pH value of 0.1 N HCl solution is approximately:

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Correct — A, 1.0. Hydrochloric acid is a strong, monoprotic acid: in dilute aqueous solution it dissociates essentially completely, HCl → H⁺ + Cl⁻. Normality counts equivalents per litre, and for an acid one equivalent is one mole of donatable H⁺, so a 0.1 N solution of any strong acid delivers 0.1 mol of hydrogen ions per litre. For HCl there is not even a conversion to do — it has a single ionisable hydrogen, so its n-factor is 1 and 0.1 N is simply 0.1 M. Put [H⁺] = 0.1 = 1 × 10⁻¹ mol/L into Sørensen's definition, pH = −log₁₀[H⁺] = −log₁₀(10⁻¹) = 1.0. The Commission published exactly this working when it disposed of candidate objections on 31 October 2025: −log(0.1) = −log(10⁻¹) = 1. Two independent checks confirm it. First, at 25 °C pOH = 14 − 1 = 13, so [OH⁻] = 10⁻¹³ mol/L — vanishingly small, as it must be in a decinormal strong acid. Second, the physical analogue: human gastric juice is roughly 0.1 M hydrochloric acid and measures about pH 1.5–3.5, the same order of magnitude as this solution. The word 'approximately' in the stem is doing honest work, because pH is strictly defined on hydrogen-ion activity rather than concentration, and the mean ionic activity coefficient of 0.1 molal HCl is about 0.80 — so a real electrode reads close to 1.1, not an exact 1.00. Note also what the arithmetic quietly assumes: complete dissociation, which is legitimate only because HCl is strong. Had the stem said 0.1 N acetic acid (Ka = 1.8 × 10⁻⁵), [H⁺] would be √(Ka·C) ≈ 1.3 × 10⁻³ M and the pH about 2.9.

Why the other options are wrong — (b) pH 11 is alkaline — a solution with [H⁺] = 10⁻¹¹ mol/L, ten billion times poorer in hydrogen ions than this one. It is roughly the pH of 0.001 M sodium hydroxide or of household ammonia. No acid can read above 7 at 25 °C, and no arithmetic on 0.1 produces 11: even the pOH of this solution, for the candidate who subtracts on the wrong side of pH + pOH = 14, is 13.  (c) Also alkaline, and the sharpest trap in the set, because it differs from the correct option only in where the decimal point sits — '1.0' against '10'. A candidate racing through a block of ten numericals can do the sum perfectly and still tick the wrong box. pH 10 means [H⁺] = 10⁻¹⁰ mol/L, about a billion-fold less acidic than 0.1 N HCl; milk of magnesia sits near it at about 10.5.  (d) The intelligent wrong answer. pH 2 belongs to 0.01 N (10⁻² M) HCl — exactly one ten-fold dilution away. You land on it by miscounting the exponent, by reading 0.1 as 0.01, or by recalling a school demonstration in which 'dilute HCl' happened to be 0.01 M. A fourth route is to invent a normality-to-molarity halving that HCl never needs: 0.1 N H₂SO₄ is indeed 0.05 M, but it still carries 0.1 mol H⁺ per litre and its pH is also 1.0.

Concept: pH is a compressed way of writing hydrogen-ion concentration. The Danish chemist S. P. L. Sørensen introduced it at the Carlsberg Laboratory in Copenhagen in 1909, working on brewing enzymes, to spare chemists from writing numbers like 0.00000001 mol/L; he defined pH = −log₁₀[H⁺], and modern IUPAC practice refines the bracket to hydrogen-ion activity, which departs from concentration only in strong or concentrated solutions. Because the definition is a base-ten logarithm, one pH unit is a ten-fold change in acidity: pH 3 is ten times more acidic than pH 4 and a hundred times more acidic than pH 5. The familiar 0–14 range is not a law of nature but a consequence of water's own self-ionisation. At 25 °C the ionic product Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14, and pure water — where [H⁺] = [OH⁻] = 10⁻⁷ M — is neutral at pH 7. Both boundaries leak: concentrated hydrochloric acid at about 12 M has a negative pH, and because Kw rises with temperature to 5.1 × 10⁻¹³ at 100 °C, boiling water is neutral at pH 6.14 while remaining perfectly neutral. The second half of the question is a units question. Normality is equivalents per litre; molarity is moles per litre; normality = molarity × n-factor, where the n-factor of an acid is the number of replaceable hydrogens. HCl, HNO₃ and CH₃COOH are monoprotic, so N = M; H₂SO₄ is diprotic, so 0.1 M H₂SO₄ is 0.2 N.

State PCS 2025 Q131 — Science & Technology PYQ with Answer

State PCS 2025 Q131 — Science & Technology PYQ with Answer