I'm stuck on a couple of similar problems involving pH and acids/bases

1) What is the final pH of a 1L solution that is initially .15 M HF and .2 M NaF after 10 mL of .02 M HCl has been added? Ka=7.2x10^-4

Do i just calculate the total mols of H and divide by the volume, then find the pH?

2.) Calculate the volume of .030 M HNO3 that must be added to a 100mL solution of .25 M NaOh to give a pH of 8.5

This question deals with a strong acid/base, so would I do the same procedure as I would do for number 1? Or would I use a different approach?

Thanks!

The first problem is a buffer problem. HF is a weak acid and NaF is the salt of the weak acid. Use the Henderson-Hasselbalch equation.

The second problem is a strong acid/strong base problem. There are no ionization constants involved and you can go from molarities to H^+ and pH.
Post your work if you get stuck.

Thanks! I figured out how to do the buffer problem. Can I use the Henderson-Hasselbalch equation for the strong acid/base problem with 0 for the pKa?

No. The H-H equation has no meaning for a strong acid/strong base because neither HNO3 nor NaOH has a Ka or Kb.

1) To find the final pH of the solution, you need to consider the dissociation of HF (hydrofluoric acid) and the reaction with HCl.

First, calculate the moles of HF initially present in the 1 liter solution:
moles of HF = concentration (M) × volume (L) = 0.15 M × 1 L = 0.15 moles

Then calculate the moles of NaF initially present in the 1 liter solution:
moles of NaF = concentration (M) × volume (L) = 0.2 M × 1 L = 0.2 moles

Since HF is a weak acid, it partially dissociates according to the equation:
HF ⇌ H+ + F-

The Ka value provided (7.2x10^-4) can be used to calculate the equilibrium concentrations of H+ and F- ions.

Next, you need to account for the reaction between HF and HCl. The equation for that reaction is:
HF + HCl → H2O + Cl- + F-

Since HF is in excess compared to HCl, only HF reacts completely. The moles of HF remaining after the reaction can be calculated by subtracting the moles of HCl reacted:
moles of HCl reacted = concentration (M) × volume (L) = 0.02 M × 0.01 L (10 mL = 0.01 L) = 0.0002 moles
moles of HF remaining = initial moles of HF - moles of HCl reacted = 0.15 moles - 0.0002 moles

Now, you can calculate the concentration of HF and F- ions:
concentration of HF = moles of HF remaining / volume (L) = moles of HF remaining / 1 L
concentration of F- = concentration of NaF + concentration of HF

Finally, you can use the concentrations of H+ and F- ions to calculate the pH of the solution using the equation:
pH = -log[H+]

2) In this problem, you need to find the volume of HNO3 (nitric acid) required to reach a specific pH.

Since HNO3 is a strong acid, it fully dissociates into H+ and NO3- ions in solution. Therefore, you can assume that the concentration of H+ is equal to the concentration of HNO3.

To find the volume of HNO3 required, you can use the Henderson-Hasselbalch equation:
pH = pKa + log ([A-]/[HA])

Here, [A-] represents the concentration of the base (OH-) and [HA] represents the concentration of the acid (HNO3). You know the pH (8.5) and the concentration of the base (NaOH), so you can solve for the concentration of the acid (HNO3).

Once you have the concentration of HNO3, you can calculate the moles of HNO3 present in the 100 mL solution. Then, using the molarity (0.03 M) of the HNO3 solution, calculate the volume required to reach the desired moles.

Remember to convert mL to L if needed in your calculations.

I hope this helps! Let me know if you have any further questions.