C2.1 Purity And Separating Mixtures

Question 1

What is relative atomic mass?(Ar)

Answer 1

The mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom

Question 2

What is relative formula mass?(Mr)

Answer 2

The mean of a unit of substance compared to 1/12 the mass of a carbon-12 atom

Question 3

what is relative molecular mass?

Answer 3

Same as relative formula mass but applied to a molecular (consists of 2 or more non-metal atoms) substance

Question 4

How do you calculate relative formula masses?

Answer 4

• find the atomic mass of each element in the periodic table
• work out the number of atoms of each element
• multiply the mass by the number for each element and then add them all together
  (CO2 = c=12 o=16 (1x12) + (2x16) = 44)

Question 5

Empirical formula:

Answer 5

The simplest whole number ratio of the elements in a molecule
(Find highest common factors and simplify - C4H10 = C2H5)

Question 6

What does purity mean?

Answer 6

A pure substance consists of just one element or compound (particle)

Question 8

What are the different purification methods?

Answer 8

Filtration, crystallisation, simple distillation, fractional distillation and chromatography

Question 9

How do you distinguish pure from impure substances?

Answer 9

• impure substances have a lower melting point than pure substances
• the melting point of a pure substance is one temperature but impure substances have a range of temperatures

Question 10

Crystallisation:

Answer 10

• produces dry sample of a soluble substance
• solution is heated
• solvent evaporates

Question 11

Simple distillation:

Answer 11

• separates a solvent from a solution

- evaporation —> cooling —> condensing

Question 12

Fractional distillation:

Answer 12

• separates a liquid from a mixture of liquids

- evaporation —> cooling at different temperatures —> condensing

Question 13

What purification methods are suitable for which situation?

Answer 13

/

Question 14

Chromatography:

Answer 14

Stationary phase - doesn’t move
Mobile phase- does move
Thin-layer/gas

Question 15

Calculating Rf values from chromatograms:

Answer 15

Rf = distance travelled by substance / distance travelled by solvent
Distance travelled by substance = base line to substance spot
Distance travelled by solvent = base line to solvent front
(Always 0-1 and no unit)

Question 16

Filtration:

Answer 16

• separates a solid from a liquid
• insoluble substances forms residue left in the filter paper
• soluble substances goes through filter paper as filtrate