Calculate the molar mass of calcium sulfate dihydrate ( CaSO4 · 2 H2O)
The Correct Answer and Explanation is:
Correct Answer:
The molar mass of calcium sulfate dihydrate (CaSO₄·2H₂O) is 172.17 g/mol.
Explanation:
To calculate the molar mass of calcium sulfate dihydrate, begin by identifying all the elements present in the compound and the number of atoms of each.
Calcium sulfate dihydrate (CaSO₄·2H₂O) contains:
- 1 calcium (Ca) atom
- 1 sulfur (S) atom
- 4 oxygen (O) atoms from the sulfate ion (SO₄²⁻)
- 2 water molecules (2 H₂O), each containing 2 hydrogen (H) atoms and 1 oxygen (O) atom
Break the calculation down into parts by using standard atomic masses:
- Calcium (Ca): 40.08 g/mol
- Sulfur (S): 32.07 g/mol
- Oxygen (O): 16.00 g/mol
- Hydrogen (H): 1.008 g/mol
Now, compute the contribution of each part:
- Calcium (1 atom):
1 × 40.08 = 40.08 g/mol - Sulfur (1 atom):
1 × 32.07 = 32.07 g/mol - Oxygen in SO₄ (4 atoms):
4 × 16.00 = 64.00 g/mol - Water (2 H₂O molecules):
- Hydrogen: 2 molecules × 2 H = 4 H → 4 × 1.008 = 4.032 g/mol
- Oxygen: 2 × 1 O = 2 O → 2 × 16.00 = 32.00 g/mol
Total from water: 4.032 + 32.00 = 36.032 g/mol
Add all parts together:
40.08 + 32.07 + 64.00 + 36.032 = 172.182 g/mol
Rounded to two decimal places, the molar mass is 172.17 g/mol.
Understanding molar mass is essential in stoichiometry and other chemical calculations, as it links grams to moles, a core concept in chemistry. For hydrated compounds like calcium sulfate dihydrate, the water molecules must always be included in the total molar mass calculation to ensure accurate results.
