- No.13. How pH of a solution is measured?
Answer. pH of the solution is measured by two methods
(i) Universal Indicator
Some indicators are used as mixtures. These mixture indicators give different colors at different pH values. Hence, it is used to measure the pH of a solution. Such a mixed indicator is called Universal Indicator or simply pH indicator. The pH of solution can be measured by dipping a piece of Universal Indicator paper in the solution. The pH is then found by comparing the colour obtained with a colour chart as shown
(ii) The pH meter
The pH of a solution can be measured with a pH meter. It consists of a pH electrode connected to a meter. The electrode is dipped into the solution and the meter shows the pH either on a scale or digitally. It is much more reliable and accurate method of measuring pH than Universal Indicator paper.
- No.14. Write down different uses of acids and bases.
Answer. Uses of Acids
(i) Sulphuric acid is used for manufacture of fertilizers (ammonium sulphate), explosives, paints, dyes, drugs and as an electrolyte in lead storage batteries.
(ii) Nitric acid is used for manufacturing of fertilizer (Ammonium nitrate), explosives, paints and drugs. Etching designs on copper plates.
(iii) Hydrochloric acid is used for cleaning metals, tanning and in printing industries.
(iv) Benzoic acid is used for food preservation.
(v) Acetic acid is used for flavoring food and food preservation. It is also used to cure the sting of wasps.
Uses of Bases
(i) Sodium hydroxide is used for manufacturing of soap.
(ii) Calcium hydroxide is used for manufacturing of bleaching powder, softening of hard water and neutralizing acidic soil and lakes due to acid rain.
(iii) Potassium hydroxide is used in alkaline batteries.
(iv) Magnesium hydroxide is used as a base to neutralize acidity in the stomach. It is also used for the treatment of bee’s stings.
(v) Aluminum hydroxide is used as foaming agent in fire extinguishers.
(vi) Ammonium hydroxide is used to remove grease stains from clothes.
Problem 10.1.A solution of hydrochloric acid is 0.0 1M. What is its pH value?
Solution: Hydrochloric acid is a strong acid so it ionizes completely.
HCl(aq) ————— > H+(aq) + Cl- (aq)
So, its solution also contains 0.01M H ions, i.e., 10-2M.
pH = -log[H+]
By putting the value of H+ ions in the above equation:
pH = -log l0-2
pH = 2
Problem.10.2. Find out the pH and pOH of 0.001M solution of KOH?
Solution: Potassium hydroxide solution is a strong base. It ionizes completely. One mole of KOH -gives one mole of OH- ions.
KOH(aq) ————— K+(aq) + OH-(aq)
Therefore, 0.001M solution of KOH produces 0.001M OH- ions.
[OH-]= 0.001 M or 10-3 M
pOH = -loglO-3 = 3
pH + pOH =14