I. Molarity
A. MOLARITY IS ONE METHOD OF QUANTITATIVELY ASSESSING A SOLUTION
1. A solution is made when a solute (the substance dissolving), usually a solid or a sometimes a liquid, dissolves in a solvent (the medium in which the substance is dissolved), which is usually water.
2. The concentration of a solution can be measured in terms of the number of grams of the solute that has been dissolved in a particular volume of the solution, or more often, in terms of the number of moles of the solute in a particular volume of the solution. Typical units for these concentrations are g/L (g L−1) or mol/L (mol L−1), respectively.
3. The method of expressing the concentration of a solution in mol L−1 is the most common and is called molarity.
4. Molarity is given the symbol, M.
5. Units of M are expressed as being molar, so, for example, a solution that has a concentration of 0.250 mol L−1 can be called a 0.250 molar solution.
6. When concentration is measured in mol L−1 (or M), and volume is measured in L, then, for solutions, the moles of solute can be calculated by applying:
moles = (concentration) × (volume)
It is very common for the concentrations of solutions to be expressed in units of M, and for volumes of those solutions to be expressed in units of mL. When this is the case, be sure to convert mL to L before multiplying by the molarity in order to correctly calculate the moles.
A. PERCENT BY MASS IS ANOTHER METHOD OF QUANTITATIVELY ASSESSING A SOLUTION
1. A solution is comprised of a solute and a solvent component.
2. As a result, the total mass of a solution is comprised of the mass of the solute plus the mass of the solvent.
3. Expressing the mass of the solute as a percentage of the total mass of the solution is called the percent by mass. See the following example.
4. Here is an example calculation. What is the percent by mass of NaCl of a solution in which 2.00 g of sodium chloride, NaCl, is dissolved in 100. mL of water? The density of water should be taken as 1.00 g/mL.
i. Mass of solvent (water) = (100. mL) × (1.00 g/mL) = 100 g.
ii. Total mass of solution = mass of solvent (water) + mass of solute (NaCl) = 100. g + 2.00 g = 102 g.
iii. %bymass of NaCl 
Percent by mass should result in an answer that is less than 100%. This is a good lesson in checking your answer to any calculation to see if it makes sense. It’s a good habit to get into.