Calculate the concentration of [H⁺] of a buffered solution containing 5.0 × 10⁻⁴ M HCN (K_a = 6.2 × 10⁻¹⁰) and 1.5 × 10⁻⁴ M NaCN. A) 3.06 × 10⁻⁹ M B) 0.22 × 10⁻⁹ M C) 2.07 × 10⁻⁹ M D) 4.12 × 10⁻⁹ M E) 4.58 × 10⁻⁹ M

Question

Quizplus · Accepted Answer

First, we need to write the balanced equation for HCN dissociation:
HCN + H2O ⇌ H3O+ + CN-
Then we can write the equilibrium constant expression:
Ka = [H3O+][CN-]/[HCN]
We can use the Henderson-Hasselbalch equation to find the concentration of CN- and HCN:
pKa = -logKa = 9.21
pH = pKa + log([CN-]/[HCN])
We know that the total concentration of CN- and HCN is 1.5 × 10-4 M + 5.0 × 10-4 M = 6.5 × 10-4 M. We can use the above equation and solve for [CN-]/[HCN]:
10^(pH - pKa) = [CN-]/[HCN]
10^(7.4 - 9.21) = [CN-]/[HCN]
[CNS1S1P0]/[HCN] = 0.207
Now we can use the fact that the total concentration of [HS1U1P1+S1S1P0] is equal to the concentration of HCN that has dissociated:
[HS1U1P1+S1S1P0] = [CN-] = 0.207*[HCN] = 0.207*5.0 × 10-4 M = 1.035 × 10-4 M
Therefore, the concentration of [HS1U1P1+S1S1P0] is 2.07 × 10-9 M.

Calculate the Concentration of [H+] of a Buffered Solution Containing

Calculate the Concentration of [H⁺] of a Buffered Solution Containing