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Which mixture forms a buffer when dissolved in 1L of water?
A. 0.2 mol NaOH + 0.2 mol HBr
B. 0.2 mol NaCl + 0.3 mol HCl
C. 0.4 mol HNO2 + 0.2 mol NaOH
D. 0.5 mol NH3 + 0.5 mol HCl

Answer
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Hint: To solve this question you should have basic knowledge about Buffer solutions. Buffer solutions are solutions which resist change in pH on dilution of small amounts of acid or alkali.

Complete answer:
The solutions which resist change in pH on dilution or with the addition of small amounts of acid or alkali are called Buffer solution. Buffer solutions of known pH can be prepared from the knowledge of pKa of the acid or pKb of base and by controlling the ratio of the salt and acid or salt and base. A mixture of acetic acid and sodium acetate acts as buffer solution around pH 4.75 and a mixture of ammonium chloride and ammonium hydroxide act as a buffer around pH 9.25.
A. 0.2 mol NaOH + 0.2 mol HBr will not form a buffer solution because it is a mixture of a strong base and a strong acid.
B. 0.2 mol NaCl + 0.3 mol HCl will not form a buffer solution because it is a mixture of a neutral salt and a strong acid.
C. 0.4 mol HNO2 + 0.2 mol NaOH will form a buffer solution because it is a mixture of a weak acid and its conjugate base. 0.2 mol of NaOH will neutralize 0.2 mol of HNO2. Hence, you will be left with 0.2 mol of HNO2 and 0.2 mol of NaNO2 (or, more specifically,NO2)
The pH will be equal to the pKa of HNO2.
Hence, 0.4 mol HNO2 + 0.2 mol NaOH will form a buffer as it is a mixture of weak acid and its conjugate base.
D. 0.5 mol NH3 + 0.5 mol HCl would have formed a buffer if you had added less HCl; however, you are adding so much HCl that it completely protonated NH3 to NH4+ .Hence, you will be left with 0.5 mol of NH4+ and 0.5 mol of Cl.

Hence, option C is the correct answer.

Note: You should have basic knowledge about strong acid, strong base and weak acid, weak base and also conjugates of acid and base.
You should remember pKa=[logKa] and pKb=[logKb]
Conjugate pairs of acid and base differ only by one proton.