Find the empirical formula for a compound only one element of which is metal. the compound percentage composition by mass is 40.0% metal, 12.0% c and 48.0% o

This is very easy if you assume you have 100g of the compound.

Do the metal last as it is unknown.

If you have 12% Carbon of 100 g, you have 12 g of Carbon = 1 mol C (Carbon's molar mass is 12)

Do the same with Oxygen. 48 g O / O molar mass (16) = 3 mol O

Next, find a metal that has a molar mass of a multiple of 40. This has to be Calcium with a molar mass of ~40g. 40 g / 40 = 1 mol Ca

Put these together and your empirical formula is CaCO3