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

Nov 26, 2015

Divide the mass of the molecular substance by its molar mass to get moles. Then multiply times $\left(6.022 \times {10}^{23} \text{molecules")/(1"mol}\right)$.

#### 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 $\left(6.022 \times {10}^{23} \text{molecules")/(1"mol}\right)$.

Example
How many molecules are contained in 25.00 g glucose $\left(\text{C"_6"H"_12"O"_6}\right)$?

Molar mass $\text{C"_6"H"_12"O"_6}$$=$$\text{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"))$=$$\text{8.357 molecules C"_6"H"_12"O"_6}$