Nyb Fall 06 Unit Test 3

CHEMISTRY 202-NYB Fall 2006 Instructor: Michael Lautman Class Test 3 - Colourless NAME: 1. Consider the titration of a weak base (NH3) with a strong acid (HCl).

a. List the major and minor species at the following points: initially, at the halfequivalence point, at the equivalence point and after 60.0 mL of acid have been added. (5 MARKS)

b. On the graph, sketch the curve for the titration of a weak acid with a strong base. (1 MARK) c. Explain why the indicator methyl red is better suited for the titration in part a than phenolphthalein. (4 MARKS)

2. You are required to make 150.0 mL of a buffer of pH = 7.88. Choose the components from the list below and calculate the volume of each required. (20 MARKS) i) 0.100 M acetic acid (Ka = 1.8 x 10-5) ii)0.075 M potassium phosphate iii) 0.144 M barium acetate iv) 0.100 potassium hydrogen phosphate (Ka = 6.3 x 10-8)

b. What will the resulting pH be when 55.0 mL of 0.100 HNO3(aq) are added.

3. A small lake has a surface area of 4.32 x 103 m2. 4.22 cm of rain of pH 3.22 falls on the lake, ignoring runoff from the surrounding land. (Assume that the acidity of the rain is due to a strong monoprotic acid). HINT: 1 m2 = 1x 104 cm2 1 cm3 = 1 mL = 1 L a) How many moles of H3O+ are in the rain falling on the lake? (5 MARKS)

b) If the lake is unbuffered with pH = 7.00, and the average depth is 2.0 m (before the rain), find the pH of the lake after the rain has fallen. (10 MARKS) HINT: The total volume of the lake (before the rain falls) is 8640 m3. 1 m3 = 1 x 106 cm3

c) If the lake contains hydrogen carbonate (HCO3 ), what mass of this ion would neutralize the acid in the rain? (5 MARKS) HINT: Assume that neutralization occurs at equivalence. -

4. A microbiologist prepares a phosphate buffer from 0.5 M KH2PO4 and 0.5 M K2HPO4. At 25 oC, the pH of the buffer is 8.23. At 37 oC the pH of the buffer is 7.83. Explain. Calculate the Ka at 37 oC. (KH2PO4 Ka (25 oC) = 6.3 x 10-8) (10 MARKS)

5. 75.5 mL 0.133 M hydrocyanic acid (HCN, Ka = 6.2 x 10-10) is titrated with 0.0100 M potassium hydroxide solution. a. What is the initial pH of the solution? (5 MARKS)

b. What is the pH of the solution at the equivalence point? (5 MARKS)