9701 · 2.3
Formulas — FAQ
Frequently asked questions for 9701 Formulas. Direct answers first, then deeper explanation — then practise with marking.
What if my mole ratio calculation gives me numbers like 1.5 or 1.25?
You cannot round these to the nearest whole number. You must find the smallest integer to multiply the entire ratio by to make all numbers whole. For 1.5 (which is 3/2), multiply by 2. For 1.25 (which is 5/4), multiply by 4. For 1.33 (which is 4/3), multiply by 3.
Can the empirical formula and molecular formula ever be the same?
Yes, frequently. This happens when the simplest ratio of atoms is also the actual ratio in the molecule. For example, water (H₂O) and methane (CH₄) have molecular formulas that cannot be simplified further, so their empirical and molecular formulas are identical.
Why is the formula for an ionic compound always an empirical formula?
Ionic compounds form giant lattices with a repeating pattern of ions, not individual molecules. The formula we write, called a formula unit, must represent the simplest whole-number ratio of ions in that continuous structure. This is, by definition, the empirical formula.
How do I find the formula of a simple ionic compound without experimental data?
You use the charges of the ions. The overall charge of the formula unit must be zero. For example, to combine Al³⁺ and O²⁻, you need two Al³⁺ ions (total charge +6) and three O²⁻ ions (total charge -6) to balance the charges. This gives the formula Al₂O₃.