Calculating empirical formulas from data

5 minAS Level2.3.32.3.5

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By the end of this lesson

By the end you can calculate an empirical formula from percentage or mass data, deal with awkward ratios like $1 : 1.5$ , and use $M_r$ to get the molecular formula.

You can't see atoms, but you can weigh them

An analyst hands you a white powder and a printout: "34.4 % iron, 65.6 % chlorine". No pictures of molecules, no formula. Just masses. Your job: decode it.

The trick is the one from the mole topic. Masses don't compare atoms; moles do. So convert each element's mass to moles, then find the simplest ratio.

The divide–divide method

Write the mass (or percentage) of each element. A percentage is just the mass in a 100 g sample.
Divide by $A_r$ to get moles of atoms of each element.
Divide by the smallest number of moles.
If the results are whole numbers (within about 0.1), that's the ratio. If not, multiply up (next section).

Worked example: Decoding the white powder

A chloride of iron contains 34.4 % Fe and 65.6 % Cl by mass. Find its empirical formula. ( $A_r$ : Fe = 55.8, Cl = 35.5)

Have a go first?

Got masses instead? Even easier

Masses work exactly like percentages: divide each element's mass in grams by its $A_r$ .

If you're given the mass of the product and of one element, get the other element by subtraction. Burn 1.20 g of magnesium to make 2.00 g of oxide, and the oxygen that combined is $2.00 - 1.20 = 0.80$ g.

Quick check

CalculateQuestion 1 · Apply

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