Question

In the reaction of calcium metal and water, 50.0 mL of hydrogen gas is collected over water at 25.0 degrees C with a total pressure of 752 mmHg. Calcualte the number of moles of hydrogen gas that was produced.

Do we just plug this into n=pv/rt ? or is there like an extra step?

Answer 1

If the total pressure is 752 mm Hg, that is the pressure of the H2 gas + the pressure of the water. So look up the vapor pressure of water at 25 degrees C (at moderate temperatures I remember that the vapor pressure of water is approximately the samae as the T reading in C---but you need to look it up). Subtract. Total P - vapor pressure H2O at 25 C) to give pressure of H2 by itself. Then use PV = nRT to calculate mols H2.