Determination of Formula

Molecular Formula: Covalent compounds and elements exist as molecules.
• Empirical Formula: It is the simplest whole number ratio of atoms of each element in a compound.
Used for giant structures of metals and ionic.
• Relative Molecular Mass: Mr. Add relative atomic mass of each element making up molecule.
• Relative Formula Mass: Add relative atomic masses of elements in empirical formula.
• Calculating Empirical Formula: May need to do some subtraction/ addition if masses not needed
given.
– Find moles and simplify ratio by dividing by smallest number.
– Multiply if .5 or .7 (multiply by 3). If .2 can multiply by 5. Round up if can round up all of them.

– Write empirical formula in order appeared in question.
• Calculating Molecular Formula:
– Find empirical formula and its Mr.
– Divide the Mr of the molecular formula by Mr of empirical formula.
– Multiply this factor by empirical formula.
• Knowing Compound but Not Element in the Compound:
– Divide Ar of element by Mr of substance to find percentage of element in whole compound.
– Multiply percentage by the mass of the compound to find out mass of the individual element.
– Use Mr Moles to find moles of element.