Question

I need to find the equivalence weight of a substance with a COOH group.

I weighed out 0.150 grams of the substance and dissolved it in 30 mL ethanol.

I titrated with 0.0999 M NaOH and used phenolptalein as an indicator.

The color completely shifted form clear to a purple-reddish at 9.8 mL 0.0999 M NaOH added.

I know that the substance only has one COOH group.

The question is how i find the equivalence weight?

Answer 1

Place a known amount of purified compound (in which you wish to determine the number of carboxyl groups; Dear Abhi I think this is your question) in a conical flask, dissolve in some ethanol and dilute with deionized water so that precipitation does not occur. Add few drops of phenolphthalein indicator and titrate with standardized NaOH until a faint pink colour appears. Note the titre.
Now you have:
w (in g), weight of compount
V (in mL), titre value of NaOH in experiment
M, molarity of NaOH (molarity and normality of NaOH are the same)
Mol Wt, molecular weight of the compound
Compute by substituting values in equation:
Number of COOH groups = VM (Mol Wt)/1000 w
Hint: w (g) of compound required VM of NaOH (VM = mL of 1M NaOH), so what volume (mL) of 1M NaOH the Mol Wt will require. Dividing it by 1000 converts it into L (litres) of 1M NaOH. Since each COOH group requires one NaOH (one mol wt per litre, 1M), the result equals the number of COOH group in the compound.