Finding the mass for a calorimetry problem

[fk378]

This is a general question... when given a calorimetry problem that states the molarity of the solution, the volume, and the density, why must the density be used in order to find the mass? Why can't the molarity be used (you can find moles and then use molar mass to convert to grams)?

 

[chemisttree]

Using the molarity and the volume you can easily find the mass of the solute. What remains is the mass of the solvent of course. There is no easy way to calculate it aside from knowing its volume and density.