Problem Lab Questions

February 15

Look here for the worked solutions to these questions to be posted after the lab period.

The lab period preceeding the mid-term examination has traditionally been used as a problem solving period. Here are a series of question you should work on before the lab, which will be held during the regular lab period in the lab room on February 15. Please come, having worked on these problems already. Be prepared to ask questions and let's clear things up.

Look here for a page with pertinent data regarding various acids and bases.

Link back the IPS*1200 Home Page.

1. Arrange the following in order of increasing mass:

  1. 1.06 mol of SF4.
  2. 117 g of CH4.
  3. 8.7x1023 molecules of Cl2O7.
  4. 417 x 1023 atoms of Ar.

2. Balance the following equations:

3. Urea is used as a commercial fertilizer because of its nitrogen content. An analysis of a 25.0 mg sample of urea showed that it contained 5.0 mg of C, 11.68 mg of N, 6.65 mg of O, and the remainder was assumed to be hydrogen. What is the empirical formula for urea?

4. Phosphorus trichloride, PCl3, is formed in the reaction between white phosphorus, P4, and chlorine gas, Cl2. A 16.4 g sample of PCl3 was collected from the reaction of 5.00 g of P4 with excess Cl2. What is the percentage yield of the reaction?

5. A solution containing 3.44 g of AgNO3 is mixed with a solution containing 4.22 g of K3PO4. A precipitate of Ag3PO4 forms. What is the theoretical yield of Ag3PO4 from this mixture?

6. Lithium metal is the only member of the alkali metals (Group I) which can react directly with N2 to form a nitride, Li3N. What mass of lithium nitride can form from 90.0 g of Li and 2.00 x 1023 molecules of N2? Which reactant is present in excess and what is its remaining mass?

7. A 50 mL vial of xenon gas at 23C and 130 kPa of pressure was placed in the refrigerator where the temperature was 3.6C. What is the pressure in the vial once it has been chilled to the refrigerator temperature?

8. A gas relief valve on an industrial storage tank should be triggered if the pressure of carbon dioxide inside exceeds 115 atm. During the winter, the tank was filled with CO2 to a pressure of 100 atm when the temperature was -10C. On one hot summer day, the temperature rose to 35C. Should the relief valve have operated?

9. A neon sign consists of a piece of glass tubing that is 11.0 m long and 1.00 cm in diameter. It is filled with neon gas to a pressure of 1.4 torr and 22C. What is the mass of neon in the tube? (Note: 760 torr = 1 atm)

10. For the reaction describing the formation of ammonia, N2(g) + 3H2(g) 2NH3(g), the equilibrium constant at 400 K is 41. What would the value of the equilibrium constants be, at the same temperature and measured in the same units of pressure, for the following reactions:

  1. 2NH3(g) N2(g) + 3H2(g)
  2. (1/2)N2(g) + (3/2)H2(g) NH3(g)

11. The following measurements were made on a vessel at 460C in which the reaction H2(g) + I2(g) 2HI(g) had been allowed to reach equilibrium. Determine the concentration equilibrium constant, Kc, for the reaction at this temperature. What is the value of the pressure equilibrium constant, Kp, under these same conditions if pressure is measured in atmospheres?

[H2], mol/L
[I2], mol/L
[HI], mol/L
6.47 x 10-3
0.594 x 10-3
0.0137
3.84 x 10-3
1.52 x 10-3
0.0169
1.43 x 10-3
1.43 x 10-3
0.0100

12. A 500 mL reaction vessel at 700 K contains 1.20 x 10-3 mol of SO2(g), 5.00 x 10-4 mol of O2(g), and 1.00 x 10-4 mol of SO3(g). At this temperature, Kp = 3.0 x 104 for the reaction 2SO2(g) + O2(g) 2SO3(g), with pressure measured in atm. Will more SO3(g) form?

13. Carbon monoxide and water vapour are both introduced into a 250 mL sealed container at 25C, each with a partial pressure of 200 torr. The vessel is heated to 700C and at equilibrium, the partial pressure of CO2 is 88 torr. Calculate the value of Kp for the reaction CO(g) + H2O(g) CO2(g) + H2(g).

14. An ester is formed in the reaction between an organic acid and an organic alcohol. When acetic acid and ethanol are mixed, they react to form ethyl acetate

In an initial mixture, the concentration of CH3COOH is 0.32 M and of CH3CH2OH is 6.3 M. What is the equilibrium concentration of the ethyl acetate going to be? (Water is not a solvent in this case.)

