For 0.250 M of weak acid (NaHS) the Kb is given by 1.22 x10-13 , calculate the pH of the weak base.
The pH of NaHS is determined by the hydrolysis of the salt.
HS^- + HOH ==> H2S + OH^-
Set up an ICE chart
Kb = (H2S)(OH^-)/(HS^-)
Solve for (OH^-) and convert to PH. Post your work if you get stuck.
To calculate the pH of a weak base, you need to find the pOH of the solution first, and then convert it to pH using the formula pH = 14 - pOH. Here's the step-by-step process:
1. Start by writing down the chemical equation representing the ionization of NaHS:
NaHS ⇌ Na+ + HS-
2. Since it is given that NaHS is a weak acid, we can assume that it partially disacociates in water. Let's assume x is the concentration of HS- ions that will form. Therefore, the concentration of Na+ ions will also be x.
3. Use the Kb value to set up an equation to find x, the concentration of HS- ions. The Kb expression for the reaction is:
Kb = [Na+][HS-] / [NaHS] = (x * x) / (0.250 - x)
4. Plug in the given Kb value and solve for x:
1.22 x 10^-13 = x^2 / (0.250 - x)
5. Simplify the equation by multiplying both sides by (0.250 - x) and rearrange:
(0.250 - x) * 1.22 x 10^-13 = x^2
3.05 x 10^-14 - 1.22 x 10^-13x = x^2
6. Rearrange the equation to set it equal to zero:
x^2 + 1.22 x 10^-13x - 3.05 x 10^-14 = 0
7. This is a quadratic equation. You can solve it using the quadratic formula:
x = [-b ± √(b^2 - 4ac)] / 2a
In this case, a = 1, b = 1.22 x 10^-13, and c = -3.05 x 10^-14.
8. Plug in the values in the quadratic formula and solve for x. You will get two values, but only the positive value is meaningful as concentration can't be negative.
9. Once you find the value of x, you can calculate the concentration of OH- ions by subtracting x from the initial concentration of NaHS. In this case, it would be 0.250 - x.
10. Calculate the pOH of the solution using the formula pOH = -log[OH-]. Take the negative logarithm of the concentration of OH- ions.
11. Finally, calculate the pH of the solution using the formula pH = 14 - pOH.
By following these steps, you can calculate the pH of the weak base.