Calculations Involving Masses - Topic 1

Question 1

Method to determine the water of crystallisation for MgSO₄xH₂O:

Answer 1

1. Set up apparatus as shown in diagram
2. Take crucible and weight is using mass balance
3. Heat the salt for 2 mins - be careful of any spitting
4. Carefully using tongs and a heatproof mat underneath as a tray reweigh the crucible and the contents using a mass balance
5. Continue to heat for about 1 minute and repeat step 4
6. Continue to do steps 5 and 6 until there is no longer a change in mass
7. Calculate x

Question 2

Results for determining the water of crystallisation for MgSO₄xH₂O:

Answer 2

Question 3

What numbers do you not use to calculate RFM?

Answer 3

the big numbers

Question 4

Symbol for relative formula mass:

Answer 4

Mr

Question 5

How do you calculate relative formula mass?

Answer 5

• Relative formula mass (Mr) of a compound: sum of the relative atomic masses of
  the atoms in the numbers shown in the formula
• remember you could have more than 1 atom of a certain element in a compound e.g. in CaCl2 there are atoms of chlorine so you need to add on 35.5 x2

Question 6

Empirical formula:

Answer 6

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

Question 7

How can the empirical formula be worked out?

Answer 7

If you know:

1. The mass of each element in the compound or
2. The % of each element in the compound

Question 8

How do you calculate empirical formula?

Answer 8

1. Find the mass/percentage and RAM of each of the elements
2. Find out no. of moles for each element by dividing mass by RAM
3. Look for simple ration - divide through by smallest number if ratio isn’t obvious

Question 9

How do you calculate empirical formula from the formula of a molecule?

Answer 9

• if you have a common multiple e.g. Fe2 O 4, the empirical formula is the simplest whole number ratio, which would be FeO2
• if there is no common multiple, you already have the empirical formula

Question 10

Molecular formula:

Answer 10

• the actual number / sum of all the atoms of each element in a molecule (or compound)
• molecular formula is a multiple of empirical formula

Question 11

How do you calculate molecular formula from empirical formula and relative molecular mass?

Answer 11

• Find relative molecular mass of the empirical formula
• Divide relative molecular mass of compound by that of the empirical formula
• Multiply the number of each type of atom in the empirical formula by this number
• e.g. if answer was 2 and the empirical formula was Fe2 O 3 then the molecular formula would be empirical formula x 2 = Fe4 O 6

Question 12

Hazards in experiment to determine empirical formula of simple compound magnesium oxide:

Answer 12

• risk of retina damage from intense light of burning magnesium - do not look at it
• if you burn yourself put the affected part under the tap for at least 5 mins

Question 13

Method for experiment to determine empirical formula of simple compound magnesium oxide:

Answer 13

1. Measure mass of crucible and lid using mass balance
2. Clean length of magnesium ribbon’s surface with emery paper, coil it loosely then place it in crucible
3. Find mass of crucible, lid and magnesium ribbon using same mass balance as before
4. Support the crucible on a pipe clay triangle placed on a tripod
5. Adjust the flame of a Bunsen so that it is non-luminous
6. Heat for 2-3 mins and then use crucible tongs to lift lid cautiously to admit a little air
7. Continue heating and raising lid at intervals until magnesium catches fire (if necessary increase flame size)
8. When the magnesium catches fire keep the magnesium burning by lifting the lid as required - don’t allow too much white smoke to escape, but on the other hand, don’t let the magnesium stop burning
9. When the magnesium no longer burns, remove the lid (stand it on a white tile), and heat the crucible strongly for 5 mins
10. Replace the lid and allow crucible to cool until you can hold it in your hand
11. Find mass of crucible, lid and contents again using the same mass balance
12. Repeat steps 9-11 until no change in mass occurs
13. If a. further change in mass has occurred, must repeat the heating, cooling and weighing operations
14. Known quantities: mass of magnesium used & mass of magnesium oxide produced
15. Required calculations:
    
    1. mass oxygen = mass magnesium oxide - mass magnesium
    2. moles magnesium = mass magnesium ÷ molar mass magnesium
    3. moles oxygen = mass oxygen ÷ molar mass oxygen
    4. calculate ratio of moles of magnesium to moles of oxygen
    5. use ratio to form empirical formula (same method as 1.44)

Question 14

Results for experiment to determine empirical formula of simple compound magnesium oxide:

Answer 14

Question 15

Why was the lid needed in this experiment (determining empirical formula of simple compound magnesium oxide)?

