What is the pH of a 2.62 x 10 -1 M HCN solution ? NB the pKa for HCN = 9.212.
4.90
9.21
0.58
11.32
13.42
-log(2.62x10^-1) = 0.58
4.90
To determine the pH of a solution, we can use the formula:
pH = pKa + log([A-]/[HA])
Where pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.
Since we have the pKa value for HCN, which is 9.212, we can substitute it into the formula:
pH = 9.212 + log([A-]/[HA])
Now, we can determine the concentration of HCN (the acid) and CN- (the conjugate base).
The HCN concentration is given as 2.62 x 10^-1 M.
To calculate the concentration of CN-, we need to use the equation for the dissociation of HCN:
HCN ⇌ H+ + CN-
From this equation, we can see that for every 1 mole of HCN that dissociates, 1 mole of CN- is formed. Therefore, the concentration of CN- is also 2.62 x 10^-1 M.
Now, substituting the values into the formula:
pH = 9.212 + log([2.62 x 10^-1]/[2.62 x 10^-1])
Simplifying:
pH = 9.212 + log(1/1)
Since the log(1/1) is equal to 0, the equation simplifies to:
pH = 9.212 + 0
Therefore, the pH of the 2.62 x 10^-1 M HCN solution is 9.212.
So, the correct answer is 9.21.
To find the pH of a solution, we need to use the formula:
pH = pKa + log([A-]/[HA])
Where pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.
In this case, the acid is HCN and the conjugate base is CN-. The given pKa for HCN is 9.212.
Since we are given the concentration of HCN (0.262 x 10^-1 M), we can assume that the concentration of CN- is negligible compared to HCN. Therefore, we can simplify the formula to:
pH = pKa + log([A-]/[HA])
≈ pKa + log(0/[HA])
Since [A-] is negligible, we can calculate the concentration of [HA]:
[HA] = 0.262 x 10^-1 M
Now we can plug in the values into the formula:
pH = 9.212 + log(0.262 x 10^-1/0.262 x 10^-1)
= 9.212 + log(1)
Since log(1) is equal to 0, the pH is equal to the pKa:
pH ≈ 9.212
Therefore, the pH of the 2.62 x 10^-1 M HCN solution is approximately 9.212.