The mole — counting the uncountable

A mole is 6.02214076×10²³ things — exact since the 2019 SI redefinition — the bridge from atoms you cannot see to grams you can weigh. The molar mass is just the atomic weights added up, sealed: water 18.015, CO2 44.009, table salt 58.443 g/mol (https://narrowhighway.com/s/54db514fbdd51c90cad455bc773f8152da3b3eb6a53119da6ddb670c9464e23b). That is why a recipe in atoms becomes a recipe on a scale: 18 grams of water IS Avogadro's number of molecules. The count is exact and the arithmetic seals; the atomic weights themselves are measured nature (isotope abundances), honestly not derived.