How to Get Formula Units in Chemistry
This guide explains how to find the number of formula units in a sample of an ionic compound. You’ll need the mass of the sample and the compound’s molar mass. The result tells you how many individual formula units (the smallest repeating unit of an ionic compound) are present.
What You’ll Need
- The mass of the ionic compound sample (in grams)
- The chemical formula of the compound
- A periodic table (to get atomic masses)
- A calculator
Step-by-Step
- Write down the chemical formula of the ionic compound. For example, sodium chloride is NaCl, calcium chloride is CaCl₂.
- Find the molar mass of the compound. Look up the atomic mass of each element on the periodic table (in grams per mole, g/mol). Multiply each by the number of atoms of that element in the formula, then add them together. Example for CaCl₂: Ca = 40.08 g/mol, Cl = 35.45 g/mol. Molar mass = 40.08 + (2 × 35.45) = 110.98 g/mol.
- Convert the given mass to moles using the formula: moles = mass (g) ÷ molar mass (g/mol). If you have 5.00 g of CaCl₂, then moles = 5.00 ÷ 110.98 = 0.04505 mol.
- Multiply moles by Avogadro’s number (6.022 × 10²³ formula units per mole). Formula units = moles × 6.022 × 10²³. For the example: 0.04505 × 6.022 × 10²³ = 2.71 × 10²² formula units.
- Write your answer with the correct unit: “formula units” (not molecules, since ionic compounds don’t form molecules).
Common Mistakes
- Using molecule instead of formula unit – ionic compounds exist as a lattice, not discrete molecules. Always say “formula units” for ionic compounds.
- Forgetting to multiply by the subscript – in CaCl₂, there are two chlorine atoms per formula unit. Missing that changes the molar mass.
- Mixing up grams and moles – the formula requires mass in grams first. If you have kilograms, multiply by 1000 before dividing.
- Rounding too early – keep at least 4 significant figures during intermediate steps to avoid a slightly off final answer.
- Using Avogadro’s number incorrectly – it’s 6.022 × 10²³, not 6.022 × 10²² or 6.02 × 10²³. Check the exponent.
Tips
- Always write the units in each step. If the units don’t cancel to give “formula units”, you’ve made an error.
- Use the most precise atomic masses you can find. A periodic table with two decimal places is usually enough.
- For hydrates (like CuSO₄·5H₂O), include the water molecules in the molar mass calculation.
- If you’re given the number of moles directly, skip straight to step 4 – you don’t need the mass.
- Double-check your final answer’s magnitude. A typical lab sample (1–10 g) gives between 10²² and 10²� formula units.
Troubleshooting
Your answer seems too small or too large. Check the exponent of Avogadro’s number. A common mistake is writing 6.022 × 10²³ as 6.022 × 10²² or 6.022 × 10²�. Also verify your molar mass – a single missed subscript can throw off the whole calculation.
You’re not sure if the compound is ionic. Formula units are used for ionic compounds (metal + nonmetal) and some network solids. If you have a covalent compound (like water or CO₂), you should use “molecules” instead, but the calculation method is identical.
You have the mass in milligrams or kilograms. Convert to grams first: 1 mg = 0.001 g, 1 kg = 1000 g. Then proceed with the same steps.
Your periodic table lacks atomic masses. Use an online periodic table or a chemistry textbook. The atomic mass is usually the number below the element symbol.
You’re getting a negative number. That shouldn’t happen. Check that you’re dividing the mass by the molar mass, not the other way around. Moles = mass ÷ molar mass, not molar mass ÷ mass.
Frequently Asked Questions
What exactly is a formula unit?
A formula unit is the smallest whole-number ratio of ions in an ionic compound. For NaCl, one formula unit is one Na� and one Cl�. It’s the ionic equivalent of a molecule.
Can I use the same method for covalent compounds?
Yes, the math is identical, but you’d call the result “molecules” instead of “formula units”. The term “formula unit” is specifically for ionic compounds.
Why is Avogadro’s number 6.022 × 10²³?
It’s the number of atoms in exactly 12 grams of carbon-12. This number connects the macroscopic scale (grams) to the atomic scale (individual particles).
Do I need to balance a chemical equation first?
No. To find formula units, you only need the mass and the molar mass of the substance as it exists. Balancing equations is for stoichiometry problems involving reactions.
What if I have the number of moles, not the mass?
Then you can skip the mass-to-moles step. Just multiply the moles by Avogadro’s number directly.
How do I handle a compound like (NH₄)₂SO₄?
Treat the polyatomic ion as a single unit. The molar mass is: 2 × (14.01 + 4×1.008) for NH₄, plus 32.07 + 4×16.00 for SO₄. That gives 132.14 g/mol. Then proceed as usual.
Related
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.