2 = v (we don't know what it is)
Then set up the equation:
50 times 5 = 125 times v
250 = 125v
the divide both sides by 125 and isolate v
2 = v
Therefore the volume is decreased to 2.
Also, Boyle's Law explains this too: Volume and pressure are inversely related, This means that when one goes up the other goes down (ie when pressure increases volume decreases and vice versa). Becuase the pressure went up from 50 KPa tp 125 KPa the volume had to decrease.