We are going to work the calculation of the pH of a buffer prepared by mixing a strong base with a weak acid. We want you to see all the steps of this type of problem in one place. 1. Compute the ph of the solution after the reaction of 42mL of 0.72M HCN(aq) with 3ml of 0.40 KOH.
HCN(aq) + KoH (aq) HoH KCN(aq)
1. Compute the moles of reactants before reaction:
Moles of acid moles of base
2. Moles of reactant and pH active product after reaction:
Excess reactant Product
Now convert moles to concentration units:
[Reactant] [Product]
Set up the ICE Table:
HCN H+ + CN-
Initial conc.
Charge +x -x +x
Equilibrium
Use the equilibrium conc. from your ICE table and make the appropriate approximation to fill in the table. Then solve for x and pH. Ka of HCN is 3.5 * 10-4. Now compute pH.