A solution has 3 moles of solute in two liters of water. What is its molarity?
The answer is unknown because molarity is over liters of solution.
If it was 3 mol of solute in 2 L of solution it would be 1.5 mol/L.
However when the solute dissolves in the water creating the solution, the volume increases. So 3 mol of solute in 2 L of water creates more than 2 L of solution.
The correct method for making a 3 mol/L solution would be to place some water into a two liter volume container. Dissolve all 3 mol of the solute into the water. Then add water to the 2 L mark. Now there is 3 mol of solute and 2 L of solution.