Lab: Synthesis of Zinc Iodide – then Determining its % Composition by Mass
H
2
O
1 Zn (s) + 1 I
2
(s)  1 ZnI
2
(aq)
Your Goals:
-to synthesize zinc iodide,
-to record mass measurements in order to calculate the actual % composition by mass of your product (ZnI
2
). “Actual % composition” means “from your own data” (the measured or calculated mass of Zn from lab data and the measured or calculated mass of I
2
from lab data).
-to calculate the theoretical % composition by mass of ZnI
2
. “Theoretical % composition” means, from the established well known chemical formula and the molar masses of its elements (from the periodic table).
-to calculate % error for your synthesis reaction.
Gather Supplies:
Stored in station drawer: metal spatula, ring
Stored in glassware cabinet: 10-mL graduated cylinder, 125-mL Erlenmeyer flask, 250-mL Erlenmeyer flask, glass funnel, small beaker, distilled water squirt bottle
Teacher’s table or desk: 1 circle of filter paper (must be kept dry until its mass is obtained), zinc powder, iodine.
Other: stand (for ring), balance, plastic cup (re-useable)
Procedure:
1.
Place the empty plastic cup on the balance. Tare (zero) the balance (while the empty cup is on the balance).
Wipe the metal spatula clean with a paper towel (a good habit to get into). Use the metal spatula to transfer between 1.80 – 2.50 grams of zinc powder into the cup (while it is still on the balance). Record the exact mass of zinc here: ________________
2.
Remove the cup with zinc from the balance. Bend the cup to form a “spout” and carefully pour all of the powder into the 125-mL empty Erlenmeyer flask. If you spill any, you will need to try again (the exact mass must be known).
3.
Use the graduated cylinder to measure approximate 10 mL of distilled water (from the squirt bottle). Pour this water into the 125-mL Erlenmeyer with the sample of zinc. Swirl briefly to disperse the zinc. Set aside.
4.
Measure the iodine: Place the empty cup back on the balance. Tare (zero) the balance (while the empty cup is
on the balance). Wipe the metal spatula clean with a paper towel. Use the spatula to transfer an amount of iodine equivalent to about half of the mass of zinc powder that you measured previously. [So, if you measured
2.50 grams of zinc previously, now you would try to measure about 1.25 grams of iodine.] You do not need to get exactly half; although you do need to know exactly what you use. Record mass of iodine:________________
5.
Bend the cup in half and carefully add the iodine to the zinc/H
2
O mixture in the 125-mL Erlenmeyer flask. Swirl until all the iodine crystals have reacted (completely disappeared). Then allow the flask to remain on the counter top for another 10+ minutes as the reaction continues, swirling occasionally. After 10 - 15 minutes, you will have synthesized zinc iodide, and all of the original elemental I
2
will be completely used up. BUT, since you measured out excess zinc (on purpose), the extra elemental zinc will still remain and need to be removed.
6.
Preliminary setup for filtration: Connect ring to stand. Place ring over the base of the stand so that the ring lies above the triangular opening of the stand’s base. Place empty glass funnel in ring. Place empty 250-mL

Erlenmeyer flask underneath funnel, so that the bottom of the funnel lies just within the mouth of the
Erlenmeyer flask.
7.
Use a pencil (not pen) to write your initials or name on the edge of a piece of filter paper. Tare (zero) the empty
balance. Obtain the mass of your filter paper: ________________. Then fold the circular paper in half once (so that your name stays visible). Fold the half-circle of paper in half again (so that your name stays visible) to form a cone. Place the filter paper cone into the glass funnel, with the point of the paper cone pointing downwards.
It won’t stay in place – that is OK for now. Your teacher will show you how to moisten the paper cone to make it stick to the glass funnel.
8.
Before proceeding on, be certain that the aqueous phase (the liquid section) of your chemical reaction has no remaining color. Seeing no color means that the reaction between zinc and iodine is completely finished, and you have successfully generated the product, zinc iodide (ZnI
2
). HOWEVER, on purpose we added extra zinc….so, your now have to remove the excess un-reacted zinc. Your teacher will show you how to transfer your reaction contents into the filtration setup so as to maximize your recovery of the un-reacted zinc. Your “% composition” calculation will only be accurate if you know for sure how much un-reacted zinc is still present.
9.
