3. gravimetric analysis

Question 1

what is gravimetric analysis?

Answer 1

an technique used to determine the mass of an element or compound in a substance

Question 2

what are the two main weighing techniques used in gravimetric analysis?

Answer 2

tared weighing
weighing by different

Question 3

describe the process of tared weighing

Answer 3

1. put weighing boat on balance and press tare/zero button
2. weigh out substance

Question 4

describe the process of weighing by difference

Answer 4

1. place beaker on balance and record mass
2. weigh out substance, record mass of beaker + substance
3. transfer solid to reaction vessel and reweigh beaker + residue

Question 5

how would you work out the mass of solid used in a reaction after weighing by difference?

Answer 5

(mass of beaker + substance) - (mass of beaker + residue)

Question 6

how does gravimetric analysis work?

Answer 6

a substance is converted into another substance of known composition
this can be easily isolated and purified, allowing us to determine its mass

Question 7

what happens in the precipitation method?

Answer 7

a solution of the substance being analysed is converted into an insoluble precipitate by the addition of a precipitation reagent

Question 8

describe a suitable precipitating reagent

Answer 8

is itself soluble but will react with the substance being analysed to produce a precipitate

Question 9

how is the precipitate separated from the rest of the solution?

Answer 9

filtration

Question 10

how is it ensured that the precipitation reaction has gone to completion?

Answer 10

the filtrate is tested with more of the precipitating reagent

Question 11

what is the final step in the precipitation method?

Answer 11

the precipitate is washed, dried to constant mass then weighed

Question 12

what does ‘weigh accurately approximately’ 1.5g mean?

Answer 12

the mass we measure out should be close to 1.5g, but we should record exactly the mass used e.g. 1.48g

Question 13

what is the principle of the volatisation method?

Answer 13

the substance is heated and any volatile products are evaporated

Question 14

the substance is heated to constant mass in the volatisation method - what does this mean?

Answer 14

the cycle of heating is continued - cooling in a desiccator and reweighing until two consecutive mass readings are the same

Question 15

what is the volatisation method often used to determine?

Answer 15

the water concentration of hydrated salts

Question 16

describe the process of the volatisation method

Answer 16

the substance is heated inside a crucible with a lid
after heating the crucible and lid are transferred to a desiccator to cool
once cooled the crucible and contents are reweighed

Question 17

why is a desicator used for cooling in the volatisation method?

Answer 17

provides a dry environment for cooling, which prevents the substance from absorbing water