Homework Answers

A swimming pool in a club is maintained sufficiently alkaline so that the

CO2 absorbed from the atmosphere produced in the pool a solution which was 2 x 10-4 M in CO3 2- after about a day of refilling the swimming. If the pool water was originally 4 x 10-3 M in Mg2+, 6 x 10-4 M in Ca2+ and 8 x 10-7 M in Fe2+, then which of the following precipitates should be formed immediately a day after refilling of the swimming pool:

(a) only MgCO3

(b) only CaCO3

(c) only FeCO3

(d) only CaCO3 and FeCO3

(e) MgCO3, CaCO3 and FeCO3

Calculate the percentage change in the number of H+ ions during the titration of an

acid initially at pH = 3, when its pH increases to 5 on addition of a small amount of

alkali during the course of titration.

Derive an expression for Entropy Change accompanying an isothermal reversible

expansion of an ideal gas.

Hydrofluoric acid has a Ka value of 7.2x10−4. What is the pH of a 0.04M solution

of hydrofluoric acid?

A Carnot engine absorbs heat from a source at a higher temperature of 800 Kelvin

and the efficiency of the Carnot engine is 50%.

To what value the higher temperature should be increased for efficiency to increase

to 80%, if the lower temperature is kept constant.

In a Laboratory, you have the following chemicals available:

1.0 M NaOH, 1.00 M HCl, 1.00 M H3PO4, NaH2PO4 (s), Na2HPO4 (s), and Na3PO4 (s).

Describe TWO ways of making 1000 mL of a buffer of pH = 12 (Use only the above

chemicals, plus distilled water.)

[Given: H3PO4 - Ka1 = 6.9 x 10-3; NaH2PO4 - Ka2 = 6.2 x 10-8 ; Na2HPO4 - Ka3 = 4.8 x 10-13]

Describe the effect on the pH that results by adding

a. NaNO2 to HNO2

b. NaNO3 to HNO3

A buffer solution is prepared by dissolving 1.50 g each of benzoic acid, C7H6O2, and

sodium benzoate, NaC7H5O2, in 150.0 mL of solution. [Ka = 6.3 x 10-5]

a. What is the pH of this buffer solution?

b. Which buffer component must be added, and in what quantity, to change the

buffer pH to 4.00?

c. What quantity of 2.0 M NaOH or 2.0 M HCl must be added to the original buffer to

change the pH to 4.00.