What are some common mistakes students make when determining formulas?

1 Answer


I would say a very common error students make is forgetting to balance the charges of the ions (positive and negative) in ionic compounds.


For example, when aluminum and oxygen react they form the compound aluminum oxide.

This is considered an ionic compound because it contains a metallic ion (#Al^(+3)#) and a nonmetallic ion (#O^(-2)#)

So the formula of aluminum oxide needs to be #Al_2O_3# which means there are 2 Al ions paired with 3 O ions.

2 x (#Al^(+3)#) = +6
3 x (#O^(-2)#) = -6
net charge = 0

For other formula/nomenclature tips, hints and reminders check out the video below.

Video from: Noel Pauller