Calculate the number of moles of solute in 1.5 L of 2 M solution.

Answer.3

Number of moles=molarity x volume of solution in litres
n= 2x 1.5=3 moles

The solution to this problem lies in the definition of molarity, which is the concentration of a solution expressed in moles of solute per liter of solution. The “M” in the problem is the symbol for molarity—that is, the solution is 2 molar, or contains 2 moles of solute in each liter.
Students may initially think molarity is a strange way to describe a solution compared to a more recipe-like statement of how much solute was added to how much solvent, especially if they have the opportunity to make up solutions in lab, where they would use volumetric flasks, mix thoroughly, and make up the precise volume with a dropper or pipette.
In a problem like this one, however, we see the value of molarity as a measure, because the moles of solute in any given volume are the product of the moles in one liter (i.e. the molarity) times the number of liters, making the calculation very simple.
We can show this algebraically, starting with the definition:
M = (moles solute)/(L solution)
M*(L solution) = moles solute.
In this case, (2 M) * (1.5 L) = 3 moles solute. There is only one significant figure.
Note that M is both the symbol for the quantity of molarity and also the symbol for the units of molarity, which are equal to moles/liter.