Our modified California State Standard: Students know how to calculate the concentration of a solute in terms of molarity, percent composition and parts per million.
Molarity describes the concentration of a solution in moles of solute divided by liters of solution. Masses of solute must first be converted to moles using the molar mass of the solute. This is the most widely used unit for concentration when preparing solutions in chemistry and biology. The units of molarity, mol/L, are usually represented by a scripted capital “M”.
Percent composition is the ratio parts of solute to one hundred parts of solution and is expressed as a percent. Determine the mass of solute and solution and then divide the mass of the solute by the total mass of the solution. This number is then multiplied by 100 and expressed as a percent. In dilute water solutions, we can assume that 1 mL of water-based solution has a mass of 1 gram, so 1 liter of solution has a mass of 1000 grams.
Parts per million (ppm), is a ratio of parts of solute to one million parts of solution, and is usually applied to very dilute solutions. It is often found in reports of concentration of water contaminants. To calculate parts per million, divide the mass of the solute by the total mass of the solution. This number is then multiplied by 106 and expressed as parts per million (ppm). In dilute water solutions, we can assume that 1 mL of water-based solution has a mass of 1 gram, so 1 liter of solution has a mass of 1000 grams.
***Notice that calculations of ppm are the same as percent composition, except that you multiply by 1 million instead of by 100. In other words, all you have to do is take your answer from % composition and move the decimal point 4 places to the right (4 powers of ten, the difference between 102 and 106).