A 50 mM buffer of monosodium dihydrogen phosphate and disodium monohydrogen phos

Question

A 50 mM buffer of monosodium dihydrogen phosphate and disodium monohydrogen phosphate at pH 7.0. The relevant pka is 6.8. What is the ionic strength? ( must figure out concentrations of all ionic species before computing the ionic strength). A 50 mM buffer of monosodium dihydrogen phosphate and disodium monohydrogen phosphate at pH 7.0. The relevant pka is 6.8. What is the ionic strength? ( must figure out concentrations of all ionic species before computing the ionic strength). A 50 mM buffer of monosodium dihydrogen phosphate and disodium monohydrogen phosphate at pH 7.0. The relevant pka is 6.8. What is the ionic strength? ( must figure out concentrations of all ionic species before computing the ionic strength).

Explanation / Answer

pH = pKa + log [Salt] / [Acid]

7.0 = 6.8 + log [Salt] / [Acid]

0.2 = log [Salt] / [Acid]

[Salt] / [Acid] = 1.585

x + 1.585 x = 0.05 M

2.585x = 0.05

x = [Acid] = 0.0193 M

[Salt] = 0.0306 M

Ionic strength, I = 1/2 × [(0.0193 x 1 x 12) + (0.0193 x 1 x 12) + (0.0306 x 2 x 12) + (0.0306 x 1 x 22)]

I = 1/2 x [ 0.2222] = 0.1111

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