Molar Mass Calculator

Frequently Asked Questions

How is molar mass calculated?

Sum each element's standard atomic weight times its count in the formula. The tool parses parentheses and hydrates (e.g. CuSO4·5H2O) and reports g/mol plus each element's mass percent.

What is the difference between molar mass and molecular weight?

Numerically the same value; "molecular weight" is dimensionless relative mass, "molar mass" carries units g/mol. Both come from the same element sums.

How do I write a formula with hydrates?

Use a dot or period for the water of crystallization, e.g. CuSO4·5H2O or CuSO4.5H2O - the parser multiplies the trailing group accordingly.

Why does molar mass matter?

It converts between grams and moles (n = m / M), the foundation of stoichiometry, solution preparation, and gas-law calculations.

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How This Calculator Works

The molar mass of a compound is the sum of the standard atomic weights of every atom in its chemical formula, expressed in grams per mole (g÷mol). The calculator tokenises the formula into element symbols and subscript counts, recursively expanding parentheses and bracketed groups by their multiplier, then it adds hydrate water using the centre-dot notation: it accepts CuSO4·5H2O or the keyboard-friendly CuSO4.5H2O, treating the "5" as a coefficient applied to the entire H2O unit. For each element it multiplies the IUPAC atomic weight by the atom count, sums those products, and finally divides each element's contribution by the total to give the mass percent composition. The relationship is M = Σ (nᵢ × Aᵢ), where nᵢ is the number of atoms of element i and Aᵢ is its atomic weight - the same identity printed on the back of every periodic table.

Worked Example: CuSO₄·5H₂O

Copper(II) sulfate pentahydrate has one Cu, one S, four O in the sulfate, plus five waters contributing 10 H and 5 O. Cu = 63.546, S = 32.06, O = 15.999, H = 1.008. So Cu: 1 × 63.546 = 63.546; S: 1 × 32.06 = 32.06; O: (4 + 5) × 15.999 = 9 × 15.999 = 143.991; H: 10 × 1.008 = 10.08. Total = 63.546 + 32.06 + 143.991 + 10.08 ≈ 249.68 g÷mol. Mass percent of water = (5 × 18.015) ÷ 249.68 ≈ 36.1%, which is why the blue crystals lose about a third of their weight when heated to the anhydrous white powder - a classic gravimetric lab.

Where Molar Mass Is Used

Stoichiometry: converting between grams and moles is the first step of nearly every quantitative chemistry problem - moles = mass ÷ M.

Solution prep: to make 1 L of 0.5 M NaCl you weigh 0.5 × 58.44 = 29.22 g; the molar mass is the bridge from molarity to the balance.

Yield & purity: percent yield and elemental analysis (CHN combustion) both compare measured mass fractions against the theoretical mass percent this tool reports.

Gas calculations: density of a gas equals PM÷RT, so molar mass directly sets buoyancy and effusion rates (Graham's law).

Common Mistakes & Edge Cases

Mis-capitalising symbols: "CO" (carbon monoxide) is not "Co" (cobalt). The second letter of a symbol is always lowercase - the parser is case-sensitive on purpose.

Forgetting hydrate water entirely, which understates CuSO₄·5H₂O by 36%. Always include the ·nH₂O term.

Confusing molar mass with molecular mass in daltons (u) - they are numerically equal but molar mass carries g÷mol units via Avogadro's number.

Polyatomic subscripts: Ca(OH)₂ has two O and two H, not one - the multiplier distributes over every atom inside the parentheses.

Isotopically labelled or charged species are not handled; this uses standard terrestrial atomic weights and ignores ionic charge (it does not affect mass appreciably).

Related Concepts & Limits

Atomic weights here are 4-significant-figure IUPAC conventional values; elements with wide isotopic variation (such as Li or S) carry small real-world uncertainty, so the last digit of M is indicative rather than exact. The tool covers the common main-group and transition elements through uranium but not the full lanthanide÷actinide series. Molar mass connects directly to the mole, Avogadro's number (6.022 × 10²³), and downstream tools for molarity, dilution, and ideal-gas density - compute M here first, then carry it forward. For empirical-formula determination you reverse the process: divide each element's mass percent by its atomic weight and scale to the smallest whole-number ratio.

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