Okay, a few days ago my chemistry class did an experiment on gravimetric analysis. Following the experiment we were given some questions to answer, though I’m having a bit of trouble answering these and just need a bit of help. Anyway, below are the instructions for the experiment conducted and the relevant questions.
Note also that I haven’t completed the experiment just yet. Last lesson we just had enough time to leave the crucible and contents in the oven to be left overnight (Step 9). So I don’t have the measurements for the weight of the crucible and contents. However, I’m mainly interested in the methods to solving the problems below.
PurposeTo find the proportion of sulfate present in a brand of commercial fertiliser.
Procedure1) Finely grind a small quantity of fertiliser using a motar and pestle. Weigh out accurately about 1.0 g of the ground fertiliser into a 100 ml beaker. Record the mass and the brand of fertiliser.
2) Add 50ml of distilled water and stir to dissolve as much of the fertiliser sample as possible. Filter the mixture into a 600ml beaker, washing the residue several times using distilled water.
3) Add about 3 mL of 2 M hydrochloric acid to the filtrate and add more water so that the total volume is about 200ml. Boil the solution.
4) Slowly add 15 mL of 0.5 M barium chloride solution drop by drop from a burette to the hot solution. A white precipitate of barium sulfate will form. Stir continuously throughout this process.
5) Boil the mixture for a further minute. Remove it form the heat and allow the precipitate to settle. Ensure that no sulfate ions remain in the solution by adding several drops of barium chloride solution. If more precipitate forms, add a further 3 mL of barium chloride solution and test again for unreacted sulfate ions.
6) Weigh a Gooch crucible fitted with filter paper.
7) Collect the precipitate in the Gooch crucible using gentle vacuum filtration. Use about 10 mL of warm distilled water to wash any precipitate remaining in the beaker into the crucible.
Collect the last drops of filtrate in a 100 mL beaker and test for the presence of chloride ions by adding a few drops of silver nitrate solution to the filtrate. If the solution becomes cloudy, wash the precipitate further with a further 10 mL of warm water and repeat the test.
9) Place the crucible and contents in an oven heated to 100 -120°C and leave overnight.
10) Weight the crucible and contents and record the mass.
____________________________________________________________________________________________________________________
Questions1) Determine the mass of
precipitate obtained.
 = 0.5 M * 0.015 L = 0.0075 mol)
 = n(BaCl_2) = 0.0075 mol)
 = 0.0075 mol * 233.4 g mol^{-1})
 = 1.505 g\approx2 g)
(rounded to the least number of significant figures)
2) Find the mass of sulfate ions present in the barium sulfate precipitate.
 + SO_4^{2-}(aq) ---> BaSO_4(s))
From question 1, we know that
 = 0.0075 mol)
, hence:
)
also equals
 = 0.0075 mol * 96.1 g mol^{-1} )
 = 0.72075 g \approx 0.7 g)
3) Determine the percentage by mass of sulfate in the fertiliser.
4) Determine the percentage by mass of sulfur (as sulfate) in the fertiliser.
5) How could the results obtained be affected if:
a) the mixture in step 5 of the procedure was not tested with more barium chloride solution.
Without the addition of barium chloride solution we would not know whether all the sulfate ions have precipitated with barium to form barium sulfate.
b) the filtrate were not tested with silver nitrate solution.
6) What sources of error might arise in this gravimetric analysis, when the precipitate was washed in step 8?
Anyway, any help regarding the above will be appreciated!
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