Acid & Base Equilibrium?

Question

What is the pH of a buffer which is prepared with 0.15 M formic acid, HCOOH and 0.20 M sodium formate, Na+HCOO- ? The Ka of HCOOH is 3.74.

Answer : 4.04

So, a buffer is something that is used to reduce or increase the pH of a chemical substance. To find the pH of the buffer, I have to find out the amount of H3O+ in both formic acid and sodium formate and then added the H3O+ together to find the pH ? But I still don't really understand the way or method to solve this question. Any help will be much appreciated !

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If 25 mL of 0.75 M HCL is titrated with 100 mL of 0.25 M NAOH, what is the final pH at this point ?

Answer : 11.8

This is what I did :

NAOH + HCL => H20 + NACL

I calculated the no of mole for NAOH and HCL which is 0.025 mole and 0.01875 mole.

So, there should be an excess of OH- which is 6.25 x 10^ -3 ?

Then, M of OH- is 0.05 ?

And then, pOH = - log 0.05 = 1.3

So, pH = 14-1.3 = 12.6

And that does not match the answer given.

Is there anything that I did wrong ?

I know I am asking way too much question on this topic but I need to finish 70 questions for this topic and I have only completed about 35 questions and I do really need some help from you all to understand some of the questions. I am also trying to do the questions by my own. Please bear with me for a while. =D Thanks for all your help !

Answer 1

Clearly you have not mastered acid base equilibrium and you do not understand the subject.  Giving you answers will not enable you to pass the test.  The problems are designed to enable you to learn.

First understand acid equilibrium.  Then master the effect of a conjugate base and you will understand how buffers work - clearly you do not at this time.

If you cannot master this now, you will not master the exam.  

You need to study the material; we cannot reprint the text herein.

Sorry about the bluntness but no one can learn for you; you must do the job.