
Many people have asked for some additional stoichiometry questions. So here you go. Note that the exam will definitely ask some questions on converting between moles and grams. In addition, the exam will likely have one or two somewhat more difficult stoichiometry calculations such as those below. Remember, there is a lot more to know for this exam than just stoichiometry questions so don't over focus on these. How many moles of hydrogen are produced when 0.1 mol of sodium reacts with 0.1 mol of water? 2 Na(s) + 2H_{2}O (l) > H_{2}(g) + 2 NaOH(aq) answer 0.05 mol How many moles of oxygen are required to completely burn 4.8 moles of butane? 2 C_{4}H_{10} + 13 O_{2} > 8 CO_{2} + 10 H_{2}O Answer: 31 moles Consider the following reaction: PbCl_{2} + Na_{2}CrO_{4} > PbCrO_{4} + 2 NaCl How many grams of NaCl are produced when 0.5 moles of Na_{2}CrO_{4} react. answer: 58.5 g How many moles of carbon dioxide are produced when 88.0 g of oxygen are reacted with an excess of butane, C_{4}H_{10}? 2 C_{4}H_{10} + 13 O_{2} > 8 CO_{2} + 10 H_{2}O answer: 74.5 moles. Chapter 7  States of Matter Today  Gases: I. About gases A. some gases you might recognize Atmosphere: 78% N_{2}, 21% O_{2}, 1% Ar CO_{2} 0.025% (1990), 0.036% today  increasing Other gases you might be familiar with: CO (carbon monoxide) Chlorofluorocarbons (CFCs): CF_{2}Cl_{2} and CFCl_{3} Cl_{2}, H_{2}, F_{2} noble gases B. Properties of gases 1. Gases are a collection of individual particles with rapid, random motion. 2. Energy of gases is kinetic energy (KE), which is energy due to motion. molecules move faster at higher temperatures so the kinetic energy of a gas increases with temperature 3. Distance between gas atoms or molecules is large compared to their sizes 1 mole H_{2}O(l) = 0.02L 1 mole H_{2}(g) = 22.4 L 4. Gases are characterized by a pressure. Pressure is a force per unit area. Pressing a sharp object into your hand hurts more than pressing a dull object into your hand. The collisions of gas molecules with the container they are in exerts a pressure on it. P = F / A where F = force and A = area common units: atmosphere (atm), mm Hg = torr 1 atm = 760 mm Hg = pressure which can support a column of mercury 76cm high. Alternatively, the pressure exerted by a column of mercury 76cm high. 1 torr = 1 mm Hg 1 atm = 760 torr At sea level, the atmosphere exerts a pressure of 1 atm = 760 mmHg Barometric pressure often measured in inches of Hg. 1 atm = 30 in Hg 5. In comparing gases and gas behavior we use STP  "standard temperature and pressure" For STP: T= 273 K, P = 1 atm Under these conditions, 1 mole of gas occupies 22.4L, independent of what the gas actually is. Avogadro's hypothesis  equal volumes of gases at the same temperature and pressure contain equal numbers of molecules or particles. Because of this, we can interpret the coefficients of chemical reactions involving gases in terms of volumes! For instance,
II. Gas Laws describe how gases behave  Interrelating temperature, pressure and volume  NOTE: FOR QUANTITATIVE CALCULATION NEED TO USE TEMPERATURE IN KELVIN 1. Boyles law, pressurevolume relation, for a given mass of gas at constant T, the volume varies inversely with pressure. T constant, increase pressure, volume goes down Mathematically: P_{1}V_{1} = P_{2}V_{2 }for constant temperature Ex: A gas initially has a volume of 22.4L at a pressure of 1 atm. It is compressed to a final volume of 11.2 L, what is the final pressure of the gas. P_{1} = 1 atm, V_{1} = 22.4 L Solve for P_{2} and plug in: P_{2} = P_{1}V_{1} / V_{2} = (1 atm)(22.4 2. Charles law, VolumeTemperature relation, for a pressure constant, the volume of a fixed mass of gas varies in direct proportion with temperature P constant increase T, volume goes up V_{1} / T_{1} = V_{2} / T_{2} for constant pressure 3. Temperaturepressure relation V constant increase T, P increases 4. Combined gas law  puts the three laws above together. P_{1}V_{1} / T_{1} = P_{2}V_{2} / T_{2} III. Ideal gas law  interrelates the pressure, volume, temperature, and number of moles of a gas. Above laws always dealt with the same amount of gas. PV = nRT n = number of moles, R = 0.0821 L atm mol^{1} K^{1}.
