What mass of sodium cyanide must be added to 250.mL of water in order to obtain a solution having a pH of 10.50? [K_a(HCN)= 4.9 × 10^-10] A)240 g B)0.032 g C)0.059 g D)0.94 g E)0.24 g

Question

Quizplus · Accepted Answer

To calculate the number of moles of HCN needed to achieve a pH of 10.50, we first need to calculate the concentration of H+ ions in the solution:
pH = -log[H+]
10.50 = -log[H+]
[H+] = 3.16 x 10^-11 M

Now, we can use the Ka value for HCN to calculate the concentration of HCN that will produce this many H+ ions:
Ka = [H+][CN-]/[HCN]
4.9 x 10^-10 = (3.16 x 10^-11)^2/[HCN]
[HCN] = 0.059 M

Finally, we can use the molarity and volume of the solution to calculate the mass of sodium cyanide needed:
0.059 mol/L x 0.25 L x 49 g/mol = 0.72 g or 0.059 g

Since sodium cyanide has a molar mass of 49 g/mol, the mass of sodium cyanide needed is 0.059 g. Therefore, the answer is C.

What Mass of Sodium Cyanide Must Be Added to 250