Chemistry 101

Recitation 10/3/03 Chapters 4 and 5

1) The following chemical reactions have helped shape Earth’s crust. Determine the oxidation numbers of all the elements in the reactants and products, and identify which of the elements are oxidized or reduced.

a) 3 SiO₂ + 2 Fe₃O₄ à 3 Fe₂SiO₄ + O₂

+4 Si –2 O +8/3 Fe –2 O à +2Fe +4 Si –2O + 0 for O₂

O is oxidized in going from –2 to O

Fe is reduced in going from 8/3 to 2. (Oxidation numbers can be fractions, but it’s rare)

b) SiO₂ + 2 Fe + O₂ à Fe₂SiO₄

+4 Si –2 O + 0 Fe + 0 for O₂ à+2 Fe +4 Si -2 for O₂

Fe oxidized (0 to +2) and O reduced (0 to –2)

c) 4 FeO + O₂ + 6 H₂O à 4 Fe (OH)₃

+2 Fe –2 O + 0 O₂ à +3 Fe –2 O

Fe is oxidized and O is reduced

2) The mean free paths of nitrogen and oxygen are about 60 nm at sea level, but increase to 10 cm in the thermosphere. Calculate the frequency of collisions of N₂ and O₂ molecules in the thermosphere (T = 2200 °C). The mean free path is the average distance a molecule travels between collisions.

3) The CO₂ that builds up in the air of a submerged submarine can be removed by reacting it with sodium peroxide:

2 Na₂O₂ (s) + 2 CO₂ (g) à 2 Na₂CO₃ (s) + O₂ (g)

If a sailor exhales 150 mL of CO₂ per minute at 20 °C and 1.02 atm, how much sodium peroxide is needed per sailor in a 24-hr period?

Strategy: Find number of moles of CO₂ per day (Ideal Gas Law) then the number of moles of Na₂O₂ per day (2 moles CO₂ per 2 moles Na₂O₂) and then the mass of Na₂O₂ using its molar mass of 78 g/mol

150 ml/min x 60 min/hour x 24 hr/day = 216000 mL = 216 L

n = PV/RT = 1.02 atm x 216 L / 0.0802 L.atm/K.mol x (273 + 20 K) = 9.17 moles CO₂

9.17 moles CO₂ x 2 moles Na₂O₂/2 moles CO₂ = 9.17 moles Na₂O₂

9.17 moles Na₂O₂ x 78 g/mol = 715 g

4) Carbon monoxide at a pressure of 680 mm Hg reacts completely with O₂ at a pressure of 340 mm Hg in a sealed vessel to produce CO₂. What is the final pressure in the flask?

5)

CO (g) + 1/2 O₂ à CO₂

since the volume and the temperature are the same for all gases, the number of moles of CO is twice the number of moles of O₂. Neither reagent is limiting and the reactants will be completely used up to produce 1 mole of CO₂ for every mole of CO. Since Pressure is proportional to the number of moles, the final pressure will be 680 mm Hg.

6) Chemists who analyze samples for trace metals may buy standard solutions that contain 1000.0 mg/L concentrations of the metals. If a chemist wished to dilute 1000. mg/L standard to prepare 500.0 mL of a “working” standard that has a concentration of 2.25 mg/L, what volume of the 1000.0 mg/L standard solution is needed?

2.25 mg/L / 1000 mg /L x (500 mL) = 1.125 mL needed