- Gravimetric analysis is the quantitative isolation of a substance by precipitation and weighing of the precipitate.
- An analyte is the substance to be analysed.
- A precipitating reagent is the reactant used to precipitate the analyte.
- Weigh the sample to be analysed
- Dissolve the sample in a suitable solvent, eg, water
- Add an excess of the precipitating reagent to precipitate the analyte
- Filter the mixture to separate the precipitate from the solution
- Wash the precipitate to remove any impurities
- Dry the precipitate by heating to remove water
- Cool the precipitate in a dessicator to prevent the precipitate absorbing moisture from the air
- Weigh the cooled precipitate
- Repeat the drying and weighing process until a constant mass for the precipitate is achieved
- Calculate the percent by mass of analyte in the sample
- General calculation of the percent by mass of analyte in a sample:
- Wite the balanced chemical equation for the precipitation reaction
- Calculate the moles of precipitate: moles = mass ÷ molar mass
- Calculate moles of analyte from the balanced chemical equation using the mole ratio of analyte : precipitate
- Calculate mass of analyte: mass = moles x molar mass
- Calculate percent by mass of analyte in sample: (mass analyte ÷ mass sample) x 100
- Incomplete precipitation results in a value for the percentage of analyte in the sample that is too low.
- Incomplete drying of the sample results in a value for the percentage of analyte in the sample that is too high
- Other ions in the sample may also be precipitated resulting in a value for the percentage of analyte in the sample that is too high
A 2.00 g sample of limestone was dissolved in hydrochloric acid and all the calcium present in the sample was converted to Ca2+(aq).
Excess ammonium oxalate solution, (NH4)2C2O4(aq), was added to the solution to precipitate the calcium ions as calcium oxalate, CaC2O4(s).
The precipitate was filtered, dried and weighed to a constant mass of 2.43 g.
Determine the percentage by mass of calcium in the limestone sample.
- Wite the balanced chemical equation for the precipitation reaction:
Ca2+(aq) + C2O42-(aq) → CaC2O4(s)
- Calculate the moles (n) of calcium oxalate precipitated.
n(CaC2O4(s)) = mass ÷ molar mass
n(CaC2O4(s)) = 2.43 ÷ (40.08 + 2 x 12.01 + 4 x 16.00)
n(CaC2O4(s)) = 2.43 ÷ 128.10
n(CaC2O4(s)) = 0.019 mol
- Find the moles of Ca2+(aq).
From the balanced chemical equation, the mole ratio of Ca2+ : CaC2O4(s) is 1 : 1
So, n(Ca2+(aq)) = n(CaC2O4(s)) = 0.019mol
- Calculate the mass of calcium in grams
mass (Ca) = moles x molar mass
mass (Ca) = 0.019 x 40.08 = 0.76 g
- Calculate the percentage by mass of calcium in the original sample:
%Ca = (mass Ca ÷ mass sample) x 100
%Ca = (0.76 ÷ 2.00) x 100 = 38%
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