Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation

Stoichiometry – Formulae

What is a Formula?

A chemical formula tells you how many atoms of each element are in one molecule of a compound.

Think of it like a recipe: the ingredients (elements) and the amounts (subscripts) give you the final dish (molecule). 🍲

Why Relative Numbers Matter

In a model or diagram, you often see a relative count of atoms – e.g., 2 hydrogen atoms for every 1 oxygen atom.

These ratios are the key to writing the formula.

Remember: the smallest whole‑number ratio is the empirical formula.

Step‑by‑Step Guide to Deduce a Formula

1. Count the atoms of each element shown in the model.
2. Write the ratio as a fraction or decimal.
3. Reduce to the simplest whole‑number ratio by dividing by the smallest number.
4. Write the empirical formula using the reduced ratio.
5. Check for common oxidation states if the compound is ionic (optional).

Example 1: Water (H₂O)

The ratio is 2 : 1, already the simplest whole numbers.

So the formula is \$H_2O\$.

Example 2: Ammonium Chloride (NH₄Cl)

The ratio is 1 : 4 : 1, already simplest.

Formula: \$NH_4Cl\$.

Practice Problems

1. Model shows 3 carbon atoms, 8 hydrogen atoms, and 1 oxygen atom.
   

   Answer: \$C3H8O\$ (propane‑1‑ol).

2. Model shows 2 sodium atoms and 1 chloride atom.
   

   Answer: \$Na_2Cl\$ (sodium chloride).

3. Model shows 4 nitrogen atoms, 12 hydrogen atoms, and 5 oxygen atoms.
   

   Answer: \$N4H{12}O_5\$ (simplify if possible).

Key Takeaways

• Count atoms carefully – even a single atom changes the formula.
• Reduce ratios to the smallest whole numbers.
• Use the empirical formula as the starting point for more complex compounds.
• Practice with different elements to build confidence.

Quick Quiz

If a model shows 5 sulfur atoms and 12 oxygen atoms, what is the empirical formula?

Hint: Divide by the smallest number (5).

Answer: \$S5O{12}\$