CHEM 211 LAB FINAL QUESTIONS AND
ANSWERS WITH CORRECT SOLUTIONS
UPDATED VERSION
Which of the following is TRUE for an aqueous solution of the weak acid H3A? I H3A is
acidic only
II H2A- is acidic only
III HA2- is amphiprotic
IV A3- is basic only
A) I, III, and IV
B) I, II, and III
C) I, II, III, and IV
D) I, II, and IV
E) II, III, and IV - CORRECT ANSWES
What is the acid / base reaction used for this titration? (L2) - CORRECT ANSWES --
NaOH(aq) + C8H5O4K(s) ----> H2O(l) + Na+(aq) + C8H4O4K(aq)-
Identify whether the acids/bases in the reaction are strong or weak. (L2) - CORRECT
ANSWES -- 0.1 M NaOH is a strong base, KHP is the weak acid
What is the primary standard? (L2) - CORRECT ANSWES -- KHP is the primary
standard because it is what's used to determine the unknown concentration
Is there a secondary standard, if so, what is it? (L2) - CORRECT ANSWES -- 0.1 M
NaOH is the secondary standard because its exact volume is known throughout the
experiment
What is the indicator used? Is the indicator in its acid or base form at the start of the
titration? (L2) - CORRECT ANSWES -- The indicator used is phenolphthalein. It is a
weak acid at the beginning of the titration and its ion is bright pink and is what indicates
the endpoint of the titration.
Why is carbon dioxide a concern for these (and most) acid/base titrations? Will it impact
the Erlenmeyer flask more at the start or end of titration? The titrant before or after the
standardization? (L2) - CORRECT ANSWES -- Carbon dioxide from air can enter the
water solution in the Erlenmeyer flask and create carbonic acid, which would in turn
adversely affect the endpoint of the titration with addition of more acid.
, The procedure infers running 3 standardization titrations and then 3 unknown titrations,
could doing this in sequential order cause a systematic error? What would be a possible
outcome of that error? (L2) - CORRECT ANSWES -- Doing the experiment in
sequential order can cause a high precision but increase the possibility of systematic
error. If the equipment I use for my experiment is calibrated slightly wrong or has a
slight error consistently, my data may have little variance and be highly precise but not
very accurate. The true value may be higher or lower than that which I calculate. If the
buret is calibrated slightly wrong, has an air leak, or the balance may be calibrated
incorrectly.
What is the acid / base reaction for lake sample titration? (L3) - CORRECT ANSWES --
H2O + H2SO42- ---> H+ + HSO43-
H+ + HSO4- ---> H+ +SO42-
Is your lake water buffered with a mono or diprotic base? (L3) - CORRECT ANSWES --
Diprotic weak base because it was titrated with a strong acid
If you doubled the concentration of the H2SO4 solution, would the equivalence H2SO4
volume change? If so, how? (L3) - CORRECT ANSWES -- If the concentration
increased, the volume would decrease because the endpoint would be reached faster.
Would the alkalinity change? If so, how? (L3) - CORRECT ANSWES -- No, the alkalinity
would not change because no matter how much acid is added, the alkalinity level of the
water remains constant.
What is the lake water acidity, alkalinity? (L3) - CORRECT ANSWES -- The lake water
acidity is the amount of hydrogen ions that are in the water. The pH of the water should
be pH of 6.5 to - 8.5. The alkalinity is the ability of the water to resist change in acidity.
The alkalinity of the water was 127 ppm. In recorded years it has been 24-127.
Why is H2CO3 not stable in water? (L3) - CORRECT ANSWES -- In water H2CO3
creates H2O + CO2. The CO2 evaporates away and is considered not stable.
Why did the theoretical pH calculations in part B not match the required moles of weak
acid? (L3) - CORRECT ANSWES -- What was used was not pure, the Molarity it is
labeled with is not exact or precise. Could have been hydrated or ions in the tap water
couple contribute to the solution.
What is the purpose of the ionic strength adjuster? (L4) - CORRECT ANSWES -- (ISA)
the ionic strength adjuster is used to create a background ionic strength and therefore
make the results more reproducible and comparable.
What is the purpose of using two electrodes? (L4) - CORRECT ANSWES -- One is the
reference, and one is for the reading and create a more accurate measurement. The
potential difference between the electrodes is what's measured.
