## Friday, April 22, 2011

### Titration Help! Ahhhhh- how do I do the titration lab?

Titration help handout:

The acid/base titration with NaOH and acetic acid is the oldest experiment in the book. I did it when I was in college (a long time ago) and both schools for which I work do the lab. I know they repeat it in higher level classes with a few more steps to make it more challenging. So it is to your advantage to try to master it at this level so you can handle more detail when they throw it at you.

Part I: Standardization of a base (NaOH)

Why would you standardize a solution? Generally it is because you need to precisely know the concentration of it.

What variables do you know in the first part? You know how many moles of acid that you started with. (KHP). You can calculate moles of acid in your starting material. You also know that the balanced chemical equation for the neutralization of KHP and NaOH reacts in a 1:1 mole ratio between acid and base.

KHP(aq) + NaOH(aq) → KNaP(aq) + H2O(l)

With your buret, you measure the volume of NaOH required to neutralize your KHP acid. You know the experiment is complete when you see your indicator turn a faint pink color. This indicates the pH of the titration is greater than 7 (slightly past the end point the pH can jump up sharply). The true equivalence between NaOH and KHP comes a few drops before you see pink in the solution but we approximate equivalence at the point the solution turns pink.

How do you calculate the concentration of the standard solution? You take moles of acid and set it equal to moles of NaOH. (You know this is true based on the balanced equation).

You can read the volume of NaOH required to complete the titration on your buret. This is the total mL (or Liters) you used to complete the titration.

Simply divide moles of base by liters of solution to get the concentration of the standard solution.

Generally you are told to pipette a certain amount of acetic acid in a flask. Let’s say you have a 5.00 mL calibrated pipette. You have 5.00 mL or 0.00500 L of acetic acid in your flask. (Notice I used three sig figs by adding two zeroes to the end of my liter value)

In this titration you now have a known concentration of NaOH from part A. (No, it is not 1:1 as I often see in my lab reports for this lab. It is the value you determined in part A). At this end of the titration, your acetic acid turns pink when moles of acetic acid is essentially equal to moles of base. How do you use this data to derive the concentration of your acid?

Let’s say you have a 0.2540 M concentration of NaOH from part A and you used 26.80 mL of your NaOH to titrate your acetic acid. You can determine moles of NaOH used for the titration by multiplying your concentration of NaOH by your volume of NaOH.

0.2540 moles/Liter NaOH X 0.02680 L NaOH= moles of NaOH (moles/Liter X Liters is always moles)

We know this is equal to moles of acetic acid by the balanced chemical equation.

What volume is this divided by to get the concentration? The original volume of acid you pipette into your flask- 5.00 mL or 0.00500 L.

## Saturday, April 02, 2011

### Triumph!!

It feels so good to have the check box for "excellent" on one's performance review. Yes, I'm going out with a bang!

I leave my job at the end of May to embark on motherhood. I was worried about finding a place to teach when I want to come back to it. Would all of my schools replace me with younger teachers, fresh out of school, brimming with enthusiasm?

Of course there are no guarantees in life but I feel reasonably assured that I have a place to teach come January when my newborn will be six months old and ready to spend a few hours a week with a babysitter.

Hurray! Bravo for that "excellent" check box on the evaluation form.