Please let me know if my answer to the following question is correct.
Question:
What is the concentration of a weak base if its Kb = 1.4 x 10-11 and its pH = 8.75?
Answer:
Converting pH to H^+ concentration using the formula H^+ = 10^-ph, therefore 10^-8.75 gives an H^+ concentration value of 1.0 x 10^-9. Now, using the formula Ka=Kw/Kb and subsitituing in the values (Ka = 1.4 x10^-14/1.4 x 10^-11) gives 1.0 x 10^-3.
Now, subtracting 1.0 x 10^-3 from 1.0x 10^-9 gives a concentration of 9.9 x 10^-4 mol/L
No, I don't think so. First, pH = 8.75 gives (H^+) = 1.77 x 10^-9.
Then I would say (OH^-) = Kw/(H^+) = ??
Then BOH <==>B^+ + OH^-
Kb = (B^+)(OH^-)/(BOH) = 1.4 x 10^-11
You now have (B^+)=(OH^-) = 5.62 x 10^-6 and you can solve for (BOH).
rats....:)
Your answer is incorrect. Let me walk you through how to solve the problem correctly.
To find the concentration of a weak base, you need to use the formula relating the equilibrium constant for the base (Kb) and the concentration of its conjugate acid (HA) and hydroxide ions (OH-).
Kb = [OH-][HA]/[B]
In this question, we are given the value of Kb as 1.4 x 10^-11. The pH of the solution is also given as 8.75. To find the concentration of [OH-], we need to convert the pH value to [H+].
[H+] = 10^(-pH)
Substituting the given pH value, we get [H+] = 10^(-8.75).
Since the solution is basic, we know that [OH-] and [H+] are related by the equation [H+][OH-] = Kw.
Kw is the ionization constant of water, which is equal to 1 x 10^-14 at 25 degrees Celsius. So we can write the equation as [H+][OH-] = 1 x 10^-14.
Now, we can substitute the values we know:
(10^(-8.75))(OH-) = 1 x 10^-14
Rearranging the equation, we get:
[OH-] = (1 x 10^-14)/(10^(-8.75))
Calculating this expression, we get [OH-] = 1.78 x 10^-7 mol/L.
Finally, we can calculate the concentration of the weak base [B] using the formula:
[B] = [HA] = Kb/[OH-]
Substituting the given value for Kb and the calculated value for [OH-], we get:
[B] = (1.4 x 10^-11)/(1.78 x 10^-7)
Calculating this expression, we get [B] = 7.9 x 10^-5 mol/L.
So, the correct concentration of the weak base is 7.9 x 10^-5 mol/L.