Physical Chemistry Answers

Explain the role of Solubility product (with atleast 2 examples) in qualitative

analysis.

Write a note on Buffer Capacity and Buffer Index.

Explain the Buffer Action of an Acidic Buffer.

Derive the Henderson’s Equation for a Basic Buffer.

Calculate the pH of 10-8M NaOH solution

Can pH of a solution be negative? Explain with relevant examples.

In our body, the pH concept finds one of its applications in the digestive system.

Our stomach secretes hydrochloric acid which activates enzymes such as trypsin,

pepsin which gets mixed with the food and help in the breakdown of the food.

Sometimes excess amount of acid is released in the stomach which gives us unusual

feeling of illness. An antacid is used to rectify tis imbalance. What is an Antacid? How

does it work?

A Laboratory assistant prepares 5 litres of 0.1N hydrochloric acid stock solution using commercial HCl available in laboratory which has percentage purity of 35% (w/w) and specific gravity of 1.18 g/ml. Then he fills five 100ml standard flasks with 50.0ml of 0.1N HCl solution and dilutes it with distilled water upto the mark. Then he takes 10ml of diluted HCl solution from each flask into a conical flask, mixes it and adds 50ml of 0.025M Sodium hydroxide solution to it. Finally, he adds few drops of phenolphthalein to the resulting solution in the conical flask.

i) What was the concentration of HCl solution prepared in 100ml standard

flasks?

ii) What do you think will be the colour of the resulting solution after addition of

the indicator?

iii) Calculate the pH of the resulting solution.

Which acidic solution will show the lowest pH value and which one has the

highest pH value?

0.710 g/L HCl

0.02 M H2SO4 (assuming both dissociations of sulphuric acid are complete)

0.1 M CH3COOH

Show the pH calculations for each of them.

Sulphuric acid is a dibasic acid. The second dissociation is an incomplete one as shown below: H2SO4is a diprotic acid, dissociating in an aqueous solution in two steps.

H2SO4+ H2O ==> H3O+ + HSO4 -

HSO4 - + H2O <==> H3O+ + SO4 - -

Calculate the pH of 10-3 M Sulphuric Solution.

Hint:

The first dissociation is complete because pK1 = - 3. The second dissociation is

partial, since pK2 = +2. If the stoichiometric molarity of the solution is C mol/L, [H+] < 2*C.

[H+] = C+x, where C is due to the first dissociation and x due to the second.

[HSO4 -] = C – x , [SO4 - -] = x

K2 = [H+][SO4 - -]/[HSO4 -] = (C+x)*x/C-x

Rearrangement gives the quadratic equation

x2 +(C+K2)x - K2*C = 0

Solve for x and then find pH.