Practice Formula Writing in Chemistry

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

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Quick Recap

The systematic process of combining element symbols and numerical subscripts to represent the exact composition of a chemical compound, ensuring that the total positive charge from the cations balances the total negative charge from the anions so the compound is electrically neutral.

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

Showing a random 20 of 50 problems.

Example 1

hard
Write the formula for ammonium dichromate (NH4+\text{NH}_4^+ and Cr2O72−\text{Cr}_2\text{O}_7^{2-}).

Example 2

medium
Why must Al2O3Al_2O_3 (not Al2O2Al_2O_2) form from Al3+Al^{3+} and O2−O^{2-}?

Example 3

hard
Write the formula for the binary covalent compound dinitrogen pentoxide.

Example 4

medium
Write the formula for ammonium sulfate (NH4+NH_4^+ and SO42−SO_4^{2-}).

Example 5

medium
Write the formula for iron(III) oxide.

Example 6

easy
Write the formula for potassium oxide (K+K^+ and O2−O^{2-}).

Example 7

medium
Write the formula for copper(I) sulfide using the Stock system.

Example 8

easy
Write the formula for Al3+Al^{3+} and O2−O^{2-}.

Example 9

easy
Write the formula for the nitrate ion.

Example 10

hard
A sample contains 40.0%40.0\% S and 60.0%60.0\% O by mass. Find its empirical formula.

Example 11

hard
Combustion analysis of 0.5000.500 g of a hydrocarbon yields 1.5711.571 g CO2_2 and 0.6430.643 g H2_2O. Find its empirical formula.

Example 12

medium
Write the formula for aluminum nitrate (Al3+\text{Al}^{3+} and NO3−\text{NO}_3^-).

Example 13

easy
Write the formula for Mg2+Mg^{2+} and Cl−Cl^-.

Example 14

easy
What charge does the ammonium ion carry?

Example 15

easy
Write the chemical formula for aluminum chloride using the charges of the ions: Al3+\text{Al}^{3+} and Cl−\text{Cl}^-.

Example 16

medium
Write the chemical formula for calcium phosphate using the crisscross method. Calcium forms Ca2+\text{Ca}^{2+} and phosphate is PO43−\text{PO}_4^{3-}.

Example 17

easy
Write the chemical formula for barium chloride (Ba2+\text{Ba}^{2+} and Cl−\text{Cl}^-).

Example 18

medium
Write the formula for iron(III) hydroxide.

Example 19

medium
Distinguish: in 3H2O3H_2O, what does the 3 represent versus the 2?

Example 20

easy
Write the formula for potassium bromide.
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