Calculate the relative molecular mass of Limestone CaCO3, (Ca = 40, C =12, O = 16)

Analysis

To find the relative molecular mass (also known as molar mass) of a compound, you must sum the relative atomic masses of all the individual atoms present in the chemical formula.

Concept

The formula for Limestone is CaCO₃. This means one molecule consists of:

• 1 atom of Calcium (Ca)
• 1 atom of Carbon (C)
• 3 atoms of Oxygen (O)

The formula to calculate the total mass is:

Step-by-Step Solution

1. Identify the atomic masses provided: • Ca = 40 • C = 12 • O = 16

2. Substitute the values into the formula: • Mass of Ca = $1 \times 40 = 40$ • Mass of C = $1 \times 12 = 12$ • Mass of O = $3 \times 16 = 48$ (Since there are 3 oxygen atoms)

3. Sum the values: • Total Mass = $40 + 12 + 48$ • Total Mass = $100$

The calculation confirms that the relative molecular mass of CaCO₃ is 100.

Exam Tip

When calculating molecular mass, always pay close attention to the subscripts (the small numbers after an element). A common mistake is forgetting to multiply the mass of Oxygen by 3. Always write out the count of each atom before you start multiplying to avoid silly errors!