Youngwillturner: Gram formula mass - Free Printable

Let's go through each compound and calculate the gram formula mass (also known as molar mass) step by step. The gram formula mass is the sum of the atomic masses of all atoms in a compound, expressed in grams per mole (g/mol). We'll use standard atomic masses from the periodic table:

- K = 39 g/mol
- Mn = 55 g/mol
- O = 16 g/mol
- Cl = 35.5 g/mol
- Na = 23 g/mol
- S = 32 g/mol
- Ca = 40 g/mol
- N = 14 g/mol
- H = 1 g/mol
- P = 31 g/mol
- Cu = 64 g/mol
- Mg = 24 g/mol
- Zn = 65 g/mol
- C = 12 g/mol
- H₂O = 18 g/mol (since H₂O = 2×1 + 16 = 18)

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1. KMnO₄

- K: 39
- Mn: 55
- O₄: 4 × 16 = 64
→ 39 + 55 + 64 = 158 g/mol ✔

✔ Correct.

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2. KCl

- K: 39
- Cl: 35.5
→ 39 + 35.5 = 74.5 g/mol

But student wrote 74 — close, but not exact.

👉 Should be 74.5, not 74.

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3. Na₂SO₄

- Na₂: 2 × 23 = 46
- S: 32
- O₄: 4 × 16 = 64
→ 46 + 32 + 64 = 142 g/mol

Student wrote 143 — off by 1.

👉 Should be 142, not 143.

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4. Ca(NO₃)₂

- Ca: 40
- N₂: 2 × 14 = 28
- O₆: 6 × 16 = 96
→ 40 + 28 + 96 = 164 g/mol ✔

✔ Correct.

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5. Al₂(SO₄)₃

- Al₂: 2 × 27 = 54
- S₃: 3 × 32 = 96
- O₁₂: 12 × 16 = 192
→ 54 + 96 + 192 = 342 g/mol

Student wrote 315 — incorrect.

✘ Incorrect. Should be 342.

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6. (NH₄)₃PO₄

- N₃: 3 × 14 = 42
- H₁₂: 12 × 1 = 12
- P: 31
- O₄: 4 × 16 = 64
→ 42 + 12 + 31 + 64 = 149 g/mol ✔

✔ Correct.

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7. CuSO₄·5H₂O

This is a hydrate: copper sulfate pentahydrate.

- CuSO₄:
- Cu: 64
- S: 32
- O₄: 64 → 64 + 32 + 64 = 160
- 5H₂O: 5 × 18 = 90
→ 160 + 90 = 250 g/mol ✔

✔ Correct.

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8. Mg₃(PO₄)₂

- Mg₃: 3 × 24 = 72
- P₂: 2 × 31 = 62
- O₈: 8 × 16 = 128
→ 72 + 62 + 128 = 262 g/mol ✔

✔ Correct.

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9. Zn(C₂H₃O₂)₂·2H₂O

This is zinc acetate dihydrate.

First, break down Zn(C₂H₃O₂)₂:
- Zn: 65
- C₄: 4 × 12 = 48
- H₆: 6 × 1 = 6
- O₄: 4 × 16 = 64
→ 65 + 48 + 6 + 64 = 183

Now add 2H₂O: 2 × 18 = 36
→ 183 + 36 = 219 g/mol ✔

✔ Correct.

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Summary of Corrections:

| Compound | Student Answer | Correct Answer | Notes |
|--------|----------------|----------------|-------|
| 1. KMnO₄ | 158 | ✔ 158 | Correct |
| 2. KCl | 74 | ✘ 74.5 | Should be 74.5 |
| 3. Na₂SO₄ | 143 | ✘ 142 | Should be 142 |
| 4. Ca(NO₃)₂ | 164 | ✔ 164 | Correct |
| 5. Al₂(SO₄)₃ | 315 | ✘ 342 | Wrong calculation |
| 6. (NH₄)₃PO₄ | 149 | ✔ 149 | Correct |
| 7. CuSO₄·5H₂O | 250 | ✔ 250 | Correct |
| 8. Mg₃(PO₄)₂ | 262 | ✔ 262 | Correct |
| 9. Zn(C₂H₃O₂)₂·2H₂O | 219 | ✔ 219 | Correct |

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Final Answers (Corrected):

1. 158
2. 74.5
3. 142
4. 164
5. 342
6. 149
7. 250
8. 262
9. 219

✔ Only #1, #4, #6, #7, #8, #9 are correct.
✘ #2, #3, #5 need correction.

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