General Chemistry/Gas Laws/Answers

Answers to Gas Laws Questions
1. The Ideal Gas Law accounts for chemical change. The Combined Gas Law accounts for changes in pressure, volume, and temperature. These are physical properties. The Ideal Gas Law accounts for these properties along with molar mass. Although molar mass is a physical property as well, the amount of molecules in an isolated gas would only change in the event of a chemical reaction. Thus, the Ideal Gas Law can account for chemical reactions.

2. Density is mass divided by volume, and the number of moles equals the mass divided by molecular weight. So: $$ D = \frac{m}{V} $$ and $$ PV = nRT $$

so

$$ \frac{n}{V} = \frac{P}{RT} $$

and

$$ D = \frac{P \times (MW)}{RT}$$

Substituting and solving gives us a density of 80.7 g/m3. Remember that hydrogen is diatomic, so its molecular mass is 2.0 g/mol.

3. Simply substitute into the Ideal Gas Law and solve. 0.144 m3.

4. Stoichometrically, one mole of H2 is needed for each mole of H2S. We must determine the number of moles in 7.4L of hydrogen sulfide, then convert that number of moles into liters of hydrogen. This is a two-step problem, both using the Ideal Gas Law.

Solving for moles of hydrogen sulfide: 0.330 mol

Solving for liters of hydrogen: 7.39 L H2