Laws of Chemical Combination: Quick Revision

A brief note on Laws of Chemical Combination. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Conservation of mass (Lavoisier): in a reaction, the total mass of the reactants equals the total mass of the products. Test it in a closed vessel.
• Constant proportions (Proust): a compound always has its elements in the same ratio by mass.
• Dalton's atomic theory: matter is made of atoms, which are not created or destroyed in reactions and join in small whole-number ratios.
• Atomic mass: $1\ \text{u}$ is one twelfth of the mass of a carbon-12 atom, $1.66 \times 10^{-24}\ \text{g}$.

Fixed ratios by mass

A worked example

$21\ \text{g}$ of nitrogen is mixed with $5\ \text{g}$ of hydrogen to make ammonia ($14 : 3$ by mass). How much ammonia forms, and what is left?

1. Hydrogen needed. $21\ \text{g} \times \dfrac{3}{14} = 4.5\ \text{g}$, and there is $5\ \text{g}$, so the nitrogen is used up first.
2. Ammonia. $21\ \text{g} + 4.5\ \text{g} = 25.5\ \text{g}$.
3. Left over. $5\ \text{g} - 4.5\ \text{g} = 0.5\ \text{g}$ of hydrogen.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Weighing in an open vessel and blaming the law when a gas escapes.
• Mixing up the ratio of atoms and of masses. Water has $2 : 1$ atoms but $1 : 8$ by mass.
• Forgetting the left-over. Only the fixed ratio reacts.
• Forgetting to add the parts before sharing a mass out.

Check yourself

1. In a sealed flask, $5\ \text{g}$ of A reacts with $9\ \text{g}$ of B to make $11\ \text{g}$ of C and some D. What is the mass of D?

2. How much carbon is there in $55\ \text{g}$ of carbon dioxide?

3. Which idea of Dalton explains the law of constant proportions?

4. How many times as heavy as a nitrogen atom is an iron atom? Use $\text{N} = 14\ \text{u}$ and $\text{Fe} = 56\ \text{u}$.