Molar mass is the mass of one mole of a substance in grams per mole, and it's nothing more than the atomic weights in the formula, added up: H2O = 2(1.008) + 15.999 = 18.015 g/mol, glucose = 180.156 g/mol, CuSO4·5H2O = 249.677 g/mol. Here are the steps, the two tricks (parentheses and hydrates), and how the number converts grams to moles.
A mole is 6.022 × 10²³ things, Avogadro's number, chosen so that the mass of one mole in grams equals the mass of one particle in atomic mass units. That's the entire magic: a single water molecule weighs 18.015 unified atomic mass units, and one mole of water weighs 18.015 grams. Molar mass is the bridge between the periodic table's per-atom numbers and the gram scale you actually weigh on, which is why every stoichiometry problem starts by computing it.
Six carbons at 12.011 = 72.066. Twelve hydrogens at 1.008 = 12.096. Six oxygens at 15.999 = 95.994. The total is 180.156 g/mol, and each line of that arithmetic is exactly what a good answer shows. Glucose also demonstrates why the breakdown matters: oxygen looks like a minor character in the formula but contributes 95.994 of the 180.156, more than half the mass.
Multiply everything inside by the subscript outside. Calcium hydroxide, Ca(OH)2, is one calcium (40.078) plus two groups of OH, where each group is 15.999 + 1.008 = 17.007: 40.078 + 2(17.007) = 74.092 g/mol. Aluminum sulfate goes a step further: Al2(SO4)3 is 2(26.982) + 3[32.06 + 4(15.999)] = 53.964 + 3(96.056) = 342.132 g/mol. Count atoms the way the parentheses do and the sums take care of themselves: that formula holds 12 oxygens.
A hydrate carries waters of hydration after a dot, and the coefficient before the H2O multiplies the entire water unit. Copper(II) sulfate pentahydrate, CuSO4·5H2O, is the classic: the anhydrous salt weighs 63.546 + 32.06 + 4(15.999) = 159.602 g/mol, and the five waters add 5 × 18.015 = 90.075, totaling 249.677 g/mol. Hydrate questions are worth real exam points because they test whether you multiplied the 5 through both the H and the O rather than treating "5H2O" as one blob of 5 × 18. It is, in fact, exactly that blob, which is why it works.
Type any formula, including nested parentheses and hydrate dots, and get the molar mass plus a per-element breakdown and grams-to-moles conversion.
Molar Mass Calculator →Divide by molar mass. For 25.0 g of glucose: 25.0 ÷ 180.156 = 0.1388 mol. Inverting is multiplication: to measure 0.75 mol of sodium chloride you weigh 0.75 × 58.440 = 43.83 g. Need particles? Moles × 6.022 × 10²³: that glucose sample holds 0.1388 × 6.022 × 10²³ ≈ 8.36 × 10²² molecules. Every stoichiometry chain is grams → moles → ratio → moles → grams, and molar mass powers both ends.
Each element's subtotal divided by the total. Water is (2 × 1.008) ÷ 18.015 = 11.19% hydrogen and 15.999 ÷ 18.015 = 88.81% oxygen. For glucose, carbon is 72.066 ÷ 180.156 = 40.00%, which is a satisfying round number and a quick self-check that your molar mass is in range. Percent composition is also the entry drug to empirical formulas, where you reverse the process from mass data. For quick element-share arithmetic, our percentage calculator handles the division once you have the subtotals.
None, but know where they live: the periodic table's atomic weights, rounded as your course prefers. A short list carries most of general chemistry: H 1.008, C 12.011, N 14.007, O 15.999, Na 22.990, S 32.06, Cl 35.45, Ca 40.078, Fe 55.845, Cu 63.546. One caution: older textbooks print sulfur at 32.065 or 32.066, so H2SO4 computes to 98.079 there and 98.072 with IUPAC 2021 values. Both are correct within rounding; just be consistent within a problem set. For everything heavier than memory, the scientific calculator and the molar mass tool above are faster than any table lookup.
Molar mass equals the sum of each element's atomic weight times its atom count: M = Σ (atomic weight × count). For H2SO4 that's 2(1.008) + 32.06 + 4(15.999) = 98.072 g/mol using IUPAC conventional weights.
They're the same number in different units or contexts. Molar mass is grams per mole. Molecular mass is the mass of one molecule in unified atomic mass units. Formula weight is the same sum applied to ionic compounds that form formula units, not molecules, like NaCl at 58.440. On exams and in labs, treat them as interchangeable arithmetic.
Multiply every element inside the parentheses by the subscript outside. Al2(SO4)3 means 2 aluminum plus 3 groups of SO4: 2(26.982) + 3(32.06 + 4 × 15.999) = 342.132 g/mol. Nested groups follow the same rule outward.
Moles = grams ÷ molar mass, and grams = moles × molar mass. For 25.0 g of glucose: 25.0 ÷ 180.156 = 0.1388 mol. For 0.75 mol of NaCl: 0.75 × 58.440 = 43.83 g. Multiply moles by 6.022 × 10²³ when you need particle counts.
Each element's share of the molar mass. For water, hydrogen is 2.016 ÷ 18.015 = 11.19 percent and oxygen is 88.81 percent. It's the same breakdown table your calculator shows, expressed as fractions of the total.