Version C: Instructions
Midterm is worth 34 points, not including extra credit, which gives you up to 5 extra points.
RIGHT NOW- fill in your ID number, the bubbles and your version letter on the answer sheet, do this now so you don’t
If you fail to fill in the front page of your exam sheet, including
version letter, you will lose 5 points.
forget.
There are multiple versions of the exam. The short answers are mostly just rearranged or words such as “increased” are
replaced with “decrease”. The long answers have similar switches as well as changing the numbers around. This makes it
so that if you copy, you are most likely copying the wrong answer, this makes cheating even more pointless than
normal, please don’t do it.
If you think someone nearby may be copying, or something is simply disturbing you, please let us know and we can
adjust people’s seats appropriately. Tell us as early as possible.
You will also need to show your ID to the TA when you turn in your exam.
Only the answer sheet will be graded. You may (and should) keep the exam sheet. Go home, retake it, see
how you did . If you write any work on scrap paper, the question sheet or anywhere other than the space given, it will
not be graded and you will not get any credit for it. Do not write on the back of the sheets, do not write anything you
want graded anywhere other than the space given. I’d suggest writing in pencil or doing scratch work elsewhere and
then copying over.
Pace yourself, pay attention to point distribution and do your best to complete it all.
Exams will be sent through eee dropbox. They will be graded the same day as you take it but the scanning department
(who have nothing to do with me or the TAs) take a week to get them back to you. It will be returned to you via the
dropbox (note: not the gradebook, the dropbox) the moment that I get the email allowing me to do so.
Finally, relax. At that point there is nothing that being stressed will do for you. So take a deep breath, pick up your
writing utensil and show me how amazing you are at chemistry!
Long Answer:
Problem 1: Titrations: Suppose that 2.25g of an unknown monoprotic weak acid, HA, is dissolved in 1L water. After the
addition of 27.0 mL, the pH of the solution was found to be 3.92. To reach the equivalence point, 54.0mL of 0.450 M
NaOH was used.. a) State how many moles of acid were initially in solution b) what is the molar mass of the acid? c)
what is the value of pKa for the acid? d) What is the pH at the equivalence point (assume that auto ionization of water and
weak acid assumption is valid)?
0.0243 mol, 92.6 g/mol, 3.92, 8.14
Problem 2: Students working in a lab design titrations to test the purity of a weak acid.
Student X dissolves 2.0 g into 10. mL of water and titrates the solution with 1.0 M NaOH.
Student Y dissolves 4.0 g into 20. mL of water and titrates the solution with 1.0 M NaOH.
Student Z dissolves 4.0g into 20. mL of water and titrates the solution with 2.0 M NaOH.
Comment on the ratio of the volume of titrant required to reach the equivalence point. Explain your answer.
y=2x and z=x The volume of the water added to the analyte doesn’t matter, but we do need to pay attention to moles.
Student y adds double the moles of student x, but same concentration of NaOH so we need double the volume. Student Z,
while adding double the moles of student x, also double the concentration of titrant, so the volume of titrant stays the
same.
Problem 3: Above each atom put the oxidation number. Then balance the reactions.
For B) additionally State which is oxidized and which is reduced, decide if the cell is spontaneous and which is the
oxidizing agent and the reducing agent and write the cell diagram (cell notation).
A) IO3- + I-→ I2
6H++IO3- + 5I-→ 3I2 + 3H2O
B) Cr3+ + MnO2→ Mn2++ CrO422H2O +2Cr3+ + 3MnO2→ 3Mn2++ 2CrO42-+ 4H+
Pt(s)|Cr3+(aq), CrO42-(aq)||Mn2+(aq)|MnO2(s)
Cr3+ is oxidized and therefore is the reducing agent.
Mn is reduced, and therefore MnO2 is the oxidizing agent.
Yes it is spontaneous.
Problem 4: Use the picture of the galvanic cell below to answer the following questions:
If x half reaction involves the Au3+/Au half reaction and the y half reaction is
Cu2+/Cu:
Label each letter with the chemical species involved. Label the anode and
cathode. Which direction is the flow of electrons through the wire (right or left)?
A) Au3+ B) Au C)Cu2+ D)Cu
Cathode=x flow of the electrons is to the left.
What is the voltage of the cell?1.06 V
What is the equilibrium constant? (note: if your calculator has an overflow error simplify the equation as much as
possible and use that as an answer.) e247 or 2.98x10107
Problem 5: You have a solution that has HgNO3 and AuNO3. Explain how you would separate the silver and gold ions
using selective precipitation.
Useful Solubility Rules :
Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and
(Hg2)2+.
Add NaCl, (or any other halide salt), this will precipitate out the Ag, leaving the Au in solution. The solution and solid
can then be separated.
Short Answer:
The titration curve for the titration of 0.100 M
H2SO3(aq) with 0.100 M KOH(aq) is given below
7) When placed in an aqueous solution, which has more
dissolved ions.
a) Silver sulfate
b) calcium sulfate,
c) impossible to tell
d) neither have any dissolved ions, they are insoluble
8) Write the K equation for the dissolution of the
insoluble compound silver carbonate. ksp=[Ag]2[CO32-]
9) What is the solubility of silver carbonate? 1.27x10-4 M
What species are present after the addition of the
following? (don’t include spectator ions)
1) 25mL? H2SO4, HSO32) 100mL? SO323) Estimate pKa1. About 2
4) You are designing a titration to find the molecular
mass of an unknown weak acid. Which of the following
should you use as the titrant (aka, the standard solution
in your buret).
A) HCl
B) NaOH
C) NH3
D) CH3COOH
E) You can’t find the molecular mass of an unknown
base via titration.
5) Would the equivalence point of the weak acid titration
in above, have a pH of 7, less than 7, or greater than 7?
>7
6) For the titration shown below, which of the following
indicators would be the best. pKa of each indicator is
listed in parenthesis. Cresol Purple (2), Methyl orange
(3.5), Phenol Red (7.9), Alizarin (11.7)
10) What is the solubility of silver carbonate in 0.1M
sodium carbonate? 4.5x10-6M
11) Gout is a condition that results in joint swelling and
pain is caused by the formation of sodium urate
(NaC5H3N4O3) crystals within tendons, cartilage and
ligaments. Sodium urate precipitates out of blood plasma
when uric acid levels become abnormally high. This can
happen as a result of eating too many rich foods and
consuming too much alcohol, which is why gout is
sometimes referred to as the “disease of kings”. If the
sodium concentration in blood plasma is 0.140 M and
the Ksp for sodium urate is 5.76x10-8, what minimum
concentration of urate results in precipitation?
4.11x10-7M
12) If 10. mL of a saturated solution of NiCl2 (soluble) is
mixed with a 1.0 M solution of hydroxide, will there be
a precipitate?yes
13) If 10. mL solution of 2x10-3 M CaCl2 is mixed with a
10. mL solution of 2x10-3 M NaOH. Will there be a
precipitate. no
14) Purification of copper can be achieved by
electrorefining copper from an impure copper anode
onto a pure copper cathode in an electrolytic cell. How
many hours will it take to plate 19.0 kg of copper onto
the cathode if the current passed through the cell is held
constant at 33.0 A? Assume the copper in the electrolytic
solution is present as Cu2+. Free point