How do I convert from moles per liter to percentage?

The first step is determining atomic masses of elements which make up the dissolved compound. For example, if the compound in the solution is sodium chloride, the atomic mass of sodium is 23 and chlorine is 35.5. 

The next step is to multiply the atomic mass by the number of atoms in the molecule, and then take a sum of the products to calculate the molar mass. So, the molar mass of sodium chloride is 23 x 1 + 35.5 x 1= 58.5. 

The next step is to multiply the molar mass of the compound by the molarity to determine the amount of the dissolved sample in one liter of the solution. As an example, 0.4 M of NaCl contains 58.5 x 0.4= 23.4 g of the salt. 

Next, one must multiply the density of the solution by 1,000 ml (1 L) to calculate the mass of the 1L of the solution. If the density of 0.4 M NaCl is 1.05 g/ml, the weight of the 1 liter of solution is 1.05 x 1000=1050 g. 

The mass of the dissolved compound must then be divided by the mass of the solution and multiplied by 100 to calculate the percentage. The solution of NaCl is (23.4/ 1050) x 100= 2.22 %.