ANSWERS WITH CORRECT SOLUTIONS
UPDATED VERSION
Which of the following is TRUE for an aqueous solution of the weak acid H3A? I H3A is
acidic only
II H2A- is acidic only
III HA2- is amphiprotic
IV A3- is basic only
A) I, III, and IV
B) I, II, and III
C) I, II, III, and IV
D) I, II, and IV
E) II, III, and IV - CORRECT ANSWES
What is the acid / base reaction used for this titration? (L2) - CORRECT ANSWES --
NaOH(aq) + C8H5O4K(s) ----> H2O(l) + Na+(aq) + C8H4O4K(aq)-
Identify whether the acids/bases in the reaction are strong or weak. (L2) - CORRECT
ANSWES -- 0.1 M NaOH is a strong base, KHP is the weak acid
What is the primary standard? (L2) - CORRECT ANSWES -- KHP is the primary
standard because it is what's used to determine the unknown concentration
Is there a secondary standard, if so, what is it? (L2) - CORRECT ANSWES -- 0.1 M
NaOH is the secondary standard because its exact volume is known throughout the
experiment
What is the indicator used? Is the indicator in its acid or base form at the start of the
titration? (L2) - CORRECT ANSWES -- The indicator used is phenolphthalein. It is a
weak acid at the beginning of the titration and its ion is bright pink and is what indicates
the endpoint of the titration.
Why is carbon dioxide a concern for these (and most) acid/base titrations? Will it impact
the Erlenmeyer flask more at the start or end of titration? The titrant before or after the
standardization? (L2) - CORRECT ANSWES -- Carbon dioxide from air can enter the
water solution in the Erlenmeyer flask and create carbonic acid, which would in turn
adversely affect the endpoint of the titration with addition of more acid.
, The procedure infers running 3 standardization titrations and then 3 unknown titrations,
could doing this in sequential order cause a systematic error? What would be a possible
outcome of that error? (L2) - CORRECT ANSWES -- Doing the experiment in
sequential order can cause a high precision but increase the possibility of systematic
error. If the equipment I use for my experiment is calibrated slightly wrong or has a
slight error consistently, my data may have little variance and be highly precise but not
very accurate. The true value may be higher or lower than that which I calculate. If the
buret is calibrated slightly wrong, has an air leak, or the balance may be calibrated
incorrectly.
What is the acid / base reaction for lake sample titration? (L3) - CORRECT ANSWES --
H2O + H2SO42- ---> H+ + HSO43-
H+ + HSO4- ---> H+ +SO42-
Is your lake water buffered with a mono or diprotic base? (L3) - CORRECT ANSWES --
Diprotic weak base because it was titrated with a strong acid
If you doubled the concentration of the H2SO4 solution, would the equivalence H2SO4
volume change? If so, how? (L3) - CORRECT ANSWES -- If the concentration
increased, the volume would decrease because the endpoint would be reached faster.
Would the alkalinity change? If so, how? (L3) - CORRECT ANSWES -- No, the alkalinity
would not change because no matter how much acid is added, the alkalinity level of the
water remains constant.
What is the lake water acidity, alkalinity? (L3) - CORRECT ANSWES -- The lake water
acidity is the amount of hydrogen ions that are in the water. The pH of the water should
be pH of 6.5 to - 8.5. The alkalinity is the ability of the water to resist change in acidity.
The alkalinity of the water was 127 ppm. In recorded years it has been 24-127.
Why is H2CO3 not stable in water? (L3) - CORRECT ANSWES -- In water H2CO3
creates H2O + CO2. The CO2 evaporates away and is considered not stable.
Why did the theoretical pH calculations in part B not match the required moles of weak
acid? (L3) - CORRECT ANSWES -- What was used was not pure, the Molarity it is
labeled with is not exact or precise. Could have been hydrated or ions in the tap water
couple contribute to the solution.
What is the purpose of the ionic strength adjuster? (L4) - CORRECT ANSWES -- (ISA)
the ionic strength adjuster is used to create a background ionic strength and therefore
make the results more reproducible and comparable.
What is the purpose of using two electrodes? (L4) - CORRECT ANSWES -- One is the
reference, and one is for the reading and create a more accurate measurement. The
potential difference between the electrodes is what's measured.
