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A) HCN and KCN B) NH3 and (NH4)2SO4 C) HF and NaF D) HCI and KCI E) HNO2 and NaNO2

Which one of the following combinations cannot function as a buffer solution and why?

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    $\begingroup$ Welcome to Chemistry.SE. Homework questions are okay but some attempt to answer/understand the question must be demonstrated. $\endgroup$
    – A.K.
    Aug 9, 2018 at 19:54

1 Answer 1

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A buffer can be made either by partially titrating an acid or having a weak acid with its conjugate base.

In options A, B, C, and E, there is a weak acid (HA) with it's conjugate base (A-). However, in D, there is HCl, a strong acid, with Cl-. The HCl/KCl system is not a buffer because the $\ce{Cl-}$ anion is the conjugate base of a strong acid, and therefore remains deprotonated; the $\ce{Cl-}$ anion does not contribute to the overall $\ce{pH}$. Since $\ce{HCl}$ is the only source of protons, and there is no other species to take up the protons, the HCl/KCl system is not a buffer.

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