Quantitative Chemistry Of Compounds

Question 1

Empirical Formula

Answer 1

The simplest ratio of elements in a compound.

Question 2

Molecular Formula

Answer 2

The no.of atoms of each element in a molecule of a compound.

Question 3

What is the formula?

Answer 3

Mass (g) = no.of moles x FM (g/mol).

Question 4

How do you work out the Empirical formula?

Answer 4

Create a table.

1. Give the name of element and the amount (percentage of grams).
2. Note down the formula mass.
3. Mass / FM
4. Work out the ratio by dividing by the smallest number in the column above this.

Question 5

What are the things to remember when working out the empirical formula?

Answer 5

You need the smallest whole number ratio, don’t round until the ratio and if it is more than 1/3 don’t round.

Question 6

How do you work out the Molecular formula?

Answer 6

The Mr Mass should be given. Work out the empirical formula of the elements and the FM of that. Then divide the Mr Mass/FM. The answer is the number of the ‘single’ one, so substitute accordingly.

Question 7

Watch out for mass!

Answer 7

1KG=1000g and 1 tonne=1000KG or 1,000,000g!

Question 8

What is the relative atomic mass?

Answer 8

• Ar
• The relative atomic mass of an element is the weighted average mass of the isotopes of the element
• It is measured on a scale on which a carbon-12 atom has a mass of exactly 12 units

Question 9

What is a mole?

Answer 9

1. The term mole represents the amount of substance

2. The term mole as the Avogadro number of particles (atoms, molecules, formulae, ions and electrons in a substance)

Question 10

What is Avogadro number?

Answer 10

6.023 x 1023, so the term molee is the Avogadro number of particles (atoms, molecules, formulae, ions and electrons) in a substance

Question 11

How can you experimentally find the formulae of water, metal oxides, water and salts containing water of crystallisation?

Answer 11

swd

Question 12

What does it mean if a salt is anhydrous or hydrated?

Answer 12

1. All solid salts consist of a lattice of positive and negative ions and in some salts the water molecules are incorporated into the lattice
2. The water in a lattice is called water of crystallisation
3. A solid salt containing water or crystallisation is hydrated and if a salt does not contain any water of crystallisation it is called anhydrous

Question 13

How many moles are in a hydrated salt?

Answer 13

• One mole of a hydrated salt always has a particular number of moles of water of crystallisation and its formula shows you how many ( it is always a whole number)
• For example hydrated copper sulphate has five moles of water for every one mole of the salt so its formula is CUSO4.5H20

Question 14

How do you calculate how much water of crystallisation a salt contains?

Answer 14

• Many salts lose their water of crystallisation when heated to become anhydrous and if you know the mass of the salt when it is hydrated and when it is anhydrous you can work out its formula:
  1. Heat hydrated magnesium sulphate MgSO4.XH2O, in a crucible to form anhydrous magnesium sulphate MgSO4
• Mass of crucible, mass of crucible and MgSO4.XH20, mass of crucible and MgSO4
  2. Work out mass of MgSO4 and MgSO4.XH20
  3. Work out number of moles of water lost
  4. Work out number of moles of anhydrous salt made
  5. Work out ratio of moles and anhydrous salt to moles of water
  6. X must be a whole number, so you can round off result

Question 15

What is an atom?

Answer 15

An atom is the smallest particle of an element that can exist. Atoms are made of protons, neutrons and electrons

Question 16

What is a molecule?

Answer 16

A molecule is a group of two or more non-metal atoms joined together by covalent bond, examples of molecules include water (H2O), oxygen (O2) , methane (CH4) and carbon dioxide (CO2)