Question

How do you calculate the number of molecules in a substance?

Answer 1

Divide the mass of the molecular substance by its molar mass to get moles. Then multiply times `(6.022xx10^23"molecules")/(1"mol")`.

Explanation:

Determine the mass of the substance, and its molar mass. Divide the given mass by its molar mass to get moles, then multiply times `(6.022xx10^23"molecules")/(1"mol")`.

Example
How many molecules are contained in 25.00 g glucose `("C"_6"H"_12"O"_6")`?

Molar mass `"C"_6"H"_12"O"_6"` `=` `"180.15588 g/mol"`
http://pubchem.ncbi.nlm.nih.gov/compound/5793

`25.00cancel("g C"_6"H"_12"O"_6)xx(1cancel("mol C"_6"H"_12"O"_6))/ (180.15588cancel("g C"_6"H"_12"O"_6))xx(6.022xx10^23"molecules")/(1cancel("mol C"_6"H"_12"O"_6"))` `=` `"8.357 molecules C"_6"H"_12"O"_6"`