SSC · Railway
The salt of a strong acid and a weak base has a pH value that is:
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Answer: Option D — Less than 7
Detailed Solution
A salt formed from a strong acid and a weak base gives an acidic solution when dissolved in water, so its pH is less than 7. A common example is ammonium chloride (NH4Cl), formed from hydrochloric acid (a strong acid) and ammonium hydroxide (a weak base).
When such a salt dissolves in water, its ions react with water in a process called hydrolysis. In ammonium chloride, the ammonium ion (NH4+) reacts with water to release hydrogen ions (H+), which makes the solution acidic. The chloride ion, which comes from a strong acid, does not react with water.
The nature of a salt solution depends on the strength of the acid and base from which it is formed:
Strong acid + strong base: neutral salt, pH 7. Example: sodium chloride (NaCl), from hydrochloric acid and sodium hydroxide.
Strong acid + weak base: acidic salt, pH less than 7. Examples: ammonium chloride (NH4Cl) and copper sulphate (CuSO4).
Weak acid + strong base: basic salt, pH more than 7. Examples: sodium carbonate (Na2CO3, washing soda), sodium bicarbonate (NaHCO3, baking soda) and sodium acetate (CH3COONa).
Weak acid + weak base: the pH depends on the relative strength of the acid and the base, such as ammonium acetate, which is nearly neutral.
The pH scale runs from 0 to 14. A pH of 7 is neutral, less than 7 is acidic and more than 7 is basic. Each step on the scale represents a tenfold change in the concentration of hydrogen ions. The pH of human blood is about 7.4 (slightly basic), and the pH of the stomach's gastric juice is about 1.5 to 2 (strongly acidic, due to hydrochloric acid).
Why not the other options?
- ✗Exactly 7 is the pH of a salt of a strong acid and a strong base, such as NaCl.
- ✗More than 7 is the pH of a salt of a weak acid and a strong base, such as Na2CO3.
- ✗Exactly 14 is the pH of a very strong base, not of such a salt.
Quick tip
Remember: strong acid + weak base = acidic salt (pH < 7, e.g. NH4Cl); weak acid + strong base = basic salt (pH > 7, e.g. Na2CO3); strong + strong = neutral (NaCl).
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Acids, Bases & Salts — Preparation
This question tests your understanding of Acids, Bases & Salts, a key topic in the General Knowledge section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
This topic covers acids, bases and salts, their properties, indicators, the pH scale, neutralisation, the acids found in natural substances, and important salts such as common salt, baking soda, washing soda, bleaching powder and plaster of Paris. Questions often ask which acid is found in which substance (citric acid in lemons, lactic acid in curd, formic acid in ant stings and tartaric acid in tamarind), the pH range of acids and bases, and the uses of everyday salts. The pH of pure water is 7, and a pH below 7 means a solution is acidic. Practise the acids, bases and salts questions below, written in the style of the general awareness sections of exams such as SSC, banking, railways, CUET GAT, MAH CET and CLAT. Each question comes with a detailed explanation that adds related facts, so every question helps you revise more than one point.