At 25 degrees celsius and 1.0 atm, 0.25g of a gas dissolves in 1.00L of water. What mass of the gas dissolves in 1.00 L of water at 25 degrees celsius and 3.0 atm?

Answer 1

Henry's Law.
P(gas) = kC
But in words, at the same T and the same volume of water, then 3 x pressure means 3 x the amount dissolved or 3 x 0.25 g.

Answer 2

0.75