Counting atoms in a formula
A subscript after brackets multiplies everything inside.
The common mistake
Applying the bracket subscript to only the last element: Ca(OH)₂ counted as 1 O and 2 H.
Instead: Multiply each atom inside the brackets by the subscript outside, then by any coefficient in front.
Worked example with Guide me
How many atoms of each element are in Ca(OH)₂?
- Sugar · Calcium?Student · 1.
- Sugar · What does the 2 after the bracket multiply?Student · Both O and H.
Answer: 1 Ca, 2 O, 2 H
Try 3 practice questions
1. Atoms in Al₂(SO₄)₃?
Show answer
2 Al, 3 S, 12 O
2. Oxygen atoms in 3H₂O?
Show answer
3
3. Hydrogen atoms in 2NH₃?
Show answer
6
Quick answers
- What is the most common mistake with counting atoms in a formula?
- Applying the bracket subscript to only the last element: Ca(OH)₂ counted as 1 O and 2 H.
- How do I get counting atoms in a formula right?
- Multiply each atom inside the brackets by the subscript outside, then by any coefficient in front.
- Can you show a worked example?
- How many atoms of each element are in Ca(OH)₂? 1 Ca, 2 O, 2 H
Related concepts
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