Help needed on a homework problem -
solution of HCN and CN- , Ka 6.2x10^-10 . Is this a buffered solution? Shouldn't the pH be greater than 7, and [OH] >[H]?
Does adding a strong acid have more effect than adding a strong base?
Thanks.
To determine if the solution of HCN and CN- is buffered, we need to consider the concentration of the acid and its conjugate base, as well as the pH of the solution.
A buffered solution is one that resists changes in pH when small amounts of acid or base are added. It contains a weak acid and its conjugate base or a weak base and its conjugate acid.
In this case, HCN is a weak acid and CN- is its conjugate base. The Ka value of HCN is given as 6.2x10^-10. Since Ka represents the acid dissociation constant, a smaller Ka value indicates a weaker acid. Therefore, HCN is a weak acid.
Now, let's address your concern about the pH of the solution. In a buffered solution, the pH is typically controlled by the weak acid and its conjugate base. If the concentrations of the weak acid and its conjugate base are similar, the pH of the solution can be maintained within a certain range.
Regarding your assumption that the pH should be greater than 7 and [OH] should be greater than [H], it is not always the case. The pH scale ranges from 0 to 14, with pH 7 considered neutral. If the pH is less than 7, the solution is considered acidic, and if it is greater than 7, the solution is considered basic (or alkaline).
In the case of a buffered solution, the pH can be either acidic or basic, depending on the specific weak acid and its conjugate base used. So, the pH could be less than 7 in this case. However, the concentration of [OH-] is not related to the pH of the solution. Higher [OH-] concentration indicates a basic solution.
Lastly, let's address your question regarding the effect of adding a strong acid versus a strong base to the solution. In general, adding a strong acid or base to a buffered solution will have different effects.
When a strong acid is added to a buffered solution, it will react with the weak base (conjugate base of the weak acid) in the solution, forming the weak acid and its conjugate acid. This reaction will absorb some of the added acid, minimizing the change in pH.
On the other hand, when a strong base is added to a buffered solution, it will react with the weak acid in the solution, forming the conjugate base of the weak acid and a strong base. This reaction will consume some of the added base, preventing a significant change in pH.
In summary, the solution of HCN and CN- can be considered a buffered solution if the concentrations of the weak acid and its conjugate base are similar. The pH of the solution can be either acidic or basic, depending on the specific weak acid used. Adding a strong acid or base to a buffered solution will have different effects, depending on the reactants involved.