[lmvent]

Titrating a solid unknown acid with 0.106 M NaOH:

Using bromothymol blue (pH 6-7.6):
0.052 g acid
4.65 mL NaOH

Using phenolphthalein (pH 8.2-10):
0.056 g acid
3.85 mL NaOH

Using methyl orange (pH 3.2-4.4):
0.052 g acid
3.36 mL NaOH

How do I calculate the equivalence weight of the acid.  This is what we are supposed to use to determine the identity of our acid.  One is 138 g and the other is 212 g.  Is it close enough to say that there are equal amounts of the acid and base using bromothymol blue?? If so, that gives me a mass of 105 g.  Is this close enough?

[Borek]

Pretty large errors.

Bromothymol blue titration seems to be completely wrong.

Calculate masses from the methyl orange and phenolophtalein titration.

Is it close enough to say that there are equal amounts of the acid and base using bromothymol blue?

No idea what you mean by that.

[lmvent]

okay...HOW do I calculate mass from the phenolphthalein and methyl orange though??

[mike]

One is 138 g and the other is 212 g

what does this mean? I don't understand the question. Are these titrations to different end points? Is this theoretical or is it an actual experiment you did? Is there any more information to go with this question?

[lmvent]

The molecular weight of one of the substances is 138g and the other is 212 g.  How do I figure out which substance it is.  All I know is the molecular weight and equivalent weight which are equal.  This was a real experiment and I need to figure out which acid my unknown was

[Borek]

All titration calulations are always done the same way - you start with a reaction equation and you assume that you know amount of titrant. Then it is simple stoichiometry - calculate moles, once you know moles and sample mass, you should have no problems with molar mass calculation.

[lmvent]

What I don't get is if the pH is say 8, are there equal amounts of the acid and the base?

[Borek]

Not exactly. pH of endpoint is either very close to 7 - when titrant and titrated substance are strong acids/bases, or is defined by hydrolysis - when acid or base titrated is weak. However, titration works in all these cases thanks to the fact that near endpoint pH changes very rapidly. If the indicator color change happens inside the area of this rapid change you will observe endpoint without problem.

Look at the three titration curves attached and think what kind of errors will you get using all three indicators in every case.

Also take a look at calculation of titration curves lecture.