Homework Help: General Chemistry-Acids/Bases

Posted by Evelyn on Wednesday, November 26, 2008 at 2:18pm.

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!

General Chemistry-Acids/Bases - DrBob222, Wednesday, November 26, 2008 at 6:29pm
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.
- General Chemistry-Acids/Bases - Evelyn, Wednesday, November 26, 2008 at 9:02pm
  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?
General Chemistry-Acids/Bases - DrBob222, Wednesday, November 26, 2008 at 9:44pm
No. The H-H equation has no meaning for a strong acid/strong base because neither HNO3 nor NaOH has a Ka or Kb.

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