How can you calculate the molecular weight of a compound?

• A. By measuring its density in water
• B. By summing the atomic weights of all atoms in the molecule
• C. By counting the number of bonds in the molecule
• D. By dividing its mass by its volume

Answer: (B)

To calculate the molecular weight of a compound, summing the atomic weights of all atoms in the molecule is the correct approach. Each element has a specific atomic weight, usually found on the periodic table, expressed in atomic mass units (amu). For a given molecule, you take into account each type of atom present, considering how many of each type are in the molecule. For example, if a molecule is made up of two carbon atoms, six hydrogen atoms, and one oxygen atom (like glucose, C6H12O6), you would sum the atomic weights of each carbon, hydrogen, and oxygen according to their quantities to get the overall molecular weight.

This method provides a direct and empirical way of calculating the molecular weight, reflecting the composition of the molecule based on the masses of the constituent atoms. The other options listed do not provide a method for accurately determining molecular weight; while density and volume may relate to other properties, they do not relate directly to the atomic composition that defines molecular weight.