Question

I need help with this question, please.

What A chemistry graduate student is given 100. mL of a 1.10 M hydrocyanic acid (HCN) solution. Hydrocyanic acid is a weak acid with Ka = - 10 =4.9 x 101 mass of KCN should the student dissolve in the HCN solution to turn it into a buffer with pH = 8.95? You may assume that the volume of the solution doesn't change when the KCN is dissolved in it. Be sure your answer has a unit symbol, and round it to 2 significant digits.

Answer 1

pKa of HCN= -log Ka= -log (4.9 X10^-10) = 9.31

According to Henderson equation

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

For given problem

8.95= 9.31 + log [KCN]/[1.1]

[KCN] = 0.48 M

Hence number of moles of KCN required = Molarity of KCN X volume of solution in L = 0.48 M X (100/1000) L = 0.048 moles

Amount of KCN required = No of moles required X mw of KCN= 0.048 mole X 65.12 g/mole = 3.1 g Answer