What molar ratio of sodium cyanide to hydrocyanic acid is necessary to prepare a buffer with...

Question

Question

What molar ratio of sodium cyanide to hydrocyanic acid is necessary to prepare a buffer with pH = 9.72? K_a of HCN = 4.9 x 10^-10

Answer 1

Answer #1

use the hander son equation

pH = pKa + log(CN-/HCN)

first convert Ka to Pka

pKa = -log(Ka) = -log(4.9 x 10^-10) = 9.31

pH is given as 9.72

now put all these values in the above equation

9.72 = 9.31 + log(CN-/HCN)

log(CN-/HCN) = 9.72 - 9.31

log(CN-/HCN) = 0.41

remove the log means

[CN-/HCN] = 10^0.41 = 2.57

Answer 2

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