2.1.2—Mole ratio

$known amount → moles → coefficient ratio → required amount$

Read the required-to-known ratio from the balanced equation. For gases at fixed temperature and pressure, the same coefficient ratio applies to volumes; always finish with the requested unit.

For N₂ + 3H₂ → 2NH₃, 0.50 mol N₂ corresponds to 1.00 mol NH₃ when H₂ is sufficient. Keep the conversion chain visible—given unit to moles, coefficient ratio, then requested unit—so molar mass, concentration or gas volume is used at the correct end.

Given or requested representation	Amount bridge	Condition/unit checkpoint
mass	n = m/M	use a consistent mass unit with molar mass
solution	n = cV	convert volume to dm³ when c is mol dm⁻³
gas at stated T/P	n = V/Vₘ, or use the given gas relation	do not mix molar volumes from different conditions
particles	n = N/Nₐ	specify atoms, molecules, ions or formula units

Convert the known representation to moles, apply the balanced-equation coefficient ratio, then convert once to the requested representation.

Worked mass example: $\ce{Mg(OH)2 + 2HCl -> MgCl2 + 2H2O}$ . For $1.00\,\mathrm{g}$ of $\ce{Mg(OH)2}$ , $n=1.00/58.33=0.0171\,\mathrm{mol}$ . The $1:2$ coefficient ratio gives $n(\ce{HCl})=2(0.0171)=0.0343\,\mathrm{mol}$ , so $m(\ce{HCl})=0.0343\times36.46=1.25\,\mathrm{g}$ . The result is the maximum neutralized mass when magnesium hydroxide reacts completely.

Representative question

[Maximum number: 3]

Calculate the volume of $2.00 \mathrm{~mol} \mathrm{dm}^{-3}$ sulfuric acid required to react completely with 10.0 g of thallium (I) hydroxide.

amount $\mathrm{TIOH}\left\langle\left\langle=\frac{10.0}{221.39}\right\rangle\right\rangle=0.0452$
amount $\mathrm{H}_{2} \mathrm{SO}_{4}=\ll 0.0452 \times 1 / 2>=0.0226$
volume of $\mathrm{H}_{2} \mathrm{SO}_{4} \ll=\frac{0.0226}{2.00} \gg$ $=0.0113 \mathrm{dm}^{3} / 11.3 \mathrm{~cm}^{3}$

Marking guidance:

Award [3] for correct final answer.

Retrieve the route: balance the equation, convert through the coefficient mole ratio, identify the limiting reactant, calculate theoretical and percentage yield, then assess atom economy.

Check formula subscripts, state symbols, ratio direction, units, the smallest normalized reactant amount, the theoretical-yield denominator, and the total reactant mass used for atom economy.

2.1.2—Mole ratio

Mole Ratios in Equations

Reacting-Quantity Calculations

Assessment in practice

Amount of Chemical Change Summary

Question 1