Section 3 - General Chemistry

Question 1

Molecular Weight

Answer 1

g / (mr x moles)

Question 2

Empirical Formula

Answer 2

The simplest whole-number ratio between the numbers of atoms of the different elements make up the compound

Question 3

Molecular Formula

Answer 3

States the exact number of the different atom make up the molecule

Question 4

General rules to assign oxidation numbers to different elements in different compounds

Answer 4

1. In elementary substances, the oxidation number of an uncombined element regardless of whether it is monatomic (1 atom), diatomic (2 atoms) or polyatomic (multiple atoms), is zero.
2. In monatomic ions, the oxidation number of the element that makes up this ion is equal to the charge of the ion.
3. In a neutral molecule, the sum of the oxidation numbers of all the elements that make up the molecule is zero.

Question 5

List some useful oxidation numbers to remember

1. H
2. O
3. Alkali metals
4. Alkaline metals
5. Aluminium
6. Group VIIA

Answer 5

1. H: +1, except in metal hydrides.
2. O: -2 in most compounds. In peroxides (e.g. in H₂O₂) the oxidation for O is -1, is is +2 in OF₂ and -1/2 in superoxides
3. Alkali metals: +1
4. Alkaline metals: +2
5. Aluminium always has an oxidation number of +3 in all its compounds.
6. Group VIIA: the oxidation of each group element ins -1; however, when it is combined with an element of higher electronegativity, the oxidation number is +1.