[3.1.2] Amount of Substance

Question 1

What’s the definition of a mole?

Answer 1

The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12.

Question 2

What’s the definition of relative atomic mass?

Answer 2

Relative atomic mass is the **average mass* of one atom compared to one twelfth of the mass of one atom of carbon-12.

Question 3

What’s the definition of relative molecular mass?

Answer 3

Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12.

Question 4

What is the equation for moles when you’re given mass and Mr? Describe the units.

Answer 4

Moles = mass ÷ Mr

• Unit of mass: grams
  
  • To convert from kg to g, x 1000.
• Unit of moles: mol
• Remember Mr must be calculated and quoted to 1dp.

Question 5

Answer 5

Question 6

What is the gas equation? Describe the units.

Answer 6

PV = nRT

• Unit of pressure (P): Pa
  
  • To convert from kPa to Pa, x 1000
• Unit of volume (V): m³
  
  • To convert from cm³ to m³, ÷ 1 000 000
  • To convert from dm³ to m³, ÷ 1000
• Unit of temperature (T): K
  
  • To convert from °C to K, + 273
• n = moles
• R = 8.31

Question 7

Answer 7

Question 8

What do you use a gas syringe for?

What are the potential errors of using one?

Draw a diagram of a gas syringe being used.

Answer 8

• Gas syringes can be used to measure the volume of gas to work out the moles of gas using PV = nRT or to follow reaction rates.

POTENTIAL ERRORS

• Gas escapes before bung inserted.
• Syringe sticks.
• Some gases like carbon dioxide or sulfur dioxide are soluble in water so the true amount of gas is not measured.

Question 9

Answer 9

Question 10

What is the equation for concentration? Describe the units and how to convert to them.

Answer 10

Concentration = moles ÷ volume

Unit of concentration = mol dm⁻³
Unit of volume = dm³
- To convert from cm³ to dm³, ÷ 1000.
- To convert from m³ to dm³, x 1000.

Question 11

Answer 11

Question 12

How many significant figures should you give your answer to?

Answer 12

• Give your answers to the same number of significant figures as the number of significant figures for the data you’re given in a question.
• If you are give a mixture of different significant figures, use the smallest.

Question 13

What is Avogadro’s constant?

Answer 13

• There are 6.022 x 10²³ in 12 grams of carbon-12.
• Therefore, in simpler terms, ‘One mole of any specific entity contains 6.022 x 10²³’.
  
  • e.g. 1 mole of copper atoms will contain 6.022 x 10²³.
• Avogadro’s constant can be used for atoms, molecules & ions.

Question 14

What equation would you use to calculate the number of particles of a substance?

Answer 14

No. of particles = moles of subsatance (in mol) x Avogadro’s constant

Question 15

Answer 15

Question 16

What is the equation for density?

Answer 16

Density = mass ÷ volume

• Density is usually given in g cm⁻³, but it can vary. Care needs to be taken if different units are used e.g. kg dm⁻³.

Question 17

Answer 17

Question 18

Define empirical formula.

Answer 18

An empirical formula is the simplest ratio of atoms of each element in the compound.

Question 19

Describe the general method to calculate an empirical formula.

Answer 19

1. Divide each mass (or % mass) by the atomic mass of the element.
2. For each of the answers from step 1, divide by the smallest one of those numbers.
3. Sometimes the numbers calculated in step 2 will need to be multiplied up to give whole numbers.
   • Those whole numbers will be the empirical formula.

Question 20

Answer 20

Question 21

Define molecular formula.

Answer 21

A molecular formula is the actual number of atoms of each element in the compound.

Question 22

Answer 22

Question 23

Describe how you would calculate the molecular formula of a compound when you’re given the empirical formula and the Mr of the compound.

Answer 23

1. Calculate the Mr of the empirical formula.
2. Divide the Mr of the empirical formula by the Mr of the compound.
3. Scale up the empirical formula using the number calculated in step 2 in order to get the molecular formula.

Question 24

What method would you use to measure mass loss in thermal decomposition reactions or mass gain when reacting magnesium in oxygen? Outline the steps in this method.

Answer 24

HEATING IN A CRUCIBLE

EXAMPLE

• The water of crystallisation in calcium sulfate crystals can be removed as water vapour (thermal decomposition) by heating in a crucible.

METHOD

1. Weigh an empty, clean, dry crucible and lid.
   
   • The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as the water would be lost when heating.
   • The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible, but should be loose fitting to allow gas to escape.
2. Add 2g of hydrated calcium to the crucible and weigh again.
   
   • Large amounts of solid should not be used as decomposition is unlikely to be complete.
   • Small amounts of solid should not be used as the % uncertainties in weighing will be too high.
3. Heat strongly with a bunsen for a couple of minutes and allow to cool
4. Weigh contents and crucible again.
5. Heat crucible again and reweigh until you reach a constant mass.
   
   • Do this to ensure that the reaction is complete.

Question 25

Answer 25

Question 26

The concentration of a solution can also be measured in terms of mass of solute per volume of solution rather than moles per volume of solution.

What is the equation for mass concentration?

How would you convert concentration measured in mol dm⁻³ into the concentration measured in g dm⁻³?

Answer 26

Mass concentration = mass ÷ volume

• Unit of mass concentration:g dm⁻³
• Unit of mass: g
• Unit of volume: dm³

Concentration in g dm⁻³ = concentration in mol dm⁻³ x Mr

Question 27

What is the formula for percentage yield?

Answer 27

Percentage yield = actual yield ÷ theoretical yield x 100

Question 28

Answer 28

Question 29

What is the formula for percentage atom economy?

Answer 29

Percentage atom economy = mass of useful products ÷ mass of all reactants x 100

• Do use balancing numbers when calculating % atom economy.

Question 30

Answer 30

Question 31

What are the economic, ethical and environmental advantages for society and for industry of developing chemical processes with a high atom economy?

Answer 31

• This means there is little or no waste product, only the desired product.
• Therefore, it means the process is more economically viable for industrial-scale manufacture.
• Having a high atom economy is also beneficial for the environment as it uses less natural resources and therefore often uses less energy.

Question 32

Why do chemists want high percentage yields and atom economies?

Answer 32

• Chemists want a high percentage yield means there has been an efficient conversion of reactants to products.
• Chemists want a high percentage atom economy so that the maximum mass of reactants ends up in the desired product (so minimising the amount of by-product).

Question 33

If you are given a quantity of one substance and are asked to work out a quantity for another substance in the reaction, what steps would you follow?

Answer 33

1. Use: moles = mass ÷ mr, PV = nRT or concentration = moles ÷ volume to convert any given quantity into moles.
2. Use the balanced equation to convert moles of initial substance into moles of second substance.
3. Convert moles of second substance into the quantity the question asked for using relevant equation i.e. mass = moles x Mr

Question 34

Answer 34

Question 35

Answer 35

Question 36

Answer 36

Question 37

Answer 37