Question

A sample of unknown acid is dissolved into 100-mL of water (assume negligible volume change) and divided into two equal volumes. One flask, labeled as flask “a”, is titrated with 15.00ml to end point with 0.20 M NaOH. The second flask, labeled as flask “b”, is not titrated. Which of the following is false?

a)Flask “a” contains some conjugate base of the acid before titration

b) The concentration of A- in the titrated flask “a” is equal to the concentration of HA in the untitrated flask “b”

c) The number of moles of A- in the titrated flask “a” is not equal to the number of moles of A- in the untitrated flask “b”

d) With the information provided, it is possible to determine the exact concentration of HA

Please also EXPLAIN your answer! This question has been asked on Chegg over three times so far and EACH time it has a DIFFERENT answer.

Answer 1

Ans. Note that the identity of acid (weak or strong) is not specified.

#Case 1: When the acid is strong:

Option A. False. Strong acid undergoes complete dissociation, there would be “All” conjugate base (A^-) but no HA.

Option B. False. Since acid is strong, all the acid is in form of A^- but no HA – irrespective of titration with NaOH. So, [A^-] is NOT equal to HA because there is no HA in either flask.

Option C. False. [A^-] in both flasks would be equal to each other irrespective of NaOH titration.

Option D. True. Assuming number of moles of NaOH = number of moles of acid at equivalence point, the calculation can be done to determine exact initial [AH].

# Case 2: When the acid is weak:

Option A. True. Due to partial dissociation of a weak acid, there would be “SOME” conjugate base (A^-) as well as HA initially.

Option B. False. In flask a, all acid would be converted into A^- after titration with strong base. In un-titrated flask, there would be some A^- and some AH. So, [A^-] in flask a would not be equal to [AH] in flask b.

Option C. True. There would be only A^- in flask a but some of both A^- and AH in flask b. So, [A^-] in flask a not equal to [A^-] in flask b.

Option D. True. Assuming number of moles of NaOH = number of moles of acid at equivalence point, the calculation can be done to determine exact initial [AH].

# Conclusion:

Compare case I and 2: There are 3 false statements in case 1 whereas only one in case 2.

Therefore, the acid is a weak acid and the options are with respect to a weak acid.

Therefore, the correct option is- option B.

Note: The difference in answers would surely appear because the identity of acid, weather it’s weak or strong, would surely arise as explained in case I and 2.