Question

If a question in the exam comes and asks me

For Example

what is the relative formaula mass of

Copper Oxide

Aluminiuim oxide

Carbon Dioxide

Is there a concept about the oxides or how do we work out the formula ?

Answer 1

It's sometimes a combination of looking at the name and looking at the position in the periodic chart. For example, for copper oxide, it must be copper(II) oxide or copper(I) oxide. Look at oxygen in the table, it is in group VIA (or 16 depending upon the system you are using) so that means the valence of oxygen is -2. So copper(II) oxide is +2 for copper and -2 for oxygen; therefore, copper(II) oxide is CuO. Copper(I) oxide is Cu2O. Al is in group IIIA (or 13) so that is Al with +3 and O with -2 so the formula is Al2O3. You are always looking for the subscripts so that the formula ends up with a zero charge; for Al2O3 that is +6 for Al and -6 for O so you know Al2O3 is correct. Carbon dioxide is a give away; the di tells you it is two; therefore you write CO2. (C is in group IVA ;or 14) so C at +4 and 2 Os at -2 will be -4 and that makes CO2 zero charge.
Sodium chloride is NaCl since Na is in group IA (or 1) and Cl is in group VIA (or 17). That means Na is +1 and Cl is -1 so NaCl is the formula. I hope this is enough to help you understand just how it is done. There are some nuances here and there but this will get you started.