Dilute it by half. You will either get +6 or +181.

Whether a molecule is optically active is an independent property relative to whether or not it has a diastereomer.

For example cis and trans 2-butene are diastereomers, but the molecules are achiral, because each molecule has a plane of symmetry (in the plane of the page).

That’s the key thing when determining whether or not a molecule is capable of being optically active. Does it have a plane of symmetry?

Generally, a molecule with only one chiral center will not have a plane of symmetry, and therefore can be optically active.
Most molecules with two chiral centers also lack a plane of symmetry, and are therefore optically active, but you need to *watch out* for meso compounds, because these are molecules which have 2 (or more) chiral centers but are arranged in such a way that the molecule lacks a plane of symmetry.

(Just one last point. Don’t forget that the terms “enanantiomers”, “diastereomers”, etc. are about relationships between two molecules, just like “brother”, “cousin”, “father” are relationships between people. It’s possible for a molecule to be both an enantiomer of one molecule and a diastereomer of another, just like it’s possible for a person to be both a brother to one person and a father to another. That’s why I say that physical properties like optical rotation are independent of those terms.)

Ha! That is a great point Dave!