To find the molecular weight (also known as molar mass or molecular mass) of a compound, you need to calculate the sum of the atomic masses of all the atoms in the molecule. Here are the steps to find the molecular weight:
Write Down the Chemical Formula: Start by writing down the chemical formula of the compound whose molecular weight you want to find. Ensure that the formula is correctly balanced and represents the actual composition of the molecule.
Determine the Number of Each Type of Atom: Count the number of each type of atom (element) present in the compound. The subscripts in the chemical formula indicate the number of atoms of each element.
Look Up Atomic Masses: Refer to the periodic table to find the atomic mass (also called atomic weight) of each element in the compound. Atomic masses are typically expressed in atomic mass units (amu) or grams per mole (g/mol).
Calculate the Contribution of Each Element: Multiply the atomic mass of each element by the number of atoms of that element in the compound. This will give you the contribution of each element to the total molecular weight.
Sum Up the Contributions: Add up all the contributions from step 4 to find the total molecular weight of the compound.
Here’s an example to illustrate the process:
Let’s find the molecular weight of glucose (C6H12O6):
Chemical formula: C6H12O6
Number of atoms of each element:
Carbon (C) = 6 atoms
Hydrogen (H) = 12 atoms
Oxygen (O) = 6 atoms
Atomic masses (from the periodic table):
Carbon (C) atomic mass ≈ 12.01 g/mol
Hydrogen (H) atomic mass ≈ 1.01 g/mol
Oxygen (O) atomic mass ≈ 16.00 g/mol
Calculate the contribution of each element:
Carbon (C): 6 atoms * 12.01 g/mol = 72.06 g/mol
Hydrogen (H): 12 atoms * 1.01 g/mol = 12.12 g/mol
Oxygen (O): 6 atoms * 16.00 g/mol = 96.00 g/mol
Sum up the contributions: Molecular weight of glucose (C6H12O6) = 72.06 g/mol + 12.12 g/mol + 96.00 g/mol = 180.18 g/mol
So, the molecular weight of glucose is approximately 180.18 grams per mole (g/mol).
The molecular weight is essential for various chemical calculations, such as determining the amount of substance in moles (using the formula: moles = mass / molecular weight) and stoichiometry in chemical reactions.