Answer 15

to keep the fire burning

Question 16

Why was the lid lifted off and then replaced on the crucible (experiment to determine empirical formula of simple compound magnesium oxide)?

Answer 16

to allow oxygen to enter and react with the magnesium and form magnesium oxide

Question 17

Law of conservation of mass:

Answer 17

• in a chemical reaction mass cannot be created or destroyed and no atoms are destroyed or made
• the total mass on one side of an equation must be the same as the total mass on the other side of the equation
• mass of products = mass of reactants

Question 18

Why can chemical reactions be represented by symbol equations due to the law of conservation of mass?

Answer 18

chemical reactions can be represented by symbol equations, which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation

Question 19

What does a balanced chemical equation indicate?

Answer 19

a balanced chemical equation indicates the correct proportions that reactants react and products are formed

Question 20

Example of law of conservation of mass in a closed system:

Answer 20

• Silver nitrate solution and sodium chloride solution are placed on the balance
• The 2 solutions are mixed together a precipitation reaction takes place
• sodium chloride (aq) + silver nitrate (aq) ——> silver chloride (s) + sodium nitrate (aq)
• as end of reaction mass of reactants = mass of products

Question 21

Example of law of conservation of mass in a non-closed system:

Answer 21

• some calcium carbonate is added to hydrochloric acid in a conical flask on a balance
• a reaction takes place and the mass reading on the balance decreases
• not because the matter is being destroyed but because the carbon dioxide gas being formed in the reaction escapes from the conical flask

Question 22

How do you calculate moles of a substance?

Answer 22

moles = mass /relative atomic mass (or relative formula mass)

Question 23

How do you calculate the masses of reactants and products from balanced equations, given the mass of one substance?

Answer 23

1. Highlight the 2 substances referred to in the question
2. Identify the ratio from the equation of the number of moles of the substances in the equation
3. Calculate the number of moles of the substance of known mass (moles = mass/relative formula mass)
4. Identify the number of moles of the other substance using the ratio
5. Calculate the mass of the unknown substance using its number of moles (mass = moles x relative formula mass)

Question 24

One mole of particles of a substance:

Answer 24

• the Avogadro constant number of particles (6.02 x 10²³ atoms/molecules/formulae/ions/formula units) of that substance
• the mass of one mole of particles is the ‘relative particle mass’ in grams

Question 25

What is measure in moles?

Answer 25

• chemical amounts are measured in moles

Question 26

Symbol for mole:

Answer 26

mol

Question 27

How is relative atomic/formula mass related to moles?

Answer 27

• the mss of one mole of a substance in grams is numerically equal to its relative formula mass
• one mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance

Question 28

How do you calculate the number of particles/formula units/ions/atoms in a given number of moles of a substance?

Answer 28

moles x avogadro’s constant

Question 29

Why in a reaction is the mass of a product formed controlled by the mass of the reactant which is not in excess?

Answer 29

• In a chemical reaction with 2 or more reactants you will often use one in excess to ensure that all of the other reactant is used
• The reactant that is used up / not in excess is called the limiting reactant since it limits the amount of products
• if a limiting reagent is used, the amount reactant in excess that actually reacts is limited to the exact amount that reacts with the amount of limiting reagent you have, so you need to use the moles/mass of the limiting reagent for any calculations
• the amount of product formed depends on the limiting reactant as this is the substance that is used up first

Question 30

Stoichiometry:

Answer 30

Stoichiometry refers to the balancing numbers in front of compounds/elements in reaction equations

Question 31

How do you deduce the stoichiometry of a reaction from the masses of the reactants and the products?

Answer 31

• Stoichiometry refers to the balancing numbers in front of compounds/elements
  in reaction equations
• Balancing numbers in a symbol equation can be calculated from the masses of
  reactants and products:
  
  • convert the masses in grams to amounts in moles (moles = mass/Mr)
  • convert the numbers of moles to simple whole number ratios
• e.g. for the reaction: Cu + O2 -> CuO (not balanced), 127 g Cu react, 32g ofoxygen react and 159g of CuO are formed. Work out the balanced equation
  using the masses given:
  • moles: (moles = mass/Mr)
  • Cu: moles = 127 / 63.5 = 2
  • O2: moles= 32 / (16 x 2) = 32/32 = 1
  • CuO moles = 159 / (16 + 63.5) = 2
• therefore you have a ratio of 2:1:2 for Cu:O2:CuO, making the overall balanced equation 2Cu + O2 -> 2CuO

Question 32

How do you calculate the mass of one atom?

Answer 32

Mass of one atom = RAM or RFM / Avogadro’s number