Titration of vinegar with NaOH?

Question

I did the lab that I put 25ml of vinegar to a beaker, and give bromothymol blue and I titrated it by adding NaOH drop by drop

until it turned into green

my results are 28.71ml and 28.67ml of NaOH

I don't know how to answer this question

1. the concentration of CH3COOH

2. The average concentration of CH3COOH

3. The pH

4.How many grams of acetic acid would we have to add to 500ml of vinegar to turn it into a 2.50M solution (use average concentration)

5. why is Bromothymol Blue used?

6. can these indicators be used in this lab, explain

a.methyl orange

b.phenol red

c.thymolphthahlein

thank you

Answer 1

The key to answering these questions is to know what the concentration of NaOH you used to titrate the vinegar, which should have been given to you from the outset.  Otherwise it is not possible to determine anything about the vinegar itself.  

For 5, BTB is used to see if your vinegar has been neutralized.  BTB is a weak acid with a pKa of 7.10.  When BTB is protonated (in acidic environments below pH=7.10), it reflects yellow light; when it is deprotonated (in basic environments above pH = 7.10) it reflects blue light.  When the pH of the environment is equal to the pKa of BTB, half of the BTB molecules will be protonated (yellow) while the other half will be deprotonated (blue), thus giving a green color.  When the BTB just starts looking like it's green, but not overtly green, the pH of the solution is just below the pKa of BTB, that is it's very near 7.0.

Since this lab is looking at neutralizing acetic acid, any indicator must show a color change near pH = 7.0, thus it's pKa must also be near 7.0.  The pKa of Methyl Orange is 4.2, phenol red's is 7.4, and thymolphthalein's is 9.9.  As such only pheol red could be used instead of BTB.