Molecular Weight Calculator
Determines the molar mass of a substance by dividing its mass in grams by the number of moles. Use it when you need to identify an unknown compound or verify purity in a chemistry lab.
About this calculator
Molecular weight (molar mass) expresses how much one mole of a substance weighs, measured in grams per mole (g/mol). The formula is straightforward: MW = mass / moles, where mass is in grams and moles is the amount of substance in mol. This relationship stems from Avogadro's number (6.022 × 10²³), which defines one mole as that many particles. Knowing molar mass is essential for stoichiometry — it lets you convert between measurable grams and chemically meaningful mole quantities. For example, if you weigh out a sample and know how many moles reacted (from a titration), you can back-calculate the compound's identity. This value is also critical for preparing solutions of precise concentration.
How to use
Suppose you dissolve a solid compound and recover 18.02 g after a reaction that consumed exactly 1.00 mol. Enter Mass = 18.02 g and Moles = 1.00 mol. The calculator computes MW = 18.02 / 1.00 = 18.02 g/mol — matching water (H₂O). Now try 58.44 g and 1.00 mol: MW = 58.44 g/mol, which corresponds to sodium chloride (NaCl). Always ensure your mass measurement is dry and your mole count is accurate for a reliable result.
Frequently asked questions
What is the difference between molecular weight and molar mass?
Molecular weight technically refers to the mass of a single molecule expressed in atomic mass units (amu), while molar mass is the mass of one mole of a substance in g/mol. Numerically, the two values are identical — water's molecular weight is 18.02 amu and its molar mass is 18.02 g/mol. In everyday lab usage the terms are used interchangeably. This calculator computes the molar mass from bulk measurements of grams and moles.
How do I find the number of moles if I only have a chemical formula?
If you know the chemical formula, sum the atomic weights of each element (found on the periodic table) multiplied by their subscript counts. For example, H₂O = 2(1.008) + 15.999 = 18.02 g/mol, so one mole weighs 18.02 g. Divide your sample's mass by this value to get moles, then enter both into the calculator to verify. This approach is useful for double-checking your formula assignment.
Why does molecular weight matter for preparing laboratory solutions?
Molar mass bridges the gap between the grams you can weigh on a balance and the moles your chemical equations require. To make a 1 M solution of NaCl, you need exactly 58.44 g per liter because that is its molar mass. Without knowing molecular weight you cannot accurately control concentration, which affects reaction yields, drug dosages, and analytical results. It is one of the first values chemists look up when designing any experiment.