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The pH of a phosphate buffer is 7.10 at 25 LaTeX: ^\circ C ∘C. What is...

The pH of a phosphate buffer is 7.10 at 25 LaTeX: ^\circ C ∘C. What is the pH of the buffer at 37 LaTeX: ^\circ C ∘C? The LaTeX: \Delta_rH^\circ ΔrH∘ for the relevant dissociation step is 3.75 kJ mol-1.

answer choices: 7.12, 7.10, 7.02 and 7.07

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Answer #1

PH of a buffer can obtained by henderson hasselbalch equation i.e.

pH=pKa+log[salt]/[acid]

In the question, ΔH of dissociation is given that can be used to get pKa of the dissociationg step.

From classical thermodynamics, ΔG= -RTlnKeq ; pKa = ΔH/2.303RT - ΔS/2.303R

As the dissociation extent is not very large (weak acid is there in buffer), we can consider entropy part to be constant (very little change). So, pKa is depended on 1/T i.e. as Temperature increases, pKa will decrease. Enthalpy part (ΔH/2.303RT) decreases from 0.657 to 0.632 as temperature change is considered. This difference of 0.025 will also be observed in pKa and hence also in pH. So, pH at 37°C is 7.07 (7.10-0.025).

Do reply for further query.

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