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

1 Answer
Nov 26, 2015

Answer:

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"#