What is the pressure of H2 gas produced?

Question

A 20.8 G sample of nickel(s) is placed in excess HCl(aq) and the gas produced is collected over water in a 2.5L container. The vapor pressure is 21.1 mmhg at 23 degrees C. After the reaction is finished, a total pressure of 3.23 atm is measured. What is the pressure of H2 gas produced?

Answer 1

Whenever gas is collected over water, the vapor pressure of the water must be subtracted from the total pressure in order to obtain a true measure of the pressure of the gas by itself.  In this problem the pressures are given in mm Hg and in atmospheres so we need to equate them.  21.1 mm Hg x 1 atm/760 mm Hg = 0.0278 atm. This will need to be deducted from the total pressure.

Pressure of H2 gas = 3.23 atm - 0.0278 atm = 3.20 atm (or