Determine the mass when given pH

What mass of sodium cyanide must be added to 250 mL of water at 25 degrees Celsius in order to obtain a solution having a pH of 10.50? (Ka(HCN)= 4.9*10^-10)

Set up your ICE table for the reaction

CN- + H2O HCN + OH-

Use the pH to calculate [H+] at equilibrium.

Use [H+] to find [OH-]

Use ka given to find kb for cyanide.

solve fo [CN-]

use concentration and volume to solve for moles of cyanide then convert to mass of NaCN.