Question

Why does .500 moles of wet H2 (H2 collected over water) have a higher volume than .500 moles of dry H2?

Answer 1

When H₂ is collected over water, some of the water vapor get dissolved in H₂ gas solvent. Hence there are gaseous entities more than 0.500 moles (i.e. 0.500 moles of H₂ along with H₂O gas​​​​_​​​​​​​^{​​​​​}​​​​^​​​). Now assuming the ideal behaviour of gases we know that PV=nRT, that means at constant pressure and temperature, volume of gases is directly proportional to mole number. So wet H₂ have a higher volume than dry H₂.