Stoichiometry of minerals:\r Calcium carbonate (CaCO3) contains what percentage by mass of calcium (Ca)?

Introduction:
Weight-percent composition is frequently used in materials balances, fertilizer analysis (e.g., Ca as CaCO3), and hardness calculations. Calcium carbonate is a common reference compound in water treatment and geology.

Given Data / Assumptions:

• Molar masses (approximate): Ca = 40 g/mol, C = 12 g/mol, O = 16 g/mol.
• Formula: CaCO3.
• Percent by mass is computed as element mass divided by formula mass times 100.

Concept / Approach:
The formula mass of CaCO3 is 40 (Ca) + 12 (C) + 3*16 (O) = 100 g/mol (to a good approximation). The calcium mass fraction is then 40/100, or 40%. Using more precise atomic weights (Ca 40.08, C 12.01, O 16.00) gives 40.08/100.09 ≈ 40.0%, confirming the rounded value.

Step-by-Step Solution:
Compute formula mass: 40 + 12 + 48 = 100 g/mol.Mass of Ca per mole: 40 g.Percent Ca = (40 / 100) * 100% = 40%.

Verification / Alternative check:
Exact atomic weights change the result only in the second decimal place, so 40% is a standard taught value.

Why Other Options Are Wrong:

• 48%: This is close to the oxygen fraction per atom but not Ca.
• 96% or 12%: Physically impossible based on the stoichiometry.
• 56%: No element in CaCO3 has this fraction.

Common Pitfalls:
Forgetting there are three oxygens or mis-adding the atomic masses.

Final Answer:
40