Molecular Weight Calculator
0.00 g/mol
| Element | Count | Atomic Weight (g/mol) | Subtotal (g/mol) | Mass % |
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How Molecular Weight Is Calculated
Molecular weight (more precisely, molar mass) is found by summing the standard atomic weights of every atom in a chemical formula, each multiplied by how many times that element appears. For glucose, C6H12O6, that means 6 carbon atoms (12.011 g/mol each), 12 hydrogen atoms (1.008 g/mol each), and 6 oxygen atoms (15.999 g/mol each), added together to get roughly 180.16 g/mol. This calculator parses standard chemical formula notation — element symbols followed by an optional subscript count, and parentheses with their own subscript for repeated groups like Ca(OH)2 or Mg(NO3)2 — then applies this same element-by-element summation.
Atomic Weights Are Standardized Averages, Not Fixed Numbers
The atomic weight used for each element is a weighted average of that element's naturally occurring isotopes, as published in the standard atomic weights table maintained by IUPAC (the International Union of Pure and Applied Chemistry). This calculator uses the commonly cited conventional values (for example, carbon at 12.011, oxygen at 15.999, chlorine at 35.45). A few elements, like lithium, boron, and sulfur, have a published range rather than a single value because their isotopic composition varies slightly depending on the natural source — for those, a representative conventional value is used, so results may differ very slightly (typically in the fourth significant figure) from a lab reference that specifies isotope ratios exactly.
Converting Between Mass and Moles
Once the molar mass is known, it becomes the conversion factor between a sample's mass and the number of moles it contains: moles = mass ÷ molar mass, and mass = moles × molar mass. This is the same relationship used constantly in stoichiometry to figure out how much of a compound is needed for a reaction. If you're working with solution concentrations rather than a raw mass, the molarity calculator carries this one step further to volume and concentration. For simple unit conversions elsewhere in your calculation, the conversion calculator may also help.
Frequently Asked Questions
How do I enter a formula with a group in parentheses, like calcium hydroxide?
Type it exactly as written in chemistry notation, such as Ca(OH)2. The calculator multiplies every element's count inside the parentheses by the subscript that follows the closing parenthesis, so it correctly reads that as 1 calcium, 2 oxygen, and 2 hydrogen atoms.
Why might this calculator's result differ slightly from a textbook or lab reference?
This calculator uses standard conventional atomic weights (IUPAC average values, e.g. carbon at 12.011). A few elements like sulfur, boron, and lithium have a natural isotopic range rather than one fixed value, so a reference that specifies exact isotope ratios for your specific sample source could differ in the third or fourth significant figure.