Calculating Molecular Weight: A Step-by-Step Guide
Atomic mass and Molecular mass • April 2026

Calculating Molecular Weight
A Step-by-Step Guide

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Written By Archive Editorial
Reading Time 5 Min Read

Understanding the Mole Concept

In chemistry, a mole represents a specific amount of substance, containing exactly $6.022 \times 10^{23}$ particles (Avogadro's number). When a problem states that "1 mole of a compound contains 1 mole of carbon (C) and 1 mole of oxygen (O)," it is providing the ratio of atoms present in a single molecule.

Breaking Down the Problem

The information given is:

  • 1 mole of compound = 1 mole of Carbon (C) + 1 mole of Oxygen (O)
  • This indicates the chemical formula of the compound is CO (Carbon Monoxide).

Step-by-Step Calculation

To find the molecular weight (molar mass) of the compound, we sum the atomic masses of all the elements present in the molecule.

Step 1: Identify the atomic masses of the elements Using the periodic table:

  • Atomic mass of Carbon (C) $\approx 12.01 \text{ g/mol}$
  • Atomic mass of Oxygen (O) $\approx 16.00 \text{ g/mol}$

Step 2: Apply the formula Since the formula is CO, we calculate the total molar mass as follows: $$\text{Molecular Weight} = (1 \times \text{Mass of C}) + (1 \times \text{Mass of O})$$

Step 3: Solve the equation $$\text{Molecular Weight} = 12.01 \text{ g/mol} + 16.00 \text{ g/mol} = 28.01 \text{ g/mol}$$

Conclusion

The molecular weight of the compound (Carbon Monoxide, CO) is approximately 28.01 g/mol.

Intuition Check

Remember that the molar mass of a compound is always the sum of the atomic masses of its constituent elements, multiplied by the number of atoms of each element present in the empirical formula. If you are ever unsure, start by writing the chemical formula based on the molar ratios provided!

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