When every bit of zinc has been transferred to the filtration setup, and all of the soluble aqueous solution has been collected in the 250-mL Erlenmeyer flask, you should test the “filtrate” (liquid in the flask) for the presence of an ionic compound. Pour a little of YOUR FILTRATE into a small beaker; and then, test it with a batterypowered conductivity tester. Since ZnI
2
is an ionic compound, and ionic compounds break apart into their ions when dissolved in water, your filtrate, if it contains ZnI
2
, should conduct electricity. If the light blinks red when you put the small electrodes of the conductivity tester into your filtrate, you have verified that you successfully made ZnI
2
, as the light will not blink red when the electrodes are only in distilled water (because there are no ions in distilled water). Result: Did your aqueous ZnI
2
conduct electricity? ________
[We could boil off the water to recover the solid product, ZnI
2
….BUT, ZnI
2
undergoes decomposition very easily when it is heated; and so the recovery step, if done correctly, would require a lot of time. Instead, your teacher will show you a previously made sample of recovered ZnI
2
.]
10.
Carefully remove (by lifting upwards) the filter paper and un-reacted zinc from the glass funnel. KEEP the paper
IN ITS CONE SHAPE, like a little pointy basket. Carry the filter paper and un-reacted zinc to the “drying oven” located next to the glassware supply cabinet. The oven’s temperature is 100 o C. Open the door of the oven; stand the filter paper cone in one of the holes in the metal shelf. Remember where you put it. Shut the oven door. Come back no sooner than 15 minutes to check for dryness. The paper and zinc must be 100% dry. If time, measure the mass in duplicate over time. This means, when you think it’s dry, record the mass. Then put the paper and zinc back in the oven and wait 5 minutes before measuring the mass a second time. Only when the mass after repeated dryings does not change, are you sure that all of the water has been removed.
Check that the balance is tared (set to zero). Then record the mass of filter paper cone + unreacted zinc.
Mass of filter paper + un-reacted zinc: ____________, ____________
Calculate the mass of “un-reacted zinc” by subtraction:
“paper + un-reacted zinc” – “paper (recorded during step 7)”:___________ - ___________ = ______________
11.
Clean Up: Toss filter paper + zinc in trash can. Lids tightly on both chemical bottles. Contents of 250-mL
Erlenmeyer dumped down drain; rinsed thoroughly. Contents of small beaker dumped down drain; rinsed thoroughly. 125-mL Erlenmeyer that contained zinc rinsed again with substantial amount of tap water. All rinsed glassware returned to glassware supply cabinet. Spatula completely cleaned and dried; placed back in drawer. Ring back in drawer. Re-use plastic cups.

Name: ___________________________
Organize your data; INCLUDE UNITS:
A.
Mass of original quantity of Zn_________________ D. Mass of unreacted Zn (B – C): ________________
B.
Mass of filter paper + unreacted Zn:_____________ E. Mass of Zn used (A – D):___ __________________
C.
Mass of filter paper alone: ________________ F. Mass of I
2
used: _______________________
G.
Sum of the Zn and I
2
used (E + F): _____________
Calculations: SHOW ALL WORK
1.
Calculate the ACTUAL % composition by mass of your product (% Zn and % I) by using Answers E/G and F/G
2.
Calculate the THEORETICAL % composition by mass of ZnI
2
(using molar masses from periodic table).
3.
Using your actual % Zn and theoretical % Zn, calculate your % error:
% error =
|𝑎𝑐𝑡𝑢𝑎𝑙 − 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙| 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
𝑥 100
4.
Using your actual % I and theoretical % I, calculate your % error:
% error =
|𝑎𝑐𝑡𝑢𝑎𝑙 − 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙| 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
𝑥 100
5.
You are a civil engineer for a major chemical producing company, and you have just been asked to evaluate the contributions of your team members. Rank each team member separately, specifying only their initials, on the following attributes. Use a scale of 1 – 5, where 1 is the worst and 5 is the best. Do NOT rank yourself!
Persons’ initials: _____________ _______________ ________________ a.
Contributed by reading the instructions and knowing “where we were” in the procedure.
______________ ________________ ________________ b.
Contributed by explaining the instructions and helping the group solve “technical” problems ourselves.
______________ ________________ c.
Contributed by maintaining a positive non-critical attitude.
________________
______________ ________________ ________________ d.
Contributed by gathering supplies, lending a hand, doing the work necessary.
______________ ________________ ________________ e.
Contributed to the awareness of safety by reminding group members to keep goggles on.
______________ ________________ ________________ f.
Contributed to the focus of the group by remaining on task and encouraging others to do the same.
______________ ________________ ________________