The molecular mass of sulphuric acid is 98. If 49 g of the acid is dissolved in water to make one litre of solution, what will be the strength of the acid?
- A.Two normal
- B.One normal
- C.0.5 normal
- D.Four normal
Correct answer
Explanation
The correct answer is B, One normal. Sulphuric acid has two replaceable hydrogen atoms, so its equivalent mass is 98 ÷ 2 = 49 g, and 49 g in one litre makes exactly one gram-equivalent per litre.
Normality is the number of gram-equivalents of a solute dissolved in one litre of solution. For an acid, the equivalent mass is its molar mass divided by its basicity, the number of H⁺ ions one molecule can give. H₂SO₄ is dibasic, so its equivalent mass is 49 g. Here 49 g ÷ 49 g = 1 gram-equivalent, dissolved in 1 litre, gives a 1 N solution. The same solution is 0.5 molar, since 49 g is half a mole; for sulphuric acid, normality is always twice the molarity.
A is wrong because 2 N would need 98 g in a litre. C is wrong because 0.5 is the molarity of this solution, not its normality. D is wrong because 4 N would need 196 g in a litre.
Exam tip: Normality = molarity × basicity; for H₂SO₄, equivalent mass 49, so N = 2M.
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