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Question

1.) A certain volume of a gas had a pressure of 800 torr at a temperature of -40 degrees celsius. What was the original volume at STP is now 450.0 cm³?

2.) Hydrogen is collected by water displacement at an atmospheric pressure of 765.0 torr and a temperature of 15 degrees celsius. What is the pressure of the hydrogen? (P_{water vapor}= 12.79 torr)

Answer 1

1) Just to clarify, should this question read, “A certain volume of a gas had a pressure of 800 torr at a temperature of -40 degrees Celsius. What was the original volume if the volume at STP is 450.0 cm³?” If so, here’s how this would set up. We’ll use the Combined Gas Law, and since we’re comparing two different states, the only unit conversion that we need to do is to have our temperatures in Kelvins. Note that STP is defined as 0 degrees Celsius (or 273 K) and 1 atm (or 760 torr) pressure.

P₁V₁/T₁ = P₂V₂/T₂; (800 torr)(V₁)/(233 K) = (760 torr)(450.0 cm³)/(273 K); V₁ = 364.9 cm³

2) Dalton’s Law of Partial Pressures is at work here. The hydrogen sample collected via water displacement has two gases in it: the hydrogen and water vapor, and the sum of the pressures of those two gases must be equal to the atmospheric pressure.

P_total = P_hydrogen + P_{water vapor}; 765.0 torr = P_hydrogen + 12.79 torr; P_hydrogen = 752.2 torr

I hope this helped!