Question

assumptions for titration of an unknown weak acid and strong base (NaOH)

We did a lab in chem and first standardized NaOH using KHP to find the actual concentration of NaOH. We then used that standardized stock to titrate our unknown acid for three trials. we calculated the molecular weight by assuming a 1:1 ratio between the acid and base. then we did a single trial of a titration of our unkown with the NaOH and used a pH probe to record the pH as we added more base. this titration was used to generate a titration curve and determine the equivalence point and average pka.

1) KHP was used to standardize NaOH but it was dried before being titrated. the assumption is that KHP had sufficiently dried. however if invalid, the mass of KHP would be artifically high. but what effect does that have on the molecular weight of the unknown acid?

2) (***)an assumption is that phenolpthalein is the right indicator for this titration (pKi is appropriate) but if thats not true, the endpoint would not reflect the equivalence point. when the pKi is both greater and less than the pH at equivalence point, what is the effect on molecular weight of the unknown acid?

if you you had to add more base that you needed to reach the endpoint bc the range of the indicator was too high (like your acid reaches eq pt at 6 and pheno. only ranges from 8-10), is the pki is greater or less than the pH at the endpoint??

Answer 1

If KHP is not dries properly, its measured weight will be artificially high. This means that when you dissolve in water the actual numner of moles dissolved (hence the molarity) will be low and amount of moles of NaOH required will be lower than the actual value. In the second step when you will use this NaOH to titrate an unknown acid, moles of acid to required will also be lower than expected. As a result the calculated Molecular weight of the acid will be more than its actual value.Because, M.Wt = Mass/moles. So, if moles of acid required is less, then calculated M.wt will be more.

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(2) if the equivalence point is reached earlier (pI is lower than pH at equivalence point), calculated M.Wt will be high. if the equivalence point is reached later (pI is greater than pH at equivalence point), calculated M.Wt will be low.

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pKI is greater than the pH at the end point