Question

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.

Answer 1

The final concentration of HCN after 0.020 mol of HCl is added is 0.180 M.

Step-by-step explanation:

When 0.020 mol of HCl is added to the buffer system, hydrocyanic acid (HCN) will react with the HCl to form more cyanide ions (CN-). The reaction is as follows:

HCl + HCN -> Cl- + CN-

Since the amount of HCl added is small compared to the initial concentrations of HCN and NaCN, we can assume that the concentration of NaCN remains essentially unchanged.

Therefore, we only need to calculate the change in the concentration of HCN.

First, calculate the initial concentration of HCN:

[HCN] = 0.200 M

Next, calculate the change in the concentration of HCN:

Change in [HCN] = -0.020 mol

Finally, calculate the final concentration of HCN:

[HCN] = 0.200 M - 0.020 mol / 1.00 L

= 0.180 M

Therefore, the final concentration of HCN after 0.020 mol of HCl is added is 0.180 M.