366 ❯ STEP 5. Build Your Test-Taking ConfidenceQuestion 3
(a) 1. Samples of iron(II) ammonium sulfate hexahydrate are weighed into beakers, and deionized water
is added to dissolve each of the samples.- A buret is rinsed with the potassium permanganate solution and then filled.
- The potassium permanganate solution is titrated into the iron(II) standard until a permanent pink
color from the permanganate solution appears. - Repeat these steps with the other samples.
Listing these will get you 2 points. Missing one or more of these will get you 1 point. Other items
are not necessary and will not change the scoring.
(b) [(Mass of Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O)/(Molar mass of Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O)] × (1 mol MnO 4 - /5 mol
Fe^2 +)/(Volume of potassium permanganate solution)
You get 2 points for this complete answer. You get 1 point if a step is missing and 0 points if two
or more steps are missing.
(c) (Volume permanganate solution) × (Concentration permanganate solution) × (5 mol Fe/1 mol
MnO 4 - ) × (Molar mass of iron) = grams iron
You get 1 point for this answer.
Percent iron = (grams iron/grams sample) × 100%
You get 1 point for this answer.
(d) The percent iron would be higher.
You get 1 point for this answer.
The titration would require more potassium permanganate solution to reach the endpoint because it
would be necessary to titrate not only the iron but also the zinc.
You get 1 point for this answer or by saying the zinc would reduce some of the iron back to iron(II),
which would require it to be re-titrated.
(e) As noted in the initial comments for this question, air oxidizes iron(II) ions. For this reason, the
iron(II) ions in iron(II) ammonium sulfate hexahydrate might undergo air oxidation before being used
to standardize the potassium permanganate.
This answer or simply stating that the compound is unstable is worth 1 point.
(f ) The electron configuration may be written as [Ar] 3d^6 or 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6. Recall that for the transi-
tion metals the s-electrons, in this case 4s^2 , are the first electrons lost in oxidation.
This answer is worth 1 point.
Total your points for the various parts. There are 10 possible points.
21-Moore_PE02_p341-370.indd 366 31/05/18 1:54 pm