Determine the wt% sulfate in a fertilizer
PLAN 1
1)
Wear PPE – gloves and goggles.
2)
Wash beaker with distilled water to remove
impurities, dry & weigh.
3)
Weigh crystals of fertilizer in the pre-weighed
beaker.
4)
Weigh a filter paper.
5)
Dissolve some fertilizer crystals in some water.
6)
Add BaCl2 solution to precipitate the
sulfate as BaSO4 (s)
7)
Use a filter funnel and the weighed filter paper
to filter out the precipitate.
8)
Wash all precipitate from the beaker into the funnel
with a wash bottle & distilled water
9)
Place the filter paper on a tray and place in an
oven to dry.
10)
Weigh the dry precipitate and filter paper and
subtract the original mass of filter paper to find the mass of precipitate
11)
Calculate the moles of precipitate and sulfate,
the mass of sulfate and the wt% sulfate in the fertilizer via wt%sulfate
= msulfate/mfertilzser x 100
12)
Repeat the experiment for reliability
PLAN 2
bold =
adjustments to the plan. The changes made to minimise error are numbered. You
need to explain the error that each is designed to overcome as part of Q2 of
your discussion.
1)
Wear PPE – gloves and goggles.
2)
Wash beaker with distilled water to remove
impurities, dry & weigh.
3)
Weigh
three pre dried(1) filter papers on a watch glass(2).
4)
Weigh 10g
of fertilizer crystals into a clean dry beaker.
5)
Make a 250ml standard solution* of this fertilizer in nitric acid(3).
6)
Use a volumetric
pipette to transfer 10ml of the solution to a clean test tube.
7)
Add 0.25 mol/L BaCl2 (aq) –
until no more ppt formation is apparent.
Let the precipitate settle 24hr. Add more BaCl2 (aq)
and repeat until no more ppt forms(4).
8)
Use a filter funnel and the three(5) pre-weighed filter papers to filter out the
precipitate.
9)
Wash all precipitate from the beaker into the
funnel with a wash bottle & distilled water
10)
Wash the
ppt with water (6).
11)
Dry the filter papers and precipitate on the watch glass in an oven to constant mass(7).
12)
Weigh the dry precipitate and filter paper and
subtract the original mass of filter paper to find the mass of precipitate.
13)
Calculate the moles of precipitate and sulfate,
the mass of sulfate and the wt% sulfate in the fertilizer via wt%sulfate
= msulfate/mfertilzser x 100.
14)
Repeat the experiment for reliability.
* Ensure the correct steps are
followed to ensure accuracy of measurement and concentration – see titration
and standard solution experiment.
The equipment used
The precipitates in different stages of settling* (most to least settled)
*sedimenting
RESULTS
All tests had 10.00 g of fertilizer dissolved into a 250.0 mL standard solution. 10.00 mL of this standard was reacted with excess aqueous barium chloride to produce a barium sulfate precipitate which was weighed.
GROUP
|
Mass ppt (g)
|
A
|
0.25
|
A
|
0.23
|
A
|
0.26
|
A
|
0.11
|
A
|
0.16
|
B
|
0.26
|
B
|
0.39
|
B
|
0.08
|
B
|
0.24
|
C
|
0.43
|
C
|
0.62
|
C
|
0.65
|
C
|
0.51
|
CHEMICALS
A = Iron (II) Sulfate - hmmm
B = Magnesium Sulfate - hmmm
C = Potassium Sulfate - hooray!
Summary Feedback
NOTE - AAS can ONLY be used for analysing metal ion conc. It cannot be used to analye sulphate ion conc
A) After 19 experiments you should know that you can’t skimp on safety. The danger of each chemical is distinct. Choose 1 Ba compound e.g. BaCl2 and HNO3 and specify the dangers (and precautions) for each. Don’t forget about disposal issues. ‘Take care’ is not sufficient instead say ‘wear gloves to avoid contact with skin’ ie say exactly what ‘taking care’ means
B) Make sure you look up a proper chemical definition of each term – use textbook? – many definitions given were too specific and thus not thorough. You need a valid, general definition for each term.
C) Detail, detail, detail – don’t just say the precipitate might pass through the filter paper – this is unusual – say why e.g. "The very fine nature of the BaSO4 ppt means it can pass through the filter paper…this would lead to an underestimate of sulfate content". NB the MOST important issues to discuss are: adding acid, adding excess BaCl2, filtering, washing, drying – be detailed for each.
D) In the conclusion, state the class average and the theoretically calculated value
E) You must show working for your calculation of the theoretical wt% sulfate in the fertiliser. Make sure you use the results posted to the Blurg (above) for your calculations for the group wt %. Also don’t forget the fertiliser was ammonium sulfate NOT barium sulfate.
F) This summary can be 2 pages!
G) Reliability means repeated with consistent results. We repeated but were the results consistent (they were to an extent but were not perfect) – what were the main sources of error? Remember that assess means you need to introduce the concept first – ie DEFINE reliability & ditto when assessing validity
H) Make sure your calculations are clearly set out. ie don’t just write "n = 3.02 / 2233.37….". Use subscripts "nBaSO4 = 3.02/233.37…"
I) Nitric acid was used as it will dissolve any phosphates in the fertilizer and also any carbonates (which are converted to CO2 and bubble out of solution). Thus the only ions that will precipitate with the Barium ions will be sulfate ions. Thus we know the ppt will be BaSO4 only (ie carbonates and phosphates will NOT ppt in acidic conditions)
But NB sulfates WILL precipitate in acidic, neutral and basic conditions – see Expt #18 results!
J) Because we added nitric acid there will be nitrate ions (and because there might be phosphate, chloride, potassium, magnesium, ammonium etc ions from the fertilizer) in the solution. These ions will be in the water surrounding the wet ppt. Unless we wash them through the filter with extra water they will crystallise when we evaporate the water from the ppt and add to the mass of the ppt … UPDATE- thank you to those that read this before posting but Don't take shortcuts. nitrate ions will not 'crystallize' upon evaporation. They will crystalise as a salt with some of the cations in the solution - eg excess barium ions, metal ions from the fertilizer etc.
K) There were 4 possible fertilizers to calculate the wt% sulfate for – don’t forget ammonium sulfate
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