2.1.3 Amount of substance

Question 1

Define amount of substance

Answer 1

the quantity that has moles as its units, used as a way of counting atoms

amount of substance can be calculated using mass (n = m/M), gas volumes (n =pV/(RT)) or solution volume and concentration (n = CV).

Question 2

Define mole (mol)

Answer 2

the amount of substance in grams that has the same number of particles as their are atoms in exactly 12g of carbon-12

Question 3

Define molar mass
(g mol -1)

Answer 3

the **mass in grams ** in one mole of a substance

MOLAR MASS IS EQUAL TO RELATIVE ATOMIC MASS
The Mr (relative atomic mass) of a substance gives the number of grams in one mole
eg. Mr of Mg is 24.3, so 24.3g of Mg is one mole
(As mole = mass/Mr)

Question 4

Define molar gas volume
( dm3 mol–1)

Answer 4

the volume occupied by one mole of a gas at a given temperature and pressure

Room temp = 1atm and 25 oC so 24 dm3 mol–1

Question 5

Define avagadros constant and give the value

Answer 5

The number of particles per mole of a substance
(6.02 x 10 ^23)

So one mole of a substance contains 6.02 x 10^23 particles
144.0g CO2 = 3.27 moles
Molecules = 3.27 x avagadros constant = 1.97 x 10^24
Atoms = 3 (3 atoms) x 1.97 x 10^24 = 5.91 x 10^24

Question 6

Define empirical formula

Answer 6

the simplest whole number ratio of atoms of each element in a compound

Question 7

How do you calculate empirical formula ?

Answer 7

• Calculate the moles of each element involved (Moles = mass/Mr)
• Divide all results by the** smallest value** (Round to whole numbers if needed)
• Use this** ratio** to write the new empirical formula

If percentages are given, treat them as if the overall value is 100g
so 56.5 % of 100g will be 56.5 g

Question 8

Define molecular formula

Answer 8

The actual number of atoms of each element in a compound

Question 9

How do you calculate molecular formula ?

Answer 9

• Calculate the empirical formula (Moles and then dividing to get a ratio)
• Calculate the Mr of the empirical
• See how many times the Mr of the empirical fits into the actual Mr
• **Multiply ** the empirical by this value

Question 10

Define hydrated

Answer 10

a crystalline compound that contains water molecules

Question 11

Define anhydrous

Answer 11

a crystalline compound containing** no** water molecules

Question 12

Define water of crystallisation

Answer 12

water molecules bonded to the crystalline structure of a hydrated compound

Question 13

Show the formula of an anhydrous vs a hydrated salt

Answer 13

• CuSO^4 .5H^2O (Hydrated)=
  CuSO^4 + 5H^2O (Anhydrous)

In a hydrated substance the amount of water is always shown after a dot
The water present is the water of crystallisation

Question 14

How do you calculate the** formula of a hydrated salt **from mass data ?

Answer 14

4.76 g Hydrated colbalt (ll) chloride is heated to constant mass 2.60 g
Calculate formula
1. Calculate** moles** of the unhydrated salt and water (Cobalt chloride is 0.02g and water is 1.12g)
2.** Divide moles **by smallest number to get ratio (2.60/0.02 = 1 and 2.16/0.02 = 6
3. These numbers are the formula
4. (COcl^2 . 6 H^2O)

Question 15

How do you calculate formula of a hydrated salt from percentage composition data ?

Answer 15

Treat the % as masses and calculate formula normally (moles then divide by smallest to get ratio)

Question 16

Define concentration ?

Answer 16

The amount of solute in moles dissolved per 1dm^3 of solution

Unit = Mol dm^3

Question 17

What is the equation to calculate concentration ?

Answer 17

Concentration (mol/g dm^3) =
moles or mass / volume (dm^3)

Question 18

How do you convert from cm^-3 to dm^-3 ?

Answer 18

Divide by 1000

Question 19

How do you convert from mol dm^-3 to g dm^-3 ?

Answer 19

Mass = mr x moles
So multiply the result by its Mr
eg 0.738 mol dm ^-3 x 74.1 = 54.7 g dm^-3

Question 20

Define molar gas volume

Answer 20

volume per mole of gas molecules at a stated temperature and pressure

Question 21

What is the molar gas volume at room temperature and pressure ?

Answer 21

24 dm ^3

Question 22

What is the equation for volume of a gas ?

This is for ROOM TEMPERATURE AND PRESSURE

Answer 22

Volume (dm^3) =
Moles (mol) x 24 dm^3

Question 23

What is the ideal gas equation ?
What units are needed ?

This is used when gases are at different temperatures or pressures, or more accuracy is needed

Answer 23

PV = nRT
1. Pressure = Pa (kPa to Pa = x1000)
2. Volume = m^3 ( if from cm^3 divide by 1,000,000, if from dm^3 divide by 1000)
3. Amount = mol
4. Gas constant = 8.314
5. Temp = K (add 273)

Question 24

How can you use the ideal gas equation to calculate the relative molecular mass of a volatile liquid ?

Answer 24

1. Rearrange the equation to get moles
2. Convert any units
3. Solve for moles
4. **Calculate Mr using Mr = mass/moles **

Question 25

What do the balancing numbers show in a balanced equation ?

Answer 25

the ratio of moles involved in a chemical reaction
(N^2 + 3H^2 = 2NH^3)
- 1 mole of nitrogen reacts with 3 moles of hydrogen to make 2 moles of ammonia)

Question 26

What can we use balancing numbers to find ?

Answer 26

1. quantities of reactants required
2. quantities of products that should be formed

Question 27

How can we use balancing numbers to calculate an unknown quantity ?

Answer 27

1. **convert **given quantities into moles (Moles = mass / Mr)
2. Use balancing numbers to establish moles of unknown chemical
3. Use moles to convert 2nd value into the desired information (mass, volume, concentration)

Question 28

Define percentage yield

Answer 28

the % ratio of the actual yield of the product compared with the theoretical yield

Theoretical yield = mass of chemical that should be made in a reaction if no chemicals are lost (ACTUAL YIELD IS ALWAYS LESS)

Question 29

What is the equation for percentage yield ?

Answer 29

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

Question 30

Give 3 examples why the maximum amount of product may not be achieved

Answer 30

1. reaction may not have gone to completion
2. other reactions may have occured
3. transfer of product may have resulted in some product being lost

Question 31

Define limiting reactant

Answer 31

completely used up in a reaction (non in excess) which puts a limit on how much product can form

Question 32

How can you calculate percentage yield while taking into account limiting reactant

Question: Calculate the % yield in this reaction
CH^3C00H + CH^3OH = CH^3COOCH^3 + H^2O

Answer 32

1. Calculate moles of both reactants to find the limiting one
   (3.0/60 = 0.05 mol and 2.8/32 = 0.0875 mol)
   -First reactant is **limiting **so only 0.05 moles of product are possible
2. Calculate maximum theoretical mass using mass = mr x mole
   (74 x 0.05 = 3.7g)
   3.**Calculate % yield **
   (actual/theoretical x100 = 86.5 %)

Question 33

Define atom economy

Answer 33

a measure of the amount of starting materials that end up as useful products

Question 34

What is the equation for atom economy ?

Answer 34

( sum of Mr of desired products/ sum of Mr of all products ) x 100

RULE: Balancing numbers must be factored into all equations when calculating the sum of molar masses (Mr)

Question 35

Give 2 reasons why a high atom economy is beneficial

Answer 35

1. Lower costs (less waste of reactants so less money spent on disposal)
2. More sustainable (preserves limited resources, less high energy demands-temp, pressure)