Empirical Formulae Flashcards

All 6 cards in this deck

What is an empirical formula?

The simplest whole number ratio of the atoms (or ions) of each element in a compound.

What is a molecular formula?

The actual number of atoms of each element in one molecule of a substance.

How do you deduce the empirical formula from a molecular formula?

e.g. ethene, $\mathrm{C_2H_4}$

Divide all the subscripts by their highest common factor.
$\mathrm{C_2H_4} \rightarrow \mathrm{CH_2}$

What are the steps to deduce a molecular formula from an empirical formula and $M_r$?

e.g. empirical formula $\mathrm{CH_2}$, $M_r = 84$

1. Find the mass of the empirical formula unit: $12 + 2 = 14$
2. Divide $M_r$ by it: $84 \div 14 = 6$
3. Multiply all subscripts by that number: $\mathrm{C_6H_{12}}$

True or false? The empirical and molecular formula of a compound can be the same.

True. e.g. water is $\mathrm{H_2O}$ for both, as the subscripts are already in the simplest ratio.

True or false? Formulae worked out from reacting masses or percentage composition are molecular formulae.

False. They are empirical formulae; you also need the $M_r$ to get the molecular formula.