Final answer:
To determine the number of moles of a compound, divide the known mass by the compound's molecular weight (a). The molecular weight is the sum of all the atomic masses in the compound's molecular formula. The formula used is n = m/M.
Step-by-step explanation:
To determine the number of moles of a compound in a known mass of the compound using its molecular formula, you must divide the mass of the compound by the molecular weight (also known as molar mass). The molecular weight is the sum of the average atomic masses of all the atoms represented in the compound's molecular formula. Use the equation: n = m/M where n is the number of moles, m is the mass of the substance in grams, and M is the molar mass of the substance in grams per mole.
For example, to find the number of moles of water (H2O) in 18 grams of water, first calculate the molecular weight of water, which is approximately 18.02 g/mol (2 x 1.01 for hydrogen + 16.00 for oxygen). Then divide the mass by the molecular weight: n = 18 g / 18.02 g/mol = 1 mol.