Verified Solution • 2025–26 Session moderate

QJEEAdv15Ch1542: JEE Advanced 2015 Chemistry - Class 11 JEEAdv15Ch1542 Solution: Paragraph When 100 mL of 1.0 M KCl was mixed with 100 mL of 1.0 …

Paragraph
When 100 mL of 1.0 M KCl was mixed with 100 mL of 1.0 M NaOH in an insulated beaker at constant pressure, a temperature increase of 5.7o C was measured for the beaker and its contents (Expt. 1). Because the enthalpy of neutralization of a strong acid with a strong base is constant (-57.0 kJ/mol), this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. 2) 100 mL of 2.0 M acetic acid (Ka = 2.0 × 10-5) was mixed with 100 mL of 1.0 M NaOH (under identical conditions to Expt. 1) where a temperature rise of 5.6o C was measured. (Consider heat capacity of all solutions as 4.2 J/gK and density of all solutions as 1.0 g m/L)
Question
The pH of the solution after Expt. 2 is

Multiple Choice Options

  • A 2.8
  • B 4.7
  • C 5
  • D 7

Step-by-Step Verified Solution

In Expt. 2, the final solution is a buffer as it contains equimolar amounts of acid and salt.

pH=pKa+log⁡(salt)(acid) ..... (1)

pKa=−log⁡(2×10−5)

=−0.3010+5

=4.699≈4.7

[Salt]=[CH3COONa]=100200×2=1M

[Acid)=[CH3COOH]=200−100200×2=100200×2=1M

Substituting the values in Eq. (1), we get

pH=4.7+log⁡11=4.7

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