Question

An approximately 0.1 M NaOH solution is prepared by adding 2.0 g of solid NaOH to

500 mL of water. The precise concentration of the solution is determined by titrating

the NaOH against weighed portions of a weak acid, pot

assium hydrogen phthalate

(KHP), obtained from the National Institute of Standards and Technology (NIST) and

certified as being 99.99% pure. The KHP samples are weighed by difference on an

analytical balance. Following its standardization, the NaOH solut

ion is used to

determine the concentration of acetic acid in a sample of commercial vinegar. How

would the calculated acetic acid concentration be affected (in comparison with the

true concentration) if each of the following three events were to occur? E

xplain your

answers.

a.)

[5 pts] The KHP was dried prior to weighing.

b.)

[5 pts] The balance was not properly zeroed, so it always reads exactly 1.00 mg

too low.

c.)

[5

pts] During the standardization of the NaOH solution, when the buret was filled

prior to each titration, an air bubble became trapped in the tip of the buret, but it

became dislodged during the titration

Answer 1

Titration and determination of unknown acetic acid concentration.

Effect of the following on measurement

(a) KHP was dried prior to weighing : This procedure removes all the adhered water present in KHP and the weight after that is the exact mass of KHP.

(b) If the balance was not prperly zeroed and read 1.00 mg too low. The concentration of all the NaOH and the KHP solutions would be lower than actual values. The final concentration of acetic acid thus calculated would be higher than actual value.

(c) Introduction of air bubble in the buret would give wrong reading for the titrant introduced. Thus higher volume would be recored and we would get lower value for acetic acid concentration.