Let’s go through each problem one by one. We’ll count how many atoms of each element are in the compound, multiplied by the number in front (the coefficient).
Remember:
- The number in front multiplies EVERYTHING in the formula.
- Subscripts tell you how many atoms of that element are in one unit.
- If there’s a parenthesis, multiply everything inside by the subscript outside the parenthesis — then multiply by the coefficient.
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1. 4K₂CO₃
- K: 2 per unit × 4 =
8
- C: 1 per unit × 4 =
4
- O: 3 per unit × 4 =
12
✔ Correct as written.
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2. 2Sr₃(PO)₂
First, look inside the parentheses: PO₄ has P=1, O=4. But it’s (PO₄)₂ → so P=2, O=8 per Sr₃(PO₄)₂ unit.
Then multiply by 2:
- Sr: 3 × 2 =
6
- P: 2 × 2 =
4
- O: 8 × 2 =
16
✔ Correct as written.
---
3. 3N₄O₁₀
- N: 4 × 3 =
12
- O: 10 × 3 =
30
✔ Correct as written.
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4. 2(NH₄)₃N
Break down (NH₄)₃N:
Inside (NH₄)₃: N=3, H=12
Plus one more N at the end → total N = 3 + 1 = 4 per unit
H = 12 per unit
Now multiply by 2:
- N: 4 × 2 =
8 ← but student wrote “8” with a circle and crossed out? Wait — let’s check again.
Wait! Actually, (NH₄)₃N means:
Each NH₄ has 1 N and 4 H → times 3 → 3 N and 12 H
Then plus one more N → total N = 4, H = 12 per formula unit.
Multiply by 2:
→ N = 8, H = 24
Student wrote N = 8 (circled), H = 24 →
✔ correct.
But they had scribbles — maybe confused themselves. Final answer is right.
---
5. 8Cl₂O
- Cl: 2 × 8 =
16
- O: 1 × 8 =
8
✔ Correct.
---
6. Ca(C₂H₃O₂)₂
No coefficient? So just 1 unit.
Inside (C₂H₃O₂)₂:
C: 2×2 = 4
H: 3×2 = 6
O: 2×2 = 4
Ca: 1
So:
- Ca =
1
- C =
4
- H =
6
- O =
4
✔ All correct.
---
7. 12NaBr
- Na: 1 × 12 =
12
- Br: 1 × 12 =
12
✔ Correct.
---
8. 4Al(OH)₃
(OH)₃ → O=3, H=3 per Al(OH)₃
Multiply by 4:
- Al: 1 × 4 =
4
- O: 3 × 4 =
12
- H: 3 × 4 =
12
✔ Correct.
---
9. 3NaHCO₃
Per unit: Na=1, H=1, C=1, O=3
×3:
- Na =
3
- H =
3
- C =
3
- O =
9
✔ Correct.
---
10. 5Ga₂(Cr₂O₇)₃
First, (Cr₂O₇)₃ → Cr=6, O=21 per Ga₂(Cr₂O₇)₃
Then Ga=2
Now multiply entire thing by 5:
- Ga: 2 × 5 =
10
- Cr: 6 × 5 =
30
- O: 21 × 5 =
105
✔ Correct.
---
11. 7C₂S₂
- C: 2 × 7 =
14
- S: 2 × 7 =
14
✔ Correct.
---
12. 4Fe₂O₃
- Fe: 2 × 4 =
8
- O: 3 × 4 =
12
✔ Correct.
---
13. 6Ba(MnO₄)₂
(MnO₄)₂ → Mn=2, O=8 per Ba(MnO₄)₂
×6:
- Ba: 1 × 6 =
6
- Mn: 2 × 6 =
12
- O: 8 × 6 =
48
✔ Correct.
---
14. 3V₂O₅
- V: 2 × 3 =
6
- O: 5 × 3 =
15
✔ Correct.
---
15. 2KNO₃
Per unit: K=1, N=1, O=3
×2:
- K =
2
- N =
2
- O =
6
Student wrote K=2, N=2, O=6 →
✔ correct.
---
16. 9MgSO₄
Per unit: Mg=1, S=1, O=4
×9:
- Mg =
9
- S =
9
- O =
36
✔ Correct.
---
17. 5Al₂(SiO₃)₂
(SiO₃)₂ → Si=2, O=6 per Al₂(SiO₃)₂
Al=2
×5:
- Al: 2 × 5 =
10
- Si: 2 × 5 =
10
- O: 6 × 5 =
30
✔ Correct.
---
18. 4Au(IO₃)₃
(IO₃)₃ → I=3, O=9 per Au(IO₃)₃
Au=1
×4:
- Au: 1 × 4 =
4
- I: 3 × 4 =
12
- O: 9 × 4 =
36
✔ Correct.
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All answers on the worksheet are actually correct! Even though some have scribbles or circles, the final numbers written are accurate.
Final Answer:
All atom counts provided in the worksheet are correct.
Parent Tip: Review the logic above to help your child master the concept of counting atoms worksheet answers.