15. The density of red phosphorus is 2.34 g/cm3. Another solid form of phosphorus, white phosphorus, has a density of 1.82 g/cm3. Which would you expect to be more favoured under conditions of increased pressure?

16. A solution of potassium hydroxide at pH=11.65 is diluted with water to eight times its original volume. What is the pH and the [OH-] concentration in the diluted solution?

17. In a solution in which 0.150 g of KNH2 has been dissolved and diluted to 250.0 mL, determine the concentrations in molarity for the following species: KNH2, NH2-, K+, H3O+, OH-. The amide ion is a strong base.

18. What is the pH, the pOH, and the percent ionization in a solution which is formally

  1. 0.10 M in formic acid
  2. 0.15 M in ammonia

19. What is the pH, the pOH, and the percent ionization in a solution which is formally

  1. 0.10 M in acetic acid
  2. 0.10 M in trichloroacetic acid

20. The pH of a 0.10 M solution of butylamine was measured as 12.04. What is the percent ionization, the Kb, and pKb for this base?

21. The percent ionization of diethylbarbituric acid is 0.14% in a solution which is formally 0.020 M in concentration. What is the pH of the solution and what is the Ka of the acid?

22. The Ka of pentafluorobenzoic acid is 0.033. When 0.100 mol is dissolved in 1.00 L of water, what is the resulting pH of the solution?

23. For each of the following buffer systems, determine the buffer pH and indicate which you think will be the better buffer.

  1. 25.0 g of benzoic acid and 25.0 grams of sodium benzoate, dissolved in 500.0 mL of water
  2. A solution which is 0.10 M in HNO3 and 0.10 M in NaNO3.
  3. A 500.0 mL solution of pyridine at a concentration of 0.100 M to which is added 250 mL of 0.0500 M HCl.

24. Morpholine is a molecule with a pKa of 8.492. In a 1.00 L solution which is buffered at pH=7.40, what is the ratio of acid-form to base-form when a few mg of morpholine are added (not enough to significantly affect the solution pH).

25. Valine is an amino acid which is diprotic. The two pKa's are 2.29 and 9.72. What is the pH of a solution which is

  1. 0.100 M in H2A
  2. 0.100 M in HA-
  3. 0.100 M in A2-

26. Salicylic acid is a diprotic acid that is related to aspirin and is sometimes taken as an analgesic, though its greater acidity can cause bleeding in the stomach. The two Ka's are 1.1 x 10-3 and 3.6 x 10-14. Determine the fraction of the acid in the stomach where the pH=1.50 which is in each of the three forms: H2A (salicylic acid), HA- (hydrogen salicylate), and A2- (salicylate ion).

27. A sample of 24.98 mL of 0.1328 M HBr is titrated with a solution of KOH having a molarity of 0.1435 M. Calculate and draw the titration curve: include the beginning concentration, the half-equivalence point, the equivalence point, a couple of points 10% and 20% beyond the equivalence point and a couple of points ±1 mL around the equivalence point.

28. A 50.00 mL sample of cyclohexylamine of an unknown concentration is titrated with a 0.1800 M solution of HCl. The equivalence point is reached at 37.32 mL of added HCl. The pH was noted to equal 10.64 when 18.66 mL of HCl had been added. What is the concentration of cyclohexylamine and what is its base ionization constant?

29. A 50.00 mL sample of sodium 8-hydroxyquinolate (pKa1=4.91 and pKa2=9.81) at a concentration of 0.1928 M is titrated with a 0.2218 M solution of HCl. Calculate the titration curve following comparable guidelines as in question 27 above, but remember that there are two equivalence points this time.

30. Consider the following titration curve.

Which of the following gave rise to this curve? Original solution volume was 50.00 mL and 0.100 M.

  1. Titration of 4-methylaniline (pKb=8.916) with 0.100 M HCl?
  2. Titration of 2-methylanilinium hydrochloride (pKa=4.447) with 0.050 M NaOH.
  3. Titration of 2-naphthoic acid (pKa=4.16) with 0.100 M NaOH?
  4. Titration of hypochlorous acid (pKa=7.53) with 0.100 M NaOH?

Last update: 17 February, 1999
Comments to: Dan Thomas