A 5.55g sample of a weak acid with Ka=1.3 x10^-4 was combined with 5.00 mL of 6.00 M NaOH and the resulting solution was diluted to 750 mL. The measured pH of the solution was 4.25. What is the molar mass of the weak acid?

..............HA + NaOH ==> NaA + H2O

..............x.....0........0.....0
add................30 mmols..........
change.......-30...-30.....+30.....+30
equil........x-30....0......+30....+30

pH = pKa + log (base)/(acid)
4.25 = 3.886 + log(30/(A)
Solve for A. I found 12.98; therefore, the value for x, the initial HA, is 12.98 + 30 = 42.98 mmoles or 0.04298 moles.
n = grams/molar mass. Solve for molar mass. You know g and n.

To find the molar mass of the weak acid, we need to use the given information about the acid's concentration, volume, and the measured pH of the resulting solution. Here are the steps to solve this problem:

Step 1: Calculate the concentration of the weak acid in the final solution.
- We need to determine the number of moles of NaOH that reacted with the weak acid. To do this, we can use the volume and concentration of NaOH.
- Moles of NaOH = (concentration of NaOH) x (volume of NaOH)
- Moles of NaOH = 6.00 mol/L x 0.005 L
- Moles of NaOH = 0.030 mol

- Since the reaction between NaOH and the weak acid occurs in a 1:1 ratio, the moles of weak acid in the final solution will also be 0.030 mol.

- Now, calculate the concentration of the weak acid in the final solution.
- Concentration of weak acid = (moles of weak acid) / (volume of solution)
- Concentration of weak acid = 0.030 mol / 0.750 L
- Concentration of weak acid = 0.040 mol/L

Step 2: Calculate the molarity of H+ ions in the solution.
- Since the weak acid is only partially ionized, we can assume that the concentration of the weak acid is approximately equal to the concentration of H+ ions produced.
- Molarity of H+ ions = Concentration of weak acid
- Molarity of H+ ions = 0.040 mol/L

Step 3: Convert the pH to H+ ion concentration, [H+].
- The pH is a measure of the negative logarithm of the H+ ion concentration.
- [H+] = 10^(-pH)
- [H+] = 10^(-4.25)
- [H+] = 5.01 x 10^(-5) mol/L

Step 4: Write the balanced equation for the ionization of the weak acid.
- Since the weak acid is monoprotic (donates one H+ ion), the balanced equation will be:
- Weak acid (HA) + H2O ⇌ H3O+ + A-

Step 5: Set up the expression for the ionization constant (Ka) of the weak acid.
- Ka = [H3O+][A-] / [HA]
- Since the initial concentration of HA equals the concentration of H+ ions, we can substitute it into the equation.
- Ka = [H+]^2 / [HA]
- Ka = (5.01 x 10^(-5))^2 / 0.040

Step 6: Solve for the molar mass of the weak acid.
- We are given the value of Ka = 1.3 x 10^(-4).
- Solve the equation Ka = (5.01 x 10^(-5))^2 / Molar mass of weak acid for the molar mass of the weak acid.
- Molar mass of weak acid = (5.01 x 10^(-5))^2 / Ka

After performing the calculations, the molar mass of the weak acid will be determined.