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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ie162

A weak acid buffer has $p K_{a} = 4.74$ and pH = 5.04 ( $lo g 2 = 0.301$ ). Select the correct statements.

A $[salt] / [acid] \approx 2$
BFraction of the total buffer species present as acid is about one third
CFraction present as salt is about 0.67
DAdding base equal to half the acid present gives pH = 5.04 + 0.30

Answer: A, B, C

The Henderson equation gives

lo g ([salt] / [acid]) = 5.04 - 4.74 = 0.30

, so the ratio is

1 0^{0.30} \approx 2

(a true). Hence acid fraction

= 1/3

(b true) and salt fraction

= 2/3 \approx 0.67

(c true). Taking acid 3 and salt 6 units, adding base worth 1.5 units gives salt 7.5 and acid 1.5, ratio 5, pH

= 4.74 + 0.70 = 5.44

, not

5.34

(d false).

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