Skip to main content

Molar Mass Calculator

Standard notation. Use parentheses for groups and · for hydrates.

What molar mass is

Molar mass is the sum of the atomic masses of every atom in a chemical formula, weighted by how many of each atom there are. This calculator parses the formula you type — including parentheses, subscripts, hydrates, and coefficients — and returns the total in g/mol along with the per-element breakdown. For something simple like H2O the calculation is one line: 2(1.008) + 15.999 = 18.015 g/mol. For a hydrate like CuSO4·5H2O the parser has to handle the dot notation; for a complex like K3[Fe(CN)6] it has to handle nested brackets.

The atomic masses used are IUPAC standard atomic weights — the conventional values reported on modern periodic tables. They are weighted averages over natural-abundance isotope distributions, not single-isotope masses, which is why H is 1.008 (not exactly 1) and Cl is 35.45 (not 35 or 37). For elements without a stable isotope (Tc, Pm, all transuranics) the atomic mass shown is the mass of the most stable known isotope, in brackets on most tables.

What the calculator does

  1. Type a chemical formula (e.g., H2SO4, Ca(OH)2, CuSO4*5H2O, K3[Fe(CN)6]).
  2. The parser walks the string and counts atoms element-by-element.
  3. Output: molar mass total, breakdown table (element, atom count, individual mass contribution, mass percent of total), and a “Show Steps” section with the arithmetic spelled out for each element.

Worked examples

Water (H2O).

  • H: 2 × 1.008 = 2.016
  • O: 1 × 15.999 = 15.999
  • Total = 18.015 g/mol

Sulfuric acid (H2SO4).

  • H: 2 × 1.008 = 2.016
  • S: 1 × 32.06 = 32.06
  • O: 4 × 15.999 = 63.996
  • Total = 98.072 g/mol

Calcium hydroxide (Ca(OH)2). Parentheses: 1 Ca, 2 O, 2 H.

  • Ca: 40.078; O: 31.998; H: 2.016
  • Total = 74.092 g/mol

Glucose (C6H12O6).

  • C: 6 × 12.011 = 72.066
  • H: 12 × 1.008 = 12.096
  • O: 6 × 15.999 = 95.994
  • Total = 180.156 g/mol

Copper(II) sulfate pentahydrate (CuSO4·5H2O). The dot adds 5 separate H2O units.

  • Cu: 63.546; S: 32.06
  • O (sulfate): 4 × 15.999 = 63.996; O (5 waters): 5 × 15.999 = 79.995 → 9 O total = 143.991
  • H (5 waters): 10 × 1.008 = 10.08
  • Total = 249.677 g/mol

Where molar mass gets used

Every gram-to-mole conversion: stoichiometry, solution preparation (grams of solute for a target molarity), percent composition, empirical-and-molecular formula determination, limiting-reagent comparisons. None of those calculations can start until you have an accurate molar mass for each species involved.

Frequently Asked Questions

What is molar mass?
Molar mass is the mass in grams of one mole — that is, 6.022 × 10^23 formula units — of a substance, reported in g/mol. It is numerically equal to the molecular weight (or formula weight) but carries explicit units. Multiplying moles by molar mass gives mass in grams; dividing grams by molar mass gives moles. Every gram-to-mole conversion in chemistry routes through this number.
How do you calculate molar mass from a chemical formula?
Identify each element and how many atoms of it appear in the formula (subscripts, parentheses, hydrate dots all factor in). Look up the atomic mass of each element from the periodic table — the IUPAC standard atomic weights are the convention. Multiply atom count by atomic mass for each element, then sum across all elements. The result is the molar mass in g/mol.
What is the difference between molar mass and molecular weight?
Numerically they are the same; they differ in units. Molar mass is in g/mol — a macroscopic quantity tied to the mole. Molecular weight (or relative molecular mass) is dimensionless, expressed as a multiple of the unified atomic mass unit (amu or u). In practice chemists use the terms interchangeably and the numbers match, but rigorous SI usage prefers 'molar mass' with explicit g/mol units.
How do you handle parentheses in a chemical formula?
Parentheses group a unit that repeats. The subscript outside applies to every atom inside. Ca(OH)2 means 1 Ca, then 2 of (OH) — so 2 O and 2 H total. Nested parentheses follow the same rule: in K3[Fe(CN)6], the 6 multiplies CN to give 6 C and 6 N, and the brackets group everything inside as one Fe(CN)6 unit. The parser walks the string character-by-character to apply these rules consistently.
Can this calculator handle hydrates?
Yes. Hydrates use a center dot or asterisk: CuSO4·5H2O or CuSO4*5H2O. The parser treats the part after the dot as a separate molecular unit multiplied by the coefficient, then adds those atoms to the running total. So CuSO4·5H2O contributes 1 Cu, 1 S, 4 O from the sulfate plus 10 H and 5 O from the five waters, totaling 9 O across the whole hydrate.