Unit 4 - Gravimetric Analysis

Question 1

What is gravimetric analysis

Answer 1

Gravimetric analysis is a method used to analyse the mass of an element or compound present in a substance.

Question 2

How is gravimetric analysis carried out

Answer 2

• the mass of an element or compound present in the substance is determined by changing that substance into another substance of known chemical composition and formula: which can then be readily isolated, purified and weighed.
• the final product must be dried completely - this is done by a process known as “heating to a constant mass”- this is heating the substance allowing it to cool in a dry atmosphere inside a desiccator then weighing it. This process is repeated until a constant mass is obtained, showing water is no longer present and the final product is completely dry.

Question 3

Heating to a constant mass

Answer 3

• heating the substance, allowing it to cool in a dry atmosphere inside a desiccator and then weighing it. The process is repeated until a constant mass (+or-0.01g) is obtained , showing water is no longer present and the final product is completely dry.

Question 4

If a solid is not heated to a constant mass, what effect does this have on the determined percentage by mass of substance

Answer 4

• the mass could be overestimated if the product is not heated to a constant mass it means that water may still be present. The purpose of the dessicator is to prevent reabsorption of water

Question 5

How can the conversion of one substance into another substance of known composition be done?

Answer 5

By precipitation or volatilisation

Question 6

One method of gravimetric analysis : Precipitation reaction

Answer 6

precipitation is the process of transforming a dissolved substance into an insoluble solid

Question 7

Precipitation : explain how lead nitrate could be analysed

Answer 7

• if the mass of lead nitrate was to be analysed, it could be reacted with potassium iodide to form a precipitate of lead iodide. The lead iodide could then be filtered, washed, dried and finally weighed. Through the stoichiometric equation, concentration or mass of lead nitrate could be determined. ( look at page 15 notes)

Question 8

When using precipitation reactions what are the requirements:

Answer 8

• precipitate should have a low solubility. This is so that all the product is precipitated
• it should also be stable at temperatures of 100- 105C so the solid can be dried in the oven
• the precipitate should have a particle size that isn’t too small to allow easy filtration

Question 9

What does the success of precipitation rely on

Answer 9

The success of this method relies on the procedure being carried out carefully

Question 10

The filtration part of precipitation and steps after filtering it :

Answer 10

• all materials need to be transferred from reaction vessel to filtration. A Büchner funnel + water pump can be used to carry out filtration (this us much faster that gravity filtration)
• the precipitate should be washed with a little solvent to remove any traces of filtrate
• it should then be transferred to an oven to dry ( a small amount of precipitate is lost during transfer, but not during drying or weighing)
• after drying the sample in an oven the sample is transferred to a desiccator to cool.
• after cooling in a desiccator, the precipitate is weighed, and the heating , cooling and weighing are repeated until the mass is constant (heating to constant mass)

Question 11

What does the accuracy of this (precipitation) gravimetric analysis method rely on

Answer 11

• The accuracy of this method relies on the use of reactants at high purity
• reactions going to completion (not being equilibria)
• Having no side reaction
• the accuracy also depends on the balance used , to obtain the final mass of the product the initial masses are recorded and the final masses are recorded and the difference between them is taken.

Question 12

Volatility meaning

Answer 12

If a substance is volatile it means it will readily evaporate / turn into a gas /vapour

Question 13

Volatilisation - how is it done

Answer 13

Another common method of gravimetric analysis involved heating to change one substance into another (heating a substance to drive off any volatile products) known as volatilisation conversion.
- this is often done by decomposition reaction or due to dehydration (loss of water of crystallisation from a salt)

Question 14

Example of decomposition reaction

Answer 14

Barium carbonate into barium oxide (+co2)

Question 15

What is : water of crystallisation

Answer 15

• these are the molecules of water that are found in some salt crystals when they are crystallised out of water.

Question 16

Water of crystallisation: copper sulphate example and formula

Answer 16

• copper sulphate has blue crystals when hydrated. The actual formula of the crystals is CuSO4.5H2O :this formula shows that five water molecules are associated with each (CuSO4) in its hydrated crystalline form.
• anhydrous copper surface is a white powdery and has the formula CuSO4

Question 17

How can water of crystallisation be driven off

Answer 17

• water of crystallisation can be driven off hydrated crystals by strong heating. This results in weight loss from which the quantity of water per mole of compound can be found

Question 18

How is volatilisation(water if crystallisation driven off)carried out

Answer 18

• weighing empty crucible then re weighing it with sample present, and masses are recorded
• sample is heated gently for 2 minutes and then strongly for 10-15 minutes
• it is allowed to cool in a desiccator and is then re weighed. A desiccator allows the crucible to cool down without absorbing more moisture.
• the completion of the reaction is checked by repeating heating and cooling followed by weighing until a constant mass is obtained. (+or-0.1g of each other)

Question 19

Errors in gravimetric analysis

Answer 19

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