Molar Mass Worksheet - Elements with problems for calculating mass and moles.

Sure! Let’s solve both parts of this Molar Mass Worksheet – Elements step by step.

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## 🔢 KEY FORMULAS:

- Mass (g) = moles × molar mass (g/mol)
- Moles = mass (g) ÷ molar mass (g/mol)

We’ll use the atomic masses from the periodic table (rounded to 2 decimal places for accuracy, as commonly used in high school chemistry):

| Element | Molar Mass (g/mol) |
|---------|---------------------|
| Na | 22.99 |
| Ca | 40.08 |
| Mg | 24.31 |
| Al | 26.98 |
| H | 1.008 |
| Hg | 200.59 |
| Li | 6.94 |
| Ne | 20.18 |
| Zn | 65.38 |
| Br | 79.90 |

*(Note: Some sources may round H to 1.01 or 1.00 — we’ll use 1.008 for precision.)*

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# ✔ PART 1: Calculate the mass in grams for each of the following:

> Use: Mass = moles × molar mass

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a. 3.00 moles Na

Mass = 3.00 mol × 22.99 g/mol = 68.97 g

b. 2.50 moles Ca

Mass = 2.50 mol × 40.08 g/mol = 100.20 g

c. 5.00 moles Mg

Mass = 5.00 mol × 24.31 g/mol = 121.55 g

d. 4.00 moles Al

Mass = 4.00 mol × 26.98 g/mol = 107.92 g

e. 3.00 moles H

Mass = 3.00 mol × 1.008 g/mol = 3.024 g → Round to 3.02 g (if using 3 significant figures)

> *Note: Since 3.00 has 3 sig figs, and 1.008 has 4, answer should have 3 sig figs → 3.02 g*

f. 0.275 moles Hg

Mass = 0.275 mol × 200.59 g/mol = 55.16225 g → Round to 55.2 g (3 sig figs)

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✔ PART 1 Answers Summary:

| | Moles | Molar Mass (g/mol) | Mass (g) |
|---|-----------|--------------------|------------|
| a | 3.00 Na | 22.99 | 68.97 |
| b | 2.50 Ca | 40.08 | 100.20 |
| c | 5.00 Mg | 24.31 | 121.55 |
| d | 4.00 Al | 26.98 | 107.92 |
| e | 3.00 H | 1.008 | 3.02 |
| f | 0.275 Hg | 200.59 | 55.2 |

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# ✔ PART 2: Calculate the number of moles for each of the following:

> Use: Moles = mass ÷ molar mass

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a. 25.0 grams Li

Moles = 25.0 g ÷ 6.94 g/mol = 3.60 mol *(3 sig figs)*

b. 60.0 grams Ne

Moles = 60.0 g ÷ 20.18 g/mol = 2.97 mol *(3 sig figs)*

c. 180.0 grams Ca

Moles = 180.0 g ÷ 40.08 g/mol = 4.491 mol → 4.49 mol *(3 sig figs)*

d. 150.0 grams Zn

Moles = 150.0 g ÷ 65.38 g/mol = 2.295 mol → 2.30 mol *(3 sig figs)*

e. 160.0 grams Br

Moles = 160.0 g ÷ 79.90 g/mol = 2.003 mol → 2.00 mol *(3 sig figs)*

f. 11.9 grams of Al

Moles = 11.9 g ÷ 26.98 g/mol = 0.441 mol *(3 sig figs)*

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✔ PART 2 Answers Summary:

| | Mass (g) | Molar Mass (g/mol) | # of Moles |
|---|-----------|--------------------|------------|
| a | 25.0 Li | 6.94 | 3.60 |
| b | 60.0 Ne | 20.18 | 2.97 |
| c | 180.0 Ca | 40.08 | 4.49 |
| d | 150.0 Zn | 65.38 | 2.30 |
| e | 160.0 Br | 79.90 | 2.00 |
| f | 11.9 Al | 26.98 | 0.441 |

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## 📝 Final Notes:

- Always pay attention to significant figures based on the given values.
- Molar masses are usually taken from the periodic table — slight variations may occur depending on your textbook or teacher’s preference (e.g., H = 1.01 vs 1.008). We used precise values here.
- The key concept is understanding the relationship:
moles ↔ mass ↔ molar mass

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