From numbers to equations: deducing stoichiometry

5 minAS Level2.4.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can turn experimental results (masses, gas volumes, titres, or a break-point graph) into a mole ratio, and use it to write the balanced equation.

Flip it around

Until now you've gone equation → numbers. Examiners love to flip it: here are the numbers, now you find the equation.

It's the same mole bridge, walked backwards. Get everything into moles, compare, and the coefficients fall out.

The method

Convert every measurement to moles: mass ÷ $M$ , gas volume ÷ $V_m$ , or $c \times V$ for a solution.
Divide each by the smallest number of moles.
Make whole numbers: if you get 1.5, double everything (1 : 1.5 = 2 : 3).
Use the ratio as coefficients, then balance the rest of the equation.

Yes, it's basically the empirical formula method, but for whole molecules in a reaction.

Worked example: What charge does aluminium take?

0.540 g of aluminium reacts with excess hydrochloric acid, giving 0.720 dm³ of hydrogen at room conditions. Deduce the equation, and the charge on the aluminium ion. ( $A_r$ : Al = 27.0; $V_m = 24.0\ \text{dm}^3\ \text{mol}^{-1}$ )

Have a go first?

Quick check

Multiple choiceQuestion 1 · Apply

Answer the 2 questions above to continue.