Free Printable Percent Composition Worksheets - Free Printable

Let's solve each question step-by-step in the Percent Composition by Mass Worksheet.

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1. What is the percent composition by mass for each element in sodium phosphate, Na₃PO₄?

Step 1: Find molar mass of Na₃PO₄
- Na: 23.0 g/mol × 3 = 69.0 g/mol
- P: 31.0 g/mol × 1 = 31.0 g/mol
- O: 16.0 g/mol × 4 = 64.0 g/mol
Total Molar Mass = 69.0 + 31.0 + 64.0 = 164.0 g/mol

Step 2: Percent composition
- % Na = (69.0 / 164.0) × 100 ≈ 42.07%
- % P = (31.0 / 164.0) × 100 ≈ 18.90%
- % O = (64.0 / 164.0) × 100 ≈ 39.02%

✔ Answer:
- Sodium (Na): 42.07%
- Phosphorus (P): 18.90%
- Oxygen (O): 39.02%

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2. What is the percent composition by mass of each element in hydrogen peroxide, H₂O₂?

Step 1: Molar mass of H₂O₂
- H: 1.0 g/mol × 2 = 2.0 g/mol
- O: 16.0 g/mol × 2 = 32.0 g/mol
Total = 34.0 g/mol

Step 2: Percent composition
- % H = (2.0 / 34.0) × 100 ≈ 5.88%
- % O = (32.0 / 34.0) × 100 ≈ 94.12%

✔ Answer:
- Hydrogen (H): 5.88%
- Oxygen (O): 94.12%

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3. Find the percent composition of Fe₃O₄

Step 1: Molar mass of Fe₃O₄
- Fe: 55.8 g/mol × 3 = 167.4 g/mol
- O: 16.0 g/mol × 4 = 64.0 g/mol
Total = 231.4 g/mol

Step 2: Percent composition
- % Fe = (167.4 / 231.4) × 100 ≈ 72.34%
- % O = (64.0 / 231.4) × 100 ≈ 27.66%

✔ Answer:
- Iron (Fe): 72.34%
- Oxygen (O): 27.66%

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4. Find the percent composition of Ca₃(PO₄)₂

Step 1: Molar mass of Ca₃(PO₄)₂
- Ca: 40.1 g/mol × 3 = 120.3 g/mol
- P: 31.0 g/mol × 2 = 62.0 g/mol
- O: 16.0 g/mol × 8 = 128.0 g/mol
Total = 120.3 + 62.0 + 128.0 = 310.3 g/mol

Step 2: Percent composition
- % Ca = (120.3 / 310.3) × 100 ≈ 38.77%
- % P = (62.0 / 310.3) × 100 ≈ 19.98%
- % O = (128.0 / 310.3) × 100 ≈ 41.25%

✔ Answer:
- Calcium (Ca): 38.77%
- Phosphorus (P): 19.98%
- Oxygen (O): 41.25%

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5. In an experiment, 2.0 moles of oxygen atoms react with 80.1 g of calcium to form a new compound. What is the percent composition of each element in the new compound?

Step 1: Find mass of oxygen
- Moles of O = 2.0 mol
- Molar mass of O = 16.0 g/mol
- Mass of O = 2.0 mol × 16.0 g/mol = 32.0 g

Step 2: Mass of calcium = 80.1 g (given)

Step 3: Total mass of compound = 80.1 g + 32.0 g = 112.1 g

Step 4: Percent composition
- % Ca = (80.1 / 112.1) × 100 ≈ 71.45%
- % O = (32.0 / 112.1) × 100 ≈ 28.55%

✔ Answer:
- Calcium (Ca): 71.45%
- Oxygen (O): 28.55%

> Note: This corresponds to CaO (calcium oxide), but we're only asked for percent composition.

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6. In an experiment, 0.05 moles of iron are mixed with 0.05 moles of sulfur and heated to form a new compound. What is the percent mass of each element in the new compound?

Step 1: Moles of Fe = 0.05 mol, Moles of S = 0.05 mol
→ Ratio is 1:1 → Compound is FeS

Step 2: Mass of each element
- Fe: 0.05 mol × 55.8 g/mol = 2.79 g
- S: 0.05 mol × 32.1 g/mol = 1.605 g

Step 3: Total mass = 2.79 + 1.605 = 4.395 g

Step 4: Percent composition
- % Fe = (2.79 / 4.395) × 100 ≈ 63.48%
- % S = (1.605 / 4.395) × 100 ≈ 36.52%

✔ Answer:
- Iron (Fe): 63.48%
- Sulfur (S): 36.52%

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✔ Final Answers Summary:

| Question | Element(s) | Percent Composition |
|--------|------------|---------------------|
| 1 | Na: 42.07%, P: 18.90%, O: 39.02% |
| 2 | H: 5.88%, O: 94.12% |
| 3 | Fe: 72.34%, O: 27.66% |
| 4 | Ca: 38.77%, P: 19.98%, O: 41.25% |
| 5 | Ca: 71.45%, O: 28.55% |
| 6 | Fe: 63.48%, S: 36.52% |

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