Question

Calculate the volumes of 0.200 M hydrocyanic acid, HCN (Ka = 4.9 x 10–10) solution, and 0.200 M of sodium cyanide, NaCN, solution needed to prepare 100.0 mL buffer solution with pH = 9.5

Answer 1

Concentration of acid HCN,[HCN] = 0.200 M Volume of the HCN = V1 mL Concentration of conjugate base, (NaCN] = 0.200 M Volume of the NaNO, = (100 - V) mL PK, of HCN=-log(4.9x10-1) = 9.31 pH of the solution = 9.5 According to Henderson equation, (NaCN] pH = pk + log [HCN] 9.5 = 9.31+ log!" (100-V)x0.200 M V x0.200 M (100-V1)x0.200 M log = 0.19 Vi x0.200 M (100-V1)x0.200 M - = 100.19 = 1.55 Vi *0.200 M (100-V1) - 1.55 (100-W) = 1.55V 2.557. = 100 ml = 39 mL Thus, volume of HCN needed = V1 = 39 mL volume of NaCN needed = (100-V) mL = (100-39) mL = 61 mL