Gravimetric Analysis Chemistry Tutorial

Key Concepts

⚛ Gravimetric analysis is also known as gravimetry.

⚛ Gravimetric analysis is the quantitative isolation of a substance by precipitation and weighing of the precipitate.⁽¹⁾

⚛ An analyte is the substance to be analysed.

⚛ A precipitating reagent is the reactant used to precipitate the analyte.⁽²⁾

⚛ The precipitate must be a pure substance of definite chemical composition.

⚛ One advantage of gravimetric analysis compared to volumetric analysis (titrimetric analysis) is that there is greater likelihood of any impurities being seen, and therefore a correction can be applied.

⚛ One disadvantage of gravimetric analysis is that it is generally more time-consuming.

⚛ Gravimetric analysis can be used to determine the:

(i) percentage of sulfate in lawn fertiliser

(ii) concentration of chloride in water

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Gravimetric Analysis Experimental Procedure and Calculations

To determine the percentage by mass of analyte in a given sample you will need to:

1. Perform an experiment to dissolve the sample, precipitate the analyte and weigh the precipitate

2. Calculate the percent by mass of analyte using the experimentally determined mass of the precipitate and the mass of the sample.

An outline of the general experimental procedure, calculations, and common sources of error in gravimetric analysis is given below.

Gravimetric Analysis General Procedure:

The following 10 steps are used in a gravimetric analysis experiment:

Step 1: Weigh the sample to be analysed
Step 2: Dissolve the sample in a suitable solvent, eg, water.
Step 3: Add an excess of the precipitating reagent to precipitate the analyte
Step 4: Filter the mixture to separate the precipitate from the solution⁽³⁾
Step 5: Wash the precipitate to remove any impurities⁽⁴⁾
Step 6: Dry the precipitate by heating to remove water
Step 7: Cool the precipitate in a dessicator to prevent the precipitate absorbing moisture from the air
Step 8: Weigh the cooled precipitate
Step 9: Repeat the drying and weighing process until a constant mass for the precipitate is achieved
Step 10: Calculate the percent by mass of analyte in the sample

General Calculation of the Percent By Mass of Analyte in a Sample:

Follow these 5 steps to calculate the percent by mass of analyte in the sample using the results of the gravimetric analysis experiment:

Step 1: Write the balanced chemical equation for the precipitation reaction
Step 2: Calculate the moles of precipitate: moles = mass ÷ molar mass
Step 3: Calculate moles of analyte from the balanced chemical equation using the mole ratio of analyte : precipitate (mole ratio also known as the stoichiometric ratio of analyte to precipitate)
Step 4:v Calculate mass of analyte: mass = moles × molar mass
Step 5: Calculate percent by mass of analyte in sample: (mass analyte ÷ mass sample) × 100

Sources of Error in Gravimetric Analysis:

Gravimetric analysis requires the experimenter to be extremely careful during each step of the procedure. Common sources of error are:

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

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Worked Example of Gravimetric Analysis Calculations

The following worked example showed you how to calculate the percent by mass of analyte in a sample using the results of a gravimtric analysis experiment. It demonstrates how to apply the 5 steps outlined in the previous section.

Question: 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)₂C₂O_4(aq), was added to the solution to precipitate the calcium ions as calcium oxalate, CaC₂O_4(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.

Solution:

Step 1: Write the balanced chemical equation for the precipitation reaction:

Ca²⁺_(aq) + C₂O₄^2-_(aq) → CaC₂O_4(s)

Step 2: Calculate the moles of calcium oxalate precipitated.

moles(CaC₂O_4(s)) = mass ÷ molar mass

moles(CaC₂O_4(s)) = 2.43 ÷ (40.08 + 2 x 12.01 + 4 x 16.00)

moles(CaC₂O_4(s)) = 2.43 ÷ 128.10

moles(CaC₂O_4(s)) = 0.0190 mol

(Note: 3 significant figures are justified)

Step 3: Find the moles of Ca²⁺_(aq).

From the balanced chemical equation, the mole ratio of Ca²⁺ : CaC₂O_4(s) is 1 : 1

So, moles(Ca²⁺_(aq)) = moles(CaC₂O_4(s)) = 0.0190 mol

Step 4: Calculate the mass of calcium in grams

mass (Ca) = moles × molar mass

mass (Ca) = 0.0190 × 40.08 = 0.762 g

(Note: 3 significant figures are justified)

Step 5: Calculate the percentage by mass of calcium in the original sample:

%Ca = (mass Ca ÷ mass sample) × 100

%Ca = (0.762 ÷ 2.00) × 100 = 38.1%

(Note: 3 significant figures are justified)

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Problem Solving for Gravimetic Analysis

The Problem: A particular water soluble fertiliser contains phosphorus in the form of phosphate ions, PO₄^3-.

A student used the following procedure to determine the percentage of phosphorus in a sample of water soluble fertiliser.

5.17 g of fertiliser was added to a 250.0 mL volumetric flask and water was added to make it up to the mark.
20.00 mL of this solution was pipetted into a conical flask.
A slight excess of precipitating agent was added to precipitate the phosphate ions as MgNH₄PO₄.
The precipitate was filtered, washed with water and then converted by heating into Mg₂P₂O₇
The mass of Mg₂P₂O₇ was 0.0312 g

Calculate the percentage by mass of phosphate in the fertiliser.

