What are common mistakes students make with empirical forumlas?

I'll get this started, hopefully other contributors will add...

An empirical formula is the lowest whole number ratio of elements in a compound

NaCl - is a 1:1 ratio of sodium ions to chloride ions
#CaCl_2# - is a 1:2 ratio of calcium ions to chloride ions
#Fe_2O_3# - is a 2:3 ratio of iron ions to oxide ions
CO - is a molecule that contains one atom of C and one atom of O
HO - is the empirical formula for hydrogen peroxide, note the formula of hydrogen peroxide is #H_2O_2# and can be reduced to a 1:1 ratio
#C_12H_22O_11# is the empirical formula for sucrose, the ratio of carbon hydrogen oxygen cannot be reduced

1. Sometimes students think that empirical formulas and molecular formulas are always the same - they can be different
2. Empirical formulas apply to both ionic and molecular compounds
3. The ratio used to determine an empirical formula is a mole:mole ratio (not a mass:mass)
   Example of 3: water has a 2:1 ratio of hydrogen to oxygen, the mass ratio of hydrogen to oxygen is 1:8). If you are determining an empirical formula from experimental data, you must convert from grams of elements to moles. Here is a link for a video which will show you how to do this.

Hope this helps!
Noel P.