Chemistry worksheet for counting atoms in compounds, with examples and student answers.

Let's go through each problem on the worksheet step by step, carefully counting the number of atoms of each element in the given compounds. We'll use the subscripts and coefficients to determine the total number of atoms.

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1. 4K₂CO₃

- K: Each K₂CO₃ has 2 K atoms, and there are 4 formula units →
$ 4 \times 2 = 8 $ → K = 8
- C: Each K₂CO₃ has 1 C atom →
$ 4 \times 1 = 4 $ → C = 4
- O: Each K₂CO₃ has 3 O atoms →
$ 4 \times 3 = 12 $ → O = 12

✔ Your answers: K = 8, C = 4, O = 12 — ✔️ Correct!

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2. 2Sr₃(PO₄)₂

This is a compound with a polyatomic ion (PO₄³⁻). Let's break it down:

- Sr: Each formula unit has 3 Sr atoms, and we have 2 formula units →
$ 2 \times 3 = 6 $ → Sr = 6
- P: Each (PO₄)₂ group has 2 P atoms, so per formula unit: 2 P →
$ 2 \times 2 = 4 $ → P = 4
- O: Each PO₄ has 4 O atoms, and there are 2 PO₄ groups →
$ 2 \times 4 = 8 $ O per formula unit →
$ 2 \times 8 = 16 $ → O = 16

✔ Your answers: Sr = 6, P = 4, O = 16 — ✔️ Correct!

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3. 3N₂O₅

- N: Each N₂O₅ has 2 N atoms, and we have 3 molecules →
$ 3 \times 2 = 6 $ → N = 6
- O: Each N₂O₅ has 5 O atoms →
$ 3 \times 5 = 15 $ → O = 15

✘ You wrote: N = 12, O = 30 — ✘ Incorrect

👉 Correction:
N = 6, O = 15

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4. 2(NH₄)₃N

This is ammonium nitride. Let's analyze:

- (NH₄)₃N means:
- 3 NH₄⁺ ions → each NH₄ has 1 N and 4 H
- Plus 1 N³⁻ ion → extra nitrogen

So per formula unit:
- N from NH₄: 3 × 1 = 3 N
- N from N³⁻: 1 N
- Total N per formula unit: 3 + 1 = 4 N
- H: 3 × 4 = 12 H

Now multiply by 2 formula units:
- N: $ 2 \times 4 = 8 $
- H: $ 2 \times 12 = 24 $

✔ So:
- N = 8
- H = 24

✘ You wrote: N = 24, H = 24 — ✘ Incorrect

👉 Correction: N = 8, H = 24

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5. 8Cl₂O

- Cl: Each Cl₂O has 2 Cl → $ 8 \times 2 = 16 $ → Cl = 16
- O: Each Cl₂O has 1 O → $ 8 \times 1 = 8 $ → O = 8

✔ Your answers: Cl = 16, O = 8 — ✔️ Correct!

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6. Ca(C₂H₃O₂)₂

This is calcium acetate. The subscript "2" outside the parentheses applies to everything inside.

Breakdown per formula unit:
- Ca: 1
- C₂H₃O₂: two of these → so:
- C: $ 2 \times 2 = 4 $
- H: $ 2 \times 3 = 6 $
- O: $ 2 \times 2 = 4 $

So:
- Ca = 1
- C = 4
- H = 6
- O = 4

✔ Your answers: Ca = 1, C = 4, H = 6, O = 4 — ✔️ Correct!

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7. 12NaBr

- Na: Each NaBr has 1 Na → $ 12 \times 1 = 12 $
- Br: Each NaBr has 1 Br → $ 12 \times 1 = 12 $

✔ Your answers: Na = 12, Br = 12 — ✔️ Correct!

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✔ Final Correct Answers:

| Problem | Element | Quantity |
|--------|--------|---------|
| 1. | K | 8 |
| | C | 4 |
| | O | 12 |
| 2. | Sr | 6 |
| | P | 4 |
| | O | 16 |
| 3. | N | 6 |
| | O | 15 |
| 4. | N | 8 |
| | H | 24 |
| 5. | Cl | 16 |
| | O | 8 |
| 6. | Ca | 1 |
| | C | 4 |
| | H | 6 |
| | O | 4 |
| 7. | Na | 12 |
| | Br | 12 |

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🔍 Summary of Corrections:

- Problem 3: You wrote N = 12, O = 30 → should be N = 6, O = 15
- Problem 4: You wrote N = 24, H = 24 → should be N = 8, H = 24

All other answers are correct.

Let me know if you'd like a visual breakdown or explanation for any specific compound!