340 ❯ STEP 5. Build Your Test-Taking Confidence(b) The hydrogen atoms are in the water molecules present in the flask; therefore it is necessary to calculate
the mole of water present, convert to moles of hydrogen atoms, and use Avogadro’s number to convert
from moles to the number of atoms.
Pwater = 25 mm Hg/760 mm Hg = 0.033 atm. T and V are the same as in part (a).
n = PV/RT = (0.033 atm × 0.5000 L)/(0.0821 L atm mol–1 K–1 × 299 K)
= 6.7 × 10 –4 mol H 2 O
Number of H atoms = ×
×
(6.7 10 mol HO)− 2 mol H
1 mol H O6.022 10 H atoms
1 mol H4
2
223
= 8.1 ×
1020 H atoms
Give yourself 1 point for the correct answer (no deduction for rounding variations).
(c) Unlike oxygen gas, ammonia gas is very soluble in water because it, like water, is capable of hydrogen
bonding. The formation of hydrogen between water molecules and ammonia molecules enhances the
solubility of ammonia in water. Oxygen, on the other hand, is nonpolar and is only weakly attracted
to water, giving oxygen a much lower solubility.
Give yourself 1 point for indicating the problem is the solubility of ammonia in water. Give yourself
1 additional point for the explanation if you invoke hydrogen bonding.
Total your points for this question. There are 4 points possible. Deduct 1 point if you reported the wrong
number of significant figures in any answer.20-Moore_PE01_p307-340.indd 340 31/05/18 1:58 pm