L4 - Gravimetric Analysis

objectives

conduct investigations and/or process data involving:
- gravimetric analysis

Principles of Gravimetric Analysis

Gravimetric analysis is used to analyse the % composition by mass of a chemical species in a sample.
The % by mass composition that is used in commercial industry is best seen in soils, fertilisers and certain foodstuffs where manufacturers tell us the % composition on labels of their products.
We could choose any ion that precipitates like PO₄^3-or SO₄^2-to find the % composition by mass of that element in the sample.
For your exam - given the environmental impacts of fertilisers in eutrophication - we will examine the Sulphate ion - as it is important for plant growth and to maintain soil acidity.

Let's test the accuracy of this manufacturer's fertiliser.

Method of Gravimetric Analysis

Step 1: Carefully weigh out a known mass of the fertiliser sample - we could use 15.0g.

Step 2: Dissolve this sample in deionised water - the amount of water does not matter as long as the sample fully dissolves.

Step 3: Add an EXCESS of a reagent that will PRECIPITATE your ion - in this case we will use Barium Chloride - as this forms a white PPT with the Sulphate Ion.

Step 4: Filter the precipitate and dispose of the filtrate

Step 5: Dry the PPT to CONSTANT MASS

Step 6: Weigh the Dry PPT

Worked Example

Let's look at an example so we can calculate the % by mass of SO₄² in our fertiliser.

We measured out 15.0g of Fertiliser
After performing the above method of gravimetric analysis we were left with 7.22g of dried Precipitate

Method of working out:

1. Show the Balanced Equation for the formation of the PPT

Ba⁺²+ SO₄^2- → BaSO₄

2. Calculate Moles of the PPT using Mass/ Mr

Mass = 7.22g Mr BaSO₄ = 233.3

7.22/233.3 = 0.0309 moles BaSO₄

3. Calculate Moles SO₄ in BaSO₄

As the equation above shows 1:1 ratio for Ba to SO₄ then 0.0309 moles of the substance would yield 0.0309 moles of SO₄

4. Calculate Mass of SO₄ in the PPT using Moles x Mr

Moles = 0.0309 Mr SO₄ = 96

0.0309 x 96 = 2.97g

5. Find % by mass of SO₄ in the sample

Mass of PPT = 2.97g Mass of Sample 15.0g

(2.97/15.0) x 100 = 19.8%

Sources of Error in Gravimetric Analysis

Too high %

It is most common to find you have a much higher mass than expected of the PPT in your gravimetric analysis. This is because:

The PPT was Not dried to constant mass and thus still contains water
The PPT contains impurities

Too Low %

Not all the PPT was filtered out of the solution
Some mass is lost in rinsing, weighing and transferring the sample
In some cases - some of the compound remains in solution and does not form the PPT

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