Question

. What A chemistry graduate student is given 450. mL of a 0.60 M hydrocyanic acid (HCN) solution. Hydrocyanic acid is a weak acid with K =4.9 10 mass of KCN should the student dissolve in the HCN solution to turn it into a buffer with pH = 9.14? 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. x 6 ?

Answer 1

For, Henderson-Hasselbalch equation for buffer

pH = pKa + log (moles of KCN/moles of HCN)

pKa = - logKa = - log (4.9×10-10) = 9.31

Given , volume of HCN solution

= 450 mL = (450/1000) L = 0.450 L

Now moles of HCN = molarity × volume

= 0.60 × 0.45

= 0.27 .

Let moles of KCN = x .

Now putting the given values

9.14 = 9.31 + log (x/0.27)

or, log (x/0.27) = 9.14 - 9.31

Or, log (x/0.27) = - 0.17

Or, (x/0.27) = 10(-0.17)

Or, (x/0.27) = 0.68

Or, x = (0.27× 0.68)

Or, x = 0.18

Now, mass of KCN need to dissolve

= moles × molar mass

= 0.18 (mol) × 65 (g/mol)

= 12 g. (2 significant figures)