Matter, Gas Laws and Chemical Calculations
Matter, Gas Laws and Chemical Calculations links kinetic theory, the gas laws and mole-based stoichiometry. The exam tests whether you can convert mass to moles and then to particles or gas volume.
Gas laws
- Boyle's law at constant temperature: p1V1 = p2V2.
- Charles's law at constant pressure: V1/T1 = V2/T2, with T in kelvin.
- Combined gas law: p1V1/T1 = p2V2/T2 for a fixed amount of gas.
- Avogadro's law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
Mole concept
- One mole contains 6.022 x 10^23 particles. Molar mass in g/mol is numerically equal to molecular mass.
- Moles = mass / molar mass. At STP, one mole of an ideal gas occupies about 22.4 L.
- Stoichiometric coefficients give mole ratios; use them to convert from one substance to another.
Worked example
Find the moles, molecules and STP volume of 4.4 g of carbon dioxide. Molar mass CO2 = 12 + 32 = 44 g/mol. Moles = 4.4/44 = 0.1 mol. Molecules = 0.1 x 6.022 x 10^23 = 6.022 x 10^22. Volume at STP = 0.1 x 22.4 = 2.24 L.
Traps
- Using Celsius temperature in Charles's law or the combined gas law.
- Confusing molecular mass with molar mass units.
- Using 22.4 L/mol at conditions other than STP.
- Ignoring limiting reactants when two amounts are given.