You measure the pH of a 0.50 M unknown acid solution using a pH meter and it is found to be 1.74. What
is Ka for the acid?
I know ph=-log{H+}
Where do I go from here?
Do you know it is a weak acid?
.......HA ==> H^+ + A^-
I....,0.5.....0.....0
C......-x.....x.....x
E.....0.5-x...x.....x
You know pH so you know x = (H^+) and A^-. Plug these into the Ka xpression and solve for Ka after evaluating 0.5-x.
To find the Ka (acid dissociation constant) for the unknown acid, we can use the equation for the dissociation of the acid:
HA ⇌ H+ + A-
The equilibrium constant expression for this reaction can be written as:
Ka = [H+][A-] / [HA]
Given that the pH of the unknown acid solution is 1.74, we can determine the concentration of H+ ions using the equation:
[H+] = 10^(-pH)
So, [H+] = 10^(-1.74)
Now, since the unknown acid is a monoprotic acid, the initial concentration of the acid is equal to the concentration of H+ ions:
[HA] = [H+] = 10^(-1.74)
Substituting the values into the equilibrium constant expression, we have:
Ka = [H+][A-] / [HA] = [H+][H+] / [H+] = [H+]
Therefore, the Ka for the acid is equal to the concentration of H+ ions, which is 10^(-1.74).
To find the Ka (acid dissociation constant) for the unknown acid, you can use the pH value and the concentration of the acid solution. The pH of a strong acid is directly related to the concentration of the H+ ions it produces in water. However, for weak acids like the one in question, there is an equilibrium between the undissociated acid and its dissociated form (H+ and the conjugate base).
The equation for the dissociation of the acid is:
HA ⇌ H+ + A-
The Ka expression for this reaction is:
Ka = [H+][A-] / [HA]
Given that the pH of the solution is 1.74, you can use the pH to calculate the concentration of H+ ions:
[H+] = 10^(-pH)
[H+] = 10^(-1.74)
Now, since the acid's concentration is given as 0.50 M, you know that the concentration of the undissociated acid ([HA]) is 0.50 M. Assuming complete dissociation, the concentrations of H+ ([H+]) and A- ([A-]) are also 0.50 M.
Plugging in these values into the Ka expression:
Ka = (0.50 M)(0.50 M) / (0.50 M)
Simplifying the expression, you get:
Ka = 0.50
Therefore, the Ka value for the unknown acid is 0.50.