Solving the Problem using the StoPGoPS model for problem solving:

STOP!	State the question.	What is the question asking you to do? Calculate the percentage by mass of phosphate in the fertiliser. %(PO₄^3-) = ? %
PAUSE!	Pause to Plan.	(1) What information (data) have you been given in the question? mass of fertiliser = m(fertiliser) = 5.17 g initial volume of solution = V_i = 250.0 mL volume of solution used in experiment = V_f = 20.00 mL mass of Mg₂P₂O₇ = m(Mg₂P₂O₇) = 0.0312 g (2) What is the relationship between what you know and what you need to find out? What steps will you follow to solve the problem? (i) moles(Mg₂P₂O₇) = mass(Mg₂P₂O₇) ÷ molar mass(Mg₂P₂O₇) (ii) 2P (from PO₄^3-) → Mg₂P₂O₇ (iii) In 20.00 mL sample solution: moles(P) = 2 × moles(Mg₂P₂O₇) (iv) moles(P) in 250.0 mL solution = moles(P) in 20.00 mL sample × 250.0/20.00 (v) each mole PO₄^3- contains 1 mole of P (vi) mass(PO₄^3-) = moles(PO₄^3-) × molar mass(PO₄^3-) (vii) %(PO₄^3-) = 100 × mass(PO₄^3-)/mass(fertiliser)
GO!	Go with the Plan.	(i) moles(Mg₂P₂O₇) = mass(Mg₂P₂O₇) ÷ molar mass(Mg₂P₂O₇) mass(Mg₂P₂O₇) = 0.0312 g (given) molar mass(Mg₂P₂O₇) = (2 × 24.31) + (2 × 30.97) + (7 × 16.00) = 222.56 g mol^-1 moles(Mg₂P₂O₇) = 0.0312 ÷ 222.56 = 1.40 × 10^-4 mol (Note: 3 significant figures are justified) (ii) 2P (from PO₄^3-) → Mg₂P₂O₇ mole ratio P : Mg₂P₂O₇ is 2:1 (iii) In 20.00 mL sample solution: moles(P) = 2 × moles(Mg₂P₂O₇) moles(P) = 2 × 1.40 × 10^-4 = 2.80 × 10^-4 mol (Note: 3 significant figures are justified) (iv) moles(P) in 250.0 mL solution = moles(P) in 20.00 mL sample × 250.0/20.00 moles(P) = 2.80 × 10^-4 × 250.0/20.00 = 3.50 × 10^-3 mol (Note: 3 significant figures are justified) (v) each mole PO₄^3- contains 1 mole of P moles(PO₄^3-) = moles(P) = 3.50 × 10^-3 mol (vi) mass(PO₄^3-) = moles(PO₄^3-) × molar mass(PO₄^3-) moles(PO₄^3-) = 3.50 × 10^-3 mol molar mass(PO₄^3-) = 30.97 + 4 × 16.00 = 94.97 g mol^-1 mass(PO₄^3-) = 3.50 × 10^-3 × 94.97 = 0.332 g (Note: 3 significant figures are justified) (vii) %(PO₄^3-) = 100 × mass(PO₄^3-)/mass(fertiliser) %(PO₄^3-) = (0.332/5.17) × 100 = 6.42 % (Note: 3 significant figures are justified)
PAUSE!	Ponder Plausability.	Have you answered the question that was asked? Yes, we have calculated percent by mass of phosphate in the sample. Is your answer plausible? Approximate by rounding off the numbers to the nearest 10 (or 5): n(Mg₂P₂O₇) ≈ (5 × 10^-2 g) ÷ (2 × 10²) g mol^-1 ≈ 2.5 × 10^-4 mol n(P) ≈ 2 × 2.5 × 10^-4 mol = 5 × 10^-4 mol n(PO₄^3-) ≈ 5 × 10^-4 mol m(PO₄^3-) ≈ 5 × 10^-4 mol × (30 + 60 g mol^-1) ≈ 5 × 10^-4 × 100 ≈ 5 × 10^-2 g %(PO₄^3-) ≈ (0.05/5) × 100 ≈ 1% Our answer of 6.42% is in the same region, is of the same magnitude, as the approximated 1% so we are reasonably confident out answer is correct.
STOP!	State the solution.	Percentage by mass of phosphate ion in fertiliser is 6.42 %

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Sample Question: Gravimetric Analysis

4.00 grams of steel was dissolved in aqua regia (a mixture of hydrochloric acid and nitric acid)
This converted all of the nickel to nickel ions, Ni²⁺(aq)
The solution was neutralised with a dilute solution of ammonia, then a solution of dimethylglyoxime was added to precipitate the Ni²⁺(aq) as red nickel dimethylgloximate, NiC₈H₁₄O₄N₄(s).
This red precipitate was washed with water, dried and weighed.
0.314 grams of solid nickel dimethylgloximate was produced.

Calculate the percentage by mass of nickel in the steel.

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Footnotes:

(1) The separation of the element or the compound containing it may done a number of different ways, precipitation is one of the most common. Other methods include volatilisation and electro-analytical methods.

(2) The precipitate formed must be stable and must be so slightly soluble that no appreciable loss occurs when the precipitate is separated by filtration and weighed.

(3) The particles making up the precipitate must be large enough so that they do not pass through the filtering medium, and, they must remain larger than the pore size of the filter medium during the washing process.
Ordinary quantitative filter paper will retain particles up to a diameter of about 10 μm.

(4) Washing removes impurities from the surface of the precipitate particles but will not remove occluded foreign substances.

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