2.1.3 amount of substance

Question 1

Mole

Answer 1

Amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12

Question 2

Relative atomic mass

Answer 2

Average mass of one atom compared to 1/12th of the mass of one atom of carbon-12

Question 3

Molar mass

Answer 3

Mass in grams of 1 mole of a substance

g mol-1

Question 4

Calculating molar mass

Answer 4

Adding up the mass numbers (from the periodic table) of each element in the compound

Question 5

Molar gas volume

Answer 5

Volume of 1 mole of a gas at a given temperature and pressure.

All gases have this same volume

At room pressure (1atm) and room temperature (25C) the molar gas volume is 24dm3 mol-1

Question 6

Avogadro’s constant

Answer 6

6.02x10^23 atoms in 12 grams of carbon-12

Question 7

Amount in mol calculation

Answer 7

Moles = grams/ RAM

Question 8

Empirical formulae

Answer 8

Simplest whole number ratio of atoms of each element in the compound

Question 9

Empirical formulae method

Answer 9

1. Divide each mass by the atomic mass of the element
2. Divide each number by the smallest one
3. Sometimes you may need to multiply to get whole numbers

Question 10

Molecular formula

Answer 10

Actual number of atoms of each element in the compound

Question 11

Hydrated salt

Answer 11

Contains water of crystallisation

Question 12

Method for measuring mass loss / mass gain when reacting magnesium with oxygen

Answer 12

Heating in a crucible

1. Weigh an empty dry crucible and lid
2. Add 2g of hydrated CaSO4 to the crucible and weigh again
3. Heat strongly with a Bunsen for a couple of minutes
4. Allow to cool
5. Weigh crucible and contents again
6. Heat crucible again and reweigh until you reach a constant mass

Question 13

Concentration calculation

Answer 13

Concentration = moles / volume

dm3

Question 14

mass concentration calculation

Answer 14

Mass concentration = mass / volume

g dm3

Question 15

Making a solution

Answer 15

Weigh sample bottle containing required mass of solid on 2dp balance

Transfer to beaker and re weigh bottle

Record difference in mass

Add 100cm3 distilled water to beaker. Use glass rod to stir to help dissolve the solid

Pour solution into 250cm3 graduated flask via a funnel

Rinse beaker and funnel and add washings from beaker and glass rod to volumetric flask

Make up to the mark with distilled water using a dropping pipette

Invert flask several times to ensure uniform solution

Question 16

Diluting a solution

Answer 16

Pipette 25cm3 of original solution into 250cm3 volumetric flask

Make up to the mark with distilled water using a pipette for last few drops

Invert flask several times to ensure uniform solution

Question 17

Calculating dilutions

Answer 17

New diluted concentration = original concentration x (original volume / new diluted volume)

Question 18

Safety and hazards

Answer 18

Irritant - dilute acid and alkalis- wear googles

Corrosive- stronger acids and alkalis wear goggles

Flammable – keep away from naked flames

Toxic – wear gloves- avoid skin contact- wash hands after use

Oxidising- Keep away from flammable / easily oxidised materials

Question 19

Gas volume calculation

Answer 19

Gas volume dm3 = amount x 24

Question 20

No particles calculation

Answer 20

Amount of substance (in mol) x avogadros constant

Question 21

Density calculation

Answer 21

Density = mass / volume

G cm-3

Question 22

Percentage yield calculation

Answer 22

Percentage yield = (actual yield/theoretical yield) x 100

1. Work out moles of reactant
2. Use balanced equation to give moles of product
3. Work out mass of product
4. Use equation

Question 23

Percentage atom economy calculation

Answer 23

Atom economy = (mass of useful products/ mass of all reactants) x 100

So, (moles x Mr of product) / (moles x mr of reactant (do all)) x100