Question

The dissociation constant of a weak acid is
1 × 10⁻⁴. In order to prepare a buffer solution with a pH = 5
the [Salt]/[Acid] ratio should be [NEET Kar. 2013]
(a) 1 : 10 (b) 4 : 5
(c) 10 : 1 (d) 5 : 4

Answer 1

answer : option (c) 10 : 1

using formula, pH = pKa + log [salt/acid ]

given, pH = 5, Ka = 1 × 10-⁴

we know, pKa = -logKa

so, pKa = -log (1 × 10-⁴) = 4

so, 5 = 4 + log [salt/acid]

⇒1 = log [salt]/[acid ]

⇒10¹ = [salt ]/[acid ]

⇒[salt ]/[acid ] = 10/1

hence option (c) is correct choice.

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Answer 2

using formula, pH = pKa + log [salt/acid ]

given, pH = 5, Ka = 1 × 10-⁴

we know, pKa = -logKa

so, pKa = -log (1 × 10-⁴) = 4

so, 5 = 4 + log [salt/acid]

⇒1 = log [salt]/[acid ]

⇒10¹ = [salt ]/[acid ]

⇒[salt ]/[acid ] = 10/1