Gravimetric analysis

- percentage sulfate in fertiliser.

A case study

Sulfate is found in lawn food fertiliser and the percentage by mass of sulfate is given by manufacturers on the label. We will analyse the fertiliser and determine the percentage, by mass, of sulfate.

Step 1) Weigh out a 200 mL beaker

Step 2 Finally crush a small sample of fertiliser with a mortar and pestle.

Step 3 Place about 2.00 g of powdered fertiliser into the beaker and weigh accurately.

Step 4 Add about 100 mL of distilled water to the beaker with fertiliser and stir to dissolve as much of the fertiliser as possible.

Step Filter the mixture and collect the filtrate in a clean 200 mL beaker

Step 5 Add about 10 mL of 1.0M HCl to the filtrate and heat to until it is close to boiling.

Step 6 Add 40 mL 0.2 M BaCl₂ slowly making sure to stir throughout

Step 7 Allow the precipitate to settle then, using an eye dropper, add a few drops of barium chloride solution. If a cloudy white precipitate forms add a further 2.0 mL of 0.2M BaCl₂.

Repeat step 7 until no precipitate is observed.

Step 8 Weigh a piece of filter paper.

Step 9 Filter the precipitate through the filter paper

Step 10 Wash the precipitate with distilled water.

Step 11 To the filtrate add a drop of 0.1M AgNO₃ . If a cloudy precipitate forms repeat step 10.

Step 12 Remove the filter paper and allow to dry overnight in an oven.

Step 13 Weigh the precipitate and filter paper.

On the right is a table of data collected by one student.

Atomic mass Ba =137.3 Cl =35.5, O =16.00, S = 32.1

1) What is the mass of fertiliser used?

2) What is the mass of the dried precipitate?

3) What is the mol of precipitate present?

4) What is the mol of SO₄^2- ions present?

5) What is the mass of SO₄^2- present?

6) What is the percentage by mass of SO₄^2- in the fertiliser?

7) What is the percentage by mass of sulfur in the fertiliser?

8)
i) What is the purpose of step 7?
ii) What would be the impact on the end result if step 7 were not conducted by a student?

9)
i) What is the purpose of step 11?
ii) What would be the impact on the end result if step 11 were not conducted by a student?