# Homework Help: Chemistry help :]

1. Jun 24, 2008

### rmarsino

1. The problem statement, all variables and given/known data

What is the pH after 12.50 mL of 0.10 M NaOH has been added to 25.00 mL of 0.10 M acetic acid?

2. Relevant equations

3. The attempt at a solution
i have no idea how to do this.

2. Jun 24, 2008

### symbolipoint

The concentration of each starting solution is the same. Acetic acid and sodium hydroxide react in a 1:1 mole ratio. One of those will be in excess when the quantities of solutions are mixed. Which one, and what concentration will it be?

3. Jun 25, 2008

### Staff: Mentor

Once you know what buffer is, and once you realize what is molar ratio of both reactants, you don't need any calculations to find out the answer.

&

4. Jun 25, 2008

### symbolipoint

The exact truth would be a little different from that. Note that the solution will have unreacted acetic acid. 37.50 ml. of entire solution containing 0.0125*0.10 moles of unneutralized acetic acid. The salt present should likely not have much effect on pH. I'm assuming that there is enough acid present to overcome any effect of the salt, but I can't be sure unless I actually perform the analytical calculation-------- wait! Maybe this really IS a buffer solution, being half the moles are the salt and half the moles are the acetic acid. There is a simple way to find pH of this buffer using pK value, right?

5. Jun 25, 2008

### Staff: Mentor

Bingo Hopefully rmarsino will get to this point as well.

6. Jun 25, 2008

### symbolipoint

In post #5, I began writing using only half my mind. The concept caught up with me as I wrote and figured. rmarsino, when you study equilibrium of weak acids and weak bases, you will learn about buffers. You will also learn how to find pH of solutions containing weak bases, weak acids, and weak acids or bases and salts of these.

I actually learned this stuff best when I was NOT enrolled in a course to learn it; but while I was reviewing on my own. For monoprotic or monofunctional compounds, this stuff is full of details, but not necessarily difficult. Learning it well takes time, patience, and much exercise and repetition.

7. Jun 25, 2008

### epenguin

These things are always done the same way from three-and-a-half principles, stated in

https://www.physicsforums.com/showpost.php?p=1754973&postcount=3

You say you have no idea how to do. In general start by writing all the chemical species present. Then write what you know about their concentrations...

It is true that in this particular case there is a (instructive) shortcut as pointed out by symbolipoint.