Consider 1.00 L of the buffer system that contains 0.200 M hydrocyanic acid(HCN) and 0.150 M sodium cyanide (NaCN). The pKa of hydrocyanic acid is 9.31.
What is the [HCN] after 0.020 mol of HCl is added?
Add 0.02 mol HCl to 1L means (H^+) = 0.02M
.......CN^- + H^+ ==> HCN
I....0.150....0.......0.2
add.........0.02.........
C...-0.02..-0.02......+0.02
E....0.130...0.......+0.22
(HCN) = 0.22M assuming the HCl was added as a gas
0.200m+0.20=0.22M correct
To find the concentration of hydrocyanic acid ([HCN]) after the addition of HCl, we need to consider the reaction that occurs.
HCl is a strong acid and will completely ionize in water, producing H+ ions. The reaction can be represented as:
HCl + H2O -> H3O+ + Cl-
Since HCN is a weak acid, it will partially ionize in water. The reaction can be represented as:
HCN + H2O ⇌ H3O+ + CN-
Given that the initial volume of the buffer system is 1.00 L and the initial concentration of HCN is 0.200 M, we can calculate the initial number of moles of HCN:
n(HCN) = [HCN] x V = 0.200 M x 1.00 L = 0.200 mol
After adding 0.020 mol of HCl, the total number of moles of HCN in the system becomes:
n(HCN) = initial moles of HCN - moles of HCN reacted
= 0.200 mol - 0.020 mol
= 0.180 mol
To find the new concentration of HCN ([HCN]), we divide the moles of HCN by the new volume of the system. Since there is no change in volume, we still have 1.00 L:
[HCN] = n(HCN) / V
= 0.180 mol / 1.00 L
= 0.180 M
Therefore, the concentration of HCN after 0.020 mol of HCl is added is 0.180 M.