Consider a buffer that is prepared from HCN and NaCN for example : HCN(aq) + H20

Question

Consider a buffer that is prepared from HCN and NaCN for example : HCN(aq) + H20(1) --------- CN- (aq) + H30+(aq)

A) explain why both HCN and CN- are necessary to prepare the buffer

B) Predict what would happen to the concenctration of HCN and CN- following the addititon of a small amount of acid

C) Given that Pka of HCN = 9.31, [HCN] = 0.200 M and [NaCN] = 0.100M, calculate the pH of this buffer

D) What will the Ph of the buffer be following a 1/10 dilution?

E) How will the buffer capacity of the buffer be affected following a 1/10 dilution?

Explanation / Answer

A) To prepare a buffer we need both a weak acid as HCN and a weak conjugate base as NaCN. So both are necessary for base preparation.

B) After the addition of a small amount of an acid, the concentration of HCN would increase whereas, the concentration of CN- would reduce.

C) Using Hendersen-Hasselbalck equation for buffer,

pH = pKa + log[NaCN]/[HCN]

     = 9.31 + log(0.1/0.2)

     = 9.01

D) pH of buffer after 1/10 dilution

would remain the same

E) Buffer capacity is directly related to concentration. thus the buffer capacity of the resulting buffer after 1/10 dilution would also reduce by a factor of 1/10.

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