What is the molarity of a 3.0-liter solution that contains 0.45 moles of solute

Moles = n/v where n is the moles of solute and v being the liters of solution.
We can put in the information provided to find the molarity.

Moles = .45/3.0 = .15
So we now know that the molarity of that solution is .15!
 I hope I helped you :). Make sure to memorize that formula because it's not that hard as long as you know what to plug in.