How many possible stereoisomers are there of propylene glycol, an organic compound found in clearasil acne cream, caladryl, colgate toothpaste, and speedstick deodorant? (fill in a number)?

Isomers are molecules with the same molecular formula, but with some difference. If that difference is in the manner the atoms are distributed in the molecule, they are called structural isomers; if the difference is the way the molecule is in space, they're called space isomers (cis/trans); and there are the stereoisomers, which the molecules are images that can be overlapping.

The stereoisomers have the property to deviate the polarized light, and, when a molecule has a chiral carbon, it means that it has stereoisomers. The number of stereoisomers is 2ⁿ, where n is the number of chiral carbons.

Chiral carbon is a carbon that bonds with four different structures. So, below is represented the molecule of propylene glycol, and it has only one chiral carbon (see the blue arrow), so, it has only 2 stereoisomers.