Practice Formula Writing in Chemistry

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

Quick Recap

The process of combining element symbols and subscripts to represent the composition of a chemical compound.

Chemical formulas are the 'spelling' of chemistry — they tell you exactly which atoms and how many of each are in a compound.

Example 1

easy
Write the chemical formula for aluminum chloride using the charges of the ions: \text{Al}^{3+} and \text{Cl}^-.

Example 2

medium
Write the chemical formula for calcium phosphate using the crisscross method. Calcium forms \text{Ca}^{2+} and phosphate is \text{PO}_4^{3-}.

Example 3

medium
Write the correct chemical formulas for: (a) iron(III) sulfate, (b) ammonium carbonate.

Example 4

hard
A student writes the formula for magnesium nitride as \text{MgN}. Explain why this is incorrect and determine the correct formula. (\text{Mg}^{2+}, \text{N}^{3-})
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