Principles of Chemistry - Chemical Formulae, Equations and Calculations

Question 1

how do you write a chemical equation?

Answer 1

• write the chemical formulae of the substances
• balance the equation so the number of atoms of each element on both sides matches

Question 2

what does the state symbol (s) mean?

Answer 2

solid

Question 3

what does the state symbol (l) mean?

Answer 3

liquid

Question 4

what does the state symbol (g) mean?

Answer 4

gas

Question 5

what does the state symbol (aq) mean?

Answer 5

aqueous solution

Question 6

how do you calculate relative formula mass?

Answer 6

add up the relative atomic masses of all atoms present in the formula

Question 7

what is the mole?

Answer 7

the unit for the amount of a substance
the mass of 1 mole of a substance is the relative formula mass (Mr) of the substance in grams

Question 8

how do you carry out calculations involving mass of substance, number of moles and relative formula mass?

Answer 8

mass = amount in moles x relative formula mass

Question 9

how do you calculate percentage yield?

Answer 9

yield = actual amount of product / theoretical amount of product

Question 10

how would you find the formula of a metal oxide?

Answer 10

the formulae of metal oxides can be found experimentally by reacting a metal with oxygen and recording the mass changes

Question 11

what is the method for finding the formula of a metal oxide experimentally?

Answer 11

example: magnesium being burned in air forming magnesium oxide
- Weigh a crucible and lid
- Place the magnesium ribbon in the crucible, replace the lid, and reweigh
- Calculate the mass of magnesium
- Heat the crucible with the lid on until the magnesium burns
- Lift the lid from time to time (this allows air to enter)
- Stop heating when there is no sign of further reaction
- Allow to cool and reweigh
- Repeat until two consecutive masses are the same
- Calculate the mass of magnesium oxide formed

Question 12

how would you find the formula of a salt containing water of crystallisation?

Answer 12

when you heat a salt that contains water of crystallisation, the water is driven off leaving the anhydrous (without water) salt behind. If the salt is strongly heated in a crucible then it will become anhydrous. the method followed is similar to that for metal oxides.

Question 13

what is empirical formula?

Answer 13

the simplest whole-number ratio between atoms/ions in a compound

Question 14

what is molecular formula?

Answer 14

the actual number of atoms of each type of element in a molecule

Question 15

how do you calculate molecular formula?

Answer 15

• calculate the relative mass of the empirical formula.
• find out the number of times the relative mass of the empirical formula goes into the Mr of the compound.
• this tells how many times bigger the molecule formula is compared to the empirical formula.

Question 16

how do you calculate empirical formula with water of crystallisation?

Answer 16

• calculate the mass of the anhydrous salt and the water driven off
• divide by Mr
• put them into a ratio and simplify to find the formula

Question 17

how do you determine the formula of a metal oxide by combustion?

Answer 17

when magnesium is burned in air, it reacts with oxygen to form magnesium oxide:
- weigh a crucible and lid
- place the magnesium ribbon in the crucible, replace the lid, and reweigh
- calculate the mass of magnesium
- heat the crucible with a lid on until the magnesium burns
- lift the lid from time to time (this allows air to enter)
- stop heating when there is no sign of further reaction
- allow to cool and reweigh
- repeat until two consecutive masses are the same
- calculate the mass of magnesium oxide formed

Question 18

how do you determine the formula of a metal oxide by reduction?

Answer 18

when copper (II) oxide is heated in a stream of methane, the oxygen is removed producing copper, carbon dioxide and water
by comparing the mass of copper produced with the mass of copper oxide used it is possible to determine the formula of the copper oxide.