A mole of a substance weighs its formula mass in grams, so 18.0 g of water, 58.4 g of salt and 180 g of glucose each hold 6.022×10²³ molecules.
The molar mass is the sum of the atomic weights in the formula: water is 2 × 1.008 + 15.999 = 18.015, sodium chloride 22.990 + 35.45 = 58.44, glucose C₆H₁₂O₆ is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.16. The mole is a count, 6.022×10²³, chosen so that the grams come out equal to these numbers, and grams divided by the molar mass give the moles.
A hydrate carries its water: copper(II) sulfate pentahydrate, CuSO₄·5H₂O, is 249.7 g/mol, not the 159.6 of the dry salt. The atomic weights are averages over the natural isotopes, so a sample enriched in one isotope differs, and the mass of a single molecule is the molar mass over 6.022×10²³.
Estimate with the rule, then check it against the calculator that models it properly.
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