Percent composition worksheet - Free Printable

The image presents Sample Problem 3, which asks to find the percent composition by mass of:

- (a) Strontium (Sr)
- (b) Oxygen (O)
- (c) Water (H₂O)

in the compound: Sr₃(PO₄)₂·3H₂O (strontium phosphate trihydrate).

Let’s go through the solution step by step, explaining each part.

---

🔬 Step 1: Understand the Compound

The compound is Sr₃(PO₄)₂·3H₂O, meaning:
- It contains Sr₃(PO₄)₂ (strontium phosphate)
- And 3 water molecules (H₂O) attached as water of hydration.

So, we need to consider:
- Atoms from Sr₃(PO₄)₂
- Plus atoms from 3 H₂O molecules

---

🧮 Step 2: Determine Molar Mass of Each Element in the Compound

We'll use atomic masses:
- Sr = 88 g/mol
- P = 31 g/mol
- O = 16 g/mol
- H = 1 g/mol

Now break down the formula:

#### ✔ Strontium (Sr):
- 3 atoms × 88 g/mol = 264 g

#### ✔ Phosphorus (P):
- 2 atoms (from PO₄) × 31 g/mol = 62 g

#### ✔ Oxygen from PO₄:
- Each PO₄ has 4 O atoms → 2 PO₄ groups → 8 O atoms
- 8 × 16 = 128 g

#### ✔ Water (3H₂O):
- Each H₂O has 2 H and 1 O
- So 3 H₂O → 6 H and 3 O
- Hydrogen: 6 × 1 = 6 g
- Oxygen: 3 × 16 = 48 g

#### 🔢 Total Molar Mass:
Add all contributions:
- Sr: 264
- P: 62
- O (from PO₄): 128
- H: 6
- O (from H₂O): 48
→ Total = 264 + 62 + 128 + 6 + 48 = 508 g/mol

✔ So, molar mass of Sr₃(PO₄)₂·3H₂O = 508 g/mol

---

📊 Step 3: Calculate Percent Composition

#### (a) Percent of Strontium (Sr)

$$
\% \text{Sr} = \frac{\text{Mass of Sr}}{\text{Molar Mass}} \times 100 = \frac{264}{508} \times 100 \approx 52.0\%
$$

✔ Answer: 52.0%

---

#### (b) Percent of Oxygen (O)

We must include all oxygen atoms:
- From PO₄: 128 g
- From H₂O: 48 g
- Total O = 128 + 48 = 176 g

$$
\% \text{O} = \frac{176}{508} \times 100 \approx 34.6\%
$$

✔ Answer: 34.6%

---

#### (c) Percent of Water (H₂O)

Water contributes:
- H: 6 g
- O: 48 g
- Total mass of water = 6 + 48 = 54 g

Note: We treat the entire water molecule as a unit for percent composition.

$$
\% \text{Water} = \frac{54}{508} \times 100 \approx 10.6\%
$$

✔ Answer: 10.6%

---

✔ Final Answers:

| Component | Percent Composition |
|---------------|---------------------|
| (a) Strontium | 52.0% |
| (b) Oxygen | 34.6% |
| (c) Water | 10.6% |

---

💡 Explanation Summary:

- Percent composition tells us what fraction of the total mass comes from each element or group.
- For hydrated compounds, water is treated as a separate component.
- The total molar mass includes all atoms, including those in water molecules.
- Always double-check subscripts and coefficients (especially in hydrates like ·3H₂O).

This method can be applied to any hydrate or complex compound.

✔ Correct and complete!