Let's solve each problem step by step to determine the
total number of atoms in each chemical formula.
We need to count
all the atoms present in one molecule or formula unit of each compound. Pay attention to subscripts and parentheses — when a subscript is outside parentheses, it applies to everything inside.
---
1. NaCl
- Na: 1
- Cl: 1
→
Total = 2 atoms
---
2. H₂SO₄
- H: 2
- S: 1
- O: 4
→
Total = 2 + 1 + 4 = 7 atoms
---
3. KNO₃
- K: 1
- N: 1
- O: 3
→
Total = 1 + 1 + 3 = 5 atoms
---
4. CaCl₂
- Ca: 1
- Cl: 2
→
Total = 1 + 2 = 3 atoms
---
5. C₂H₆
- C: 2
- H: 6
→
Total = 2 + 6 = 8 atoms
---
6. Ba(OH)₂
- Ba: 1
- (OH)₂ means 2 O and 2 H
→ O: 2, H: 2
→
Total = 1 + 2 + 2 = 5 atoms
---
7. NH₄Br
- N: 1
- H: 4
- Br: 1
→
Total = 1 + 4 + 1 = 6 atoms
---
8. Ca₃(PO₄)₂
- Ca: 3
- PO₄ group appears twice → P: 2, O: 4 × 2 = 8
→
Total = 3 + 2 + 8 = 13 atoms
---
9. Al₂(SO₄)₃
- Al: 2
- SO₄ group appears three times → S: 3, O: 4 × 3 = 12
→
Total = 2 + 3 + 12 = 17 atoms
---
10. Mg(NO₃)₂
- Mg: 1
- NO₃ appears twice → N: 2, O: 3 × 2 = 6
→
Total = 1 + 2 + 6 = 9 atoms
---
11. Cu(NO₃)₂
- Cu: 1
- NO₃ appears twice → N: 2, O: 6
→
Total = 1 + 2 + 6 = 9 atoms
---
12. KMnO₄
- K: 1
- Mn: 1
- O: 4
→
Total = 1 + 1 + 4 = 6 atoms
---
13. H₂O₂
- H: 2
- O: 2
→
Total = 2 + 2 = 4 atoms
---
14. H₃PO₄
- H: 3
- P: 1
- O: 4
→
Total = 3 + 1 + 4 = 8 atoms
---
15. (NH₄)₃PO₄
- NH₄ appears three times → N: 3, H: 4 × 3 = 12
- P: 1
- O: 4
→
Total = 3 + 12 + 1 + 4 = 20 atoms
---
16. Fe₂O₃
- Fe: 2
- O: 3
→
Total = 2 + 3 = 5 atoms
---
17. NaC₂H₃O₂
- Na: 1
- C: 2
- H: 3
- O: 2
→
Total = 1 + 2 + 3 + 2 = 8 atoms
---
18. Mg(C₂H₃O₂)₂
- Mg: 1
- (C₂H₃O₂)₂ → C: 2×2 = 4, H: 3×2 = 6, O: 2×2 = 4
→
Total = 1 + 4 + 6 + 4 = 15 atoms
---
19. Hg₂Cl₂
- Hg: 2 (the subscript applies to Hg₂)
- Cl: 2
→
Total = 2 + 2 = 4 atoms
---
20. K₂SO₃
- K: 2
- S: 1
- O: 3
→
Total = 2 + 1 + 3 = 6 atoms
---
✔ Final Answers:
| # | Formula | Total Atoms |
|---|-------------------|-------------|
| 1 | NaCl | 2 |
| 2 | H₂SO₄ | 7 |
| 3 | KNO₃ | 5 |
| 4 | CaCl₂ | 3 |
| 5 | C₂H₆ | 8 |
| 6 | Ba(OH)₂ | 5 |
| 7 | NH₄Br | 6 |
| 8 | Ca₃(PO₄)₂ | 13 |
| 9 | Al₂(SO₄)₃ | 17 |
|10 | Mg(NO₃)₂ | 9 |
|11 | Cu(NO₃)₂ | 9 |
|12 | KMnO₄ | 6 |
|13 | H₂O₂ | 4 |
|14 | H₃PO₄ | 8 |
|15 | (NH₄)₃PO₄ | 20 |
|16 | Fe₂O₃ | 5 |
|17 | NaC₂H₃O₂ | 8 |
|18 | Mg(C₂H₃O₂)₂ | 15 |
|19 | Hg₂Cl₂ | 4 |
|20 | K₂SO₃ | 6 |
---
✔ Explanation Summary:
- Count each atom using subscripts.
- If there are
parentheses, multiply the atoms inside by the subscript outside.
- Add all atoms together for the total per formula unit.
Let me know if you'd like this as a printable answer key!
Parent Tip: Review the logic above to help your child master the concept of chemical formula worksheet.
