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JEE Advanced levelSingle correct optionDifficulty 3 of 3ev038

Two rain samples are mixed: 1.0 L of pH 4.0 and 9.0 L of pH 5.0. Ignoring any reaction other than mixing and using $lo g 1.9 = 0.279$ , the pH of the mixture is

A4.72
B4.50
C4.28
D5.00

Answer: A

Moles of

H X^{+}

:

1.0 \times 1 0^{- 4} + 9.0 \times 1 0^{- 5} = 1.9 \times 1 0^{- 4}

mol in 10 L, so

[H X^{+}] = 1.9 \times 1 0^{- 5}

M. pH =

5 - 0.279 = 4.72

. Averaging the pH values (4.50) is wrong because pH is logarithmic.

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