What is the answer to the buffer solutions NEET 2026 question?
B) 4 is the correct answer to the buffer solutions NEET 2026 question. The given constant belongs to the conjugate base, so convert it into the acid constant before calculating pH.
The problem gives a buffer at 298 K containing equal concentrations of weak acid HX and its conjugate base:
You must determine the buffer’s pH. The supplied choices are:
This single-correct question belongs to Chemistry → Equilibrium → Buffer Solutions. The question bank classifies it as medium, with an expected solving time of 60 seconds, not a measured student average.
How do you convert the given base constant into the acid constant for HX?
Divide the water ionisation constant by the given base constant to obtain HX’s acid constant. HX is the weak acid, and the supplied base constant belongs to its conjugate base, not to HX. The buffer equation needs the acid’s pKa, so identify which species owns the constant before calculating.
The acid dissociation equilibrium is:
For this conjugate acid–base pair:
At the stated temperature:
Following the official solution, convert the supplied base constant into the acid constant:
When dividing powers with the same base, subtract the denominator’s exponent. Here, subtracting a negative exponent increases the exponent by ten.
Now take the negative base-ten logarithm:
The stated temperature supplies the condition under which this water ionisation constant applies. Do not treat that value as temperature-independent: the constant changes with temperature.
How do equal concentrations give the buffer’s pH?
Equal acid and conjugate-base concentrations make the logarithmic term zero, so the pH equals the acid’s pKa. No absolute concentration is needed for the supplied calculation. The Henderson–Hasselbalch equation uses the concentration ratio, which the question already fixes.
Write the equation with conjugate base in the numerator and acid in the denominator:
The given equality gives:
Substitute the calculated acid pKa and this ratio:
Therefore, B) 4 is the single correct answer. Equal concentrations do not automatically mean a neutral solution: this buffer’s pH equals its acid pKa, which is four, not seven. Equal amounts of a weak acid and its conjugate base are not the same as equal hydrogen-ion and hydroxide-ion concentrations.
Do not invent a molarity or stop because none is supplied. The concentration ratio is sufficient for this buffer calculation.
The official route is: identify the given base constant → calculate the acid constant → calculate pKa → substitute the ratio.
Why does option D give 10 instead of the correct pH?
Option D follows from calculating pKb correctly but then using it as pKa. The logarithm itself is not the error. The mistake is assigning its result to the wrong member of the conjugate pair.
The incorrect route begins with:
It then wrongly proceeds as though:
What was actually calculated is:
As a check after the official solution, at 298 K:
The correctly solved buffer does have a pOH of ten:
That does not make its pH ten. The two quantities describe different ion concentrations.
Label every equilibrium constant with its species before taking a logarithm. This keeps the base constant attached to the conjugate base rather than the acid.
Can you solve three related Equilibrium practice questions?
The three answers are pH five, pH three and pOH ten, respectively. These are author-created practice variations, not additional NEET PYQs or official question-bank entries. Attempt each variation before checking its working, covering the solutions below if needed.
All three retain the original temperature and conjugate-base constant, so the acid pKa already established remains valid:
- Author-created practice 1: Find pH when the conjugate-base-to-acid ratio is ten.
The base concentration is ten times the acid concentration:
Substitute into the same buffer equation:
The logarithmic correction is positive because the ratio exceeds one.
- Author-created practice 2: Find pH when the conjugate-base-to-acid ratio is one-tenth.
The acid concentration is now ten times the base concentration:
The logarithmic correction is negative because the ratio is below one.
- Author-created practice 3: Find pOH for the original equal-concentration buffer.
First retain the correctly calculated pH, then convert:
Compared with the original equal-concentration buffer, increasing the conjugate-base-to-acid ratio raises pH; decreasing it lowers pH. Changing the requested quantity from pH to pOH changes the final conversion, not the buffer calculation.
Before selecting an option, underline which species owns the constant and circle whether the question asks for pH or pOH.
Next step: the past-paper archive on NEET JEEnius AI and search past NEET papers by year, subject or chapter, each with a worked solution (100 free searches a month).
For a worked example of the same idea, see Oscillations and Waves NEET 2026: Phase Difference.
Frequently asked questions
What is the answer to the buffer solutions NEET 2026 question?
The correct answer is B) 4 for the buffer with equal concentrations of HX and X− and Kb(X−) = 10^-10 at 298 K. Using Ka = Kw/Kb gives Ka = 10^-4 and pKa = 4. Since the conjugate-base-to-acid ratio is one, pH = pKa = 4.
How do I calculate Ka when Kb is given?
For a conjugate acid–base pair, Ka × Kb = Kw, so Ka = Kw/Kb. At 298 K, Kw = 10^-14; therefore, Kb = 10^-10 gives Ka = 10^-4. Check which species owns the supplied constant before taking its logarithm.
Why is the pH not 7 when acid and conjugate base concentrations are equal?
Equal acid and conjugate-base concentrations make the logarithmic term in the Henderson–Hasselbalch equation zero, so pH equals pKa, not necessarily 7. In this question, pKa = 4, so pH = 4. Equal concentrations of a weak acid and its conjugate base do not imply equal hydrogen-ion and hydroxide-ion concentrations.
Why is option D, pH 10, wrong?
Taking −log10 of the given Kb gives pKb = 10, not pKa = 10. At 298 K, pKa + pKb = 14, so pKa and the equal-concentration buffer's pH are both 4. The value 10 is the buffer's pOH, not its pH.