we know that the molarity of a solution is equal to the moles of solute divided by the liters of solution however one of these values is practical to know offhand while the other is not can you guess which let's say we are going to make a solution of sugar in water i think if you try to measure the volume of the resulting solution as well as count the number of sugar molecules you'll find that the latter task is quite a bit more difficult than the former so we don't have a way of directly measuring a number of moles but this is no problem because we know how to calculate the number of moles of a substance given the mass of the substance present this is quite a bit easier to measure since we might have a balance lying around let's say we take 50 grams of sugar which is sucrose or c12h22o11 and we place it in 200 milliliters of water what will be the concentration of the resulting solution well first let's take our mass of sugar and convert it into moles if we add up the masses of all the carbon hydrogen and oxygen atoms we get a molar mass of about 342 grams per mole 50 grams times one mole over 342 grams gives us 0.146 moles and since the molarity is moles per liter we simply take that number of moles and divide by 0.2 liters and we get 0.731 molar as the concentration of the resulting solution so calculating the concentration of a solution given the mass of solute is only a two-step process first we convert the mass into moles using the molar mass of the substance and then we divide the number of moles by the volume of the solution to get the concentration of that solution in molarity