Empirical Formula Flashcards

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Note: We may include questions that have multiple correct answers. It's useful to remember specific examples to understand these concepts better.

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What is an empirical formula?

It is the simplest whole-number ratio of atoms of each element in a compound.

How is an empirical formula different from a molecular formula?

A molecular formula shows the actual numbers of each atom in a molecule, but an empirical formula shows only the simplest whole-number ratio. For example, glucose is C6H12O6, but its empirical formula is CH2O.

What information do you need from an experiment to work out an empirical formula?

You need the mass or percentage by mass of each element in the compound.

What is the first step when finding an empirical formula from the masses of elements?

Divide the mass of each element by its relative atomic mass (Ar) to find the amount in moles.

After you have worked out the moles of each element, what do you do next to get the empirical formula?

Divide all the mole values by the smallest mole value to get a simple ratio of atoms.

What should you do if your mole ratio is not in whole numbers, for example 1.0 : 1.5?

Multiply all the numbers in the ratio by the same factor to make them whole numbers, then use these in the empirical formula.

If a compound has the molecular formula C2H4, what is its empirical formula and why?

Its empirical formula is CH2, because you can divide both 2 and 4 by 2 to get the simplest ratio 1 : 2.

If a compound has the molecular formula CO2, what is its empirical formula and why?

Its empirical formula is CO2, because the ratio 1 carbon to 2 oxygen atoms is already the simplest whole-number ratio.

Why can you use percentage composition to find an empirical formula?

You can pretend you have 100 g of the compound, so the percentages become masses in grams. The ratios of the elements stay the same, so you can use them to find the empirical formula.

Why are empirical formulas useful to chemists?

They show the simplest composition of a compound, which helps compare different compounds and write balanced chemical equations.