Verified Solution • 2025–26 Session moderate

QJEEM08Chabe5: JEE Main 2008 Chemistry - Class 11 JEEM08Chabe5 Solution: The pK a of a weak acid, HA, is 4.80. The pK b of a weak base,…

The pKa of a weak acid, HA, is 4.80. The pKb of a weak base, BOH, is 4.78. The pH of an aqueous solution of the corresponding salt, BA, will be

Multiple Choice Options

  • A 9.58
  • B 4.79
  • C 7.01
  • D 9.22

Step-by-Step Verified Solution

In aqueous solution BABA (salt) hydrolyses to give

BA  +  H2O ⇌ BOH  +  HABA\,\, + \,\,{H_2}O\,\rightleftharpoons\,BOH\,\, + \,\,HA

                            \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,                             \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, Base acid

Now pHpH is given by

pH=12pKw+12pKa−12pKbpH = {1 \over 2}p{K_w} + {1 \over 2}pKa - {1 \over 2}p{K_b}

Substituting given values, we get

pH=12(14+4.80−4.78)=7.01pH = {1 \over 2}\left( {14 + 4.80 - 4.78} \right) = 7.01

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