Please help me. Than What volume (in mL) of a 0.50 M NH_a Cl solution must be ad

Question

Please help me. Than What volume (in mL) of a 0.50 M NH_a Cl solution must be added to 50.0 mL of a 0.50 M NH_3 solution in order to prepare a buffer with a pH of 9.5? (k_b of NH_3 is 1.8 Times 10^-5). For best results, what should be the relationship between the desired pH of a buffer, and the pKa of the acid compressing the buffer? The pKa should be within 4 pH units of the desired pH of the buffer. The pKa should be within 2 pH units of the desired pH of the buffer. The pKa should be within 1 pH units of the desired pH of the buffer.

Explanation / Answer

1)

Ka = 1 x 10^-14/1.8 x 10^-5 = 5.55 x 10^-10

pKa = 9.25

let x amount of NH4Cl is added

Using Hendersen-Hasselbalck equation,

pH = pKa + log([base]/[acid])

9.5 = 9.25 + log[(0.5 x 50 - x)/(x)]

1.78x = 25 - x

x = 9.00 mmol

Volume of 0.5 M NH4Cl to be added = 9.00/0.5 = 18.00 ml

2) For best results, the relationship between pKa and pH of buffer must be,

e. The pKa should be withing 1 pH unit of the desired pH of the buffer.

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