Acid-base chemistry: buffer calculation: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Acid-base chemistry: buffer calculation

Sample item

One litre of a phosphate buffer contains 0.10 mol of HPO4(2-) and 0.01 mol of H2PO4(-); the pKa of the pair is 6.8. Then 0.005 mol of HCl is added and mixed without volume change. What is the approximate final pH? (log 6.33 is about 0.80.)

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. 7.6

The initial pH is 6.8 + log(0.10/0.01) = 7.8. The added H+ converts base to acid: HPO4(2-) falls to 0.095 mol and H2PO4(-) rises to 0.015 mol, a ratio of 6.33. The new pH is 6.8 + 0.80 = 7.6, so the buffer resisted a large change.

Option by option

  • A. 7.6 (correct)

    Correct: pH = 6.8 + log(0.095/0.015) = 6.8 + 0.80 = 7.6.

  • B. 7.2

    A pH of 7.2 would need a ratio of about 2.5, which requires far more acid than was added.

  • C. 7.8

    A pH of 7.8 is the starting value and ignores the small shift in the base-to-acid ratio.

  • D. 2.3

    A pH of 2.3 results from treating 0.005 M as free strong acid, ignoring the buffer base that neutralises it.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, 7.6. The initial pH is 6.

Why is "7.2" not the answer?

A pH of 7.2 would need a ratio of about 2.5, which requires far more acid than was added.

Why is "7.8" not the answer?

A pH of 7.8 is the starting value and ignores the small shift in the base-to-acid ratio.

Why is "2.3" not the answer?

A pH of 2.3 results from treating 0.005 M as free strong acid, ignoring the buffer base that neutralises it.

What concept does this question test?

It tests Acid-base chemistry: buffer calculation in Biochemistry. 8 + log(0.10/0.01) = 7.8. The added H+ converts base to acid: HPO4(2-) falls to 0.095 mol and H2PO4(-) rises to 0.015 mol, a ratio of 6.33. The new pH is 6.8 + 0.80 = 7.6, so the buffer resisted a large change.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Acid-base chemistry: buffer calculation?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.