I took 25 mL of an unknown weak acid and added it to 10 mL of NaOH solution. I measured the pH and got 2.88 with concentration of NaOH @ .0098 and weak acid at 0.0102. What is pKa for the acid?

pH = pKa + log (base)/(acid)

Set up an ICE chart and use the above equation.
25.0 x 0.0102 = 0.255 mols
10 x 0.0098 0.098 mols
.........HA + OH^- ==> H2O + A^-
I......0.255.....0........0.....0
added.........0.098...........
C....-0.098.-0.098......0.098
E ....0.157...0..........0.098...

Post your work if you get stuck.

To calculate the pKa for the weak acid, we can use the Henderson-Hasselbalch equation. The equation is as follows:

pH = pKa + log([A-]/[HA])

where pH is the measured pH, pKa is the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.

In this case, the weak acid reacts with NaOH to form the conjugate base. So, we need to calculate the concentrations of the conjugate base and the weak acid.

First, let's calculate the moles of NaOH and the weak acid:

Moles of NaOH = volume (in L) x concentration (in mol/L)
Moles of NaOH = 10 mL / 1000 mL/L x 0.0098 mol/L = 0.000098 mol

Moles of weak acid = volume (in L) x concentration (in mol/L)
Moles of weak acid = 25 mL / 1000 mL/L x 0.0102 mol/L = 0.000255 mol

The reaction between NaOH and the weak acid is a 1:1 ratio, so the moles of the weak acid that reacted with NaOH are equal to the moles of NaOH.

Now, let's calculate the concentrations of the conjugate base and the weak acid:

Concentration of [A-] = moles of NaOH / total volume (in L)
Concentration of [A-] = 0.000098 mol / (10 mL + 25 mL) / 1000 mL/L = 0.00137 mol/L

Concentration of [HA] = (initial moles of weak acid - moles of weak acid reacted with NaOH) / total volume (in L)
Concentration of [HA] = (0.000255 mol - 0.000098 mol) / (10 mL + 25 mL) / 1000 mL/L = 0.004534 mol/L

Now, substitute these values into the Henderson-Hasselbalch equation:

2.88 = pKa + log(0.00137 / 0.004534)

Rearrange the equation to solve for pKa:

pKa = pH - log(0.00137 / 0.004534) = 2.88 - log(0.00137 / 0.004534)

Using a calculator, solve for pKa:

pKa = 2.88 - (-0.845) = 2.88 + 0.845 = 3.725

Therefore, the pKa for the unknown weak acid is approximately 3.725.

To determine the pKa of the weak acid, we can use the Henderson-Hasselbalch equation, which relates the pH, pKa, and the concentrations of the acid and its conjugate base:

pH = pKa + log([conjugate base]/[acid])

In this case, the acid is the weak acid and the conjugate base is the NaOH solution, which is a strong base. However, we need to convert the concentration of NaOH to its molar concentration (M).

To calculate the molar concentration of NaOH, we can use the formula:

Molarity (M) = moles/volume (L)

Given that the volume of the NaOH solution is 10 mL (0.01 L) and the concentration of NaOH is 0.0098 M, we can calculate the moles of NaOH as follows:

moles NaOH = 0.0098 M * 0.01 L = 0.000098 moles

Now let's calculate the molar concentration of the weak acid:

Molarity of weak acid = 0.0102 M

Now we can substitute the values into the Henderson-Hasselbalch equation:

2.88 = pKa + log(0.000098/0.0102)

By rearranging the equation, we can solve for pKa:

pKa = 2.88 - log(0.000098/0.0102)

Using a calculator, we can find that the pKa for the weak acid is approximately 6.15.