PREVIOUSLY ASKED IN:
WBPSC Miscellaneous Preliminary 2019
Answer
40
Explanation
To find the percentage of Calcium in Calcium Carbonate, we first determine its chemical formula, which is CaCO3. Then, we calculate the total molar mass of the compound. Atomic mass of Calcium (Ca) = 40 u Atomic mass of Carbon (C) = 12 u Atomic mass of Oxygen (O) = 16 u (Since there are 3 oxygen atoms, 16 × 3 = 48 u) Total Molecular Mass of CaCO3 = 40 + 12 + 48 = 100 u. The percentage of Calcium is calculated as: (Mass of Ca / Total Mass) × 100 = (40 / 100) × 100 = 40%.
Key Points
- > Calcium Carbonate (CaCO3) naturally occurs as limestone, chalk, and marble.
- > The shells of marine organisms, snails, and eggshells are predominantly made of calcium carbonate.
- > It reacts with acids (like in acid rain) to release carbon dioxide, which is why marble structures deteriorate over time.
- > Heating CaCO3 produces Calcium Oxide (quicklime) and Carbon Dioxide gas (thermal decomposition).
- > It is an active ingredient in agricultural lime, used to neutralize acidic soil.
Additional Information
Common Compounds & Their Uses
| Common Name | Chemical Name | Formula | Major Use |
|---|---|---|---|
| Limestone / Marble | Calcium Carbonate | CaCO3 | Cement manufacturing |
| Quicklime | Calcium Oxide | CaO | Making bleaching powder |
| Slaked Lime | Calcium Hydroxide | Ca(OH)2 | Whitewashing walls |
| Baking Soda | Sodium Bicarbonate | NaHCO3 | Baking, Antacids |
Memory Tips
- Magic Number 100: The molar mass of CaCO3 is exactly 100 g/mol. This makes calculating percentages trivial. Whatever the atomic mass of the element is in the molecule, that is precisely its percentage. (Ca=40, therefore 40%).