Unit 3: Gravimetric Analysis

Question 1

What does gravimetric analysis do?

Answer 1

Gravimetric analysis is used to determine the mass of an element or compound in a substance

Question 2

What must happen to the starting substance for gravimetric analysis to take place?

Answer 2

The substance is converted into another substance of known chemical composition, which can be readily isolated and purified.

Question 3

The substance can be converted either by precipitation or volatilisation/decomposition.

What is the difference?

Answer 3

In precipitation conversion the substance undergoes a precipitation reaction. The precipitate is separated from the filtrate.

In volatilisation conversion the substance is heated and any volatile products (often water) are evaporated.

Question 4

What does the term “weighing accurately approximately 1g of the solid” mean?

Answer 4

The mass you measure must be around 1g (i.e. “approximately” 1g) but you must have an accurate value for the mass that you do measure, e.g. 0.96 g or 1.026g etc

Question 5

What is meant by the term “weighing by difference”?

Answer 5

The mass of the crucible is measured before adding the substance and the final mass of the substance is determined by subtracting the mass of the crucible from the mass of the crucible and dried substance.

Question 6

How would the final product be heated to a constant mass?

Answer 6

heat the substance

allowing to cool in a desiccator to prevent absorption of water

weighing

repeating the steps of heating, cooling and weighing until no further changes in mass are observed

Question 7

Why should a blue flame be used when heating to a constant mass?

Answer 7

To avoid a build up of soot on the outside of the crucible which could affect the mass