Molarity Method

That Pesky Mole

• Molarity is measured in chemistry using moles, but what does a chemist mean by mole? Nothing to do with the garden pest. A mole, also sometimes spelled mol, is the amount of a substance that would have the same number of particles as a given weight of a particular carbon atom. Another name for mole is Avogadro's number. It's the number 6.022141527, with the decimal place then moved 23 places to the right. It's named for Amedeo Avogadro (1776 to 1856), who came up with the idea but didn't have the technology to measure it. That had to wait until the late 19th century. Put simply, molar mass (or molecular weight) depends on whether the substance weighs more, less or the same as that carbon atom. Water is about 18 g per mole. Heavyweight radium is 226 g per mole. Lighter-than-air hydrogen is 1 g per mole.

Molarity Formula

• Molarity is a measure of the concentration of a solution. Every solution contains a solute, which is being dissolved, and a solvent, which is doing the dissolving. In saltwater, salt is the solute and water is the solvent. You may have medicine in your bathroom that is iodine solute dissolved in alcohol solvent, hence the sting when it's applied. Divide the number of moles of solute by the total volume of the solution, measured in liters, to determine the solution's molarity.

By the Numbers

• Let's say you've carefully combined hydrochloric acid with water in the laboratory (always wear goggles when mixing acid). You've used 50 g of hydrochloric acid, or HCl, and end up with 200 ml of solution in water, H20. What's the molarity of this mix? The molar mass of HCl is 36 g per mole, rounded off. First, divide the number of grams of HCl by total milliliters of H2O, giving you 0.25. Next, divide the molar mass of HCl into 1 (because that's the mass per mole), giving you 0.028, rounded off. Now multiply the two figures together: 0.25 multiplied by 0.028 yields 0.007. That's the molarity per milliliter. To convert that number into liters, move the decimal point three places to the right. The molarity of this solution is 7 moles per liter.

Molality vs. Molarity

• Molality tells us how many moles of a solute are dissolved in 1 kilogram (not 1 liter) of solvent (not total solution). It's a special kind of molarity, except that chemists write it with a small "m" instead of a capital "M." Divide the number of moles of solute by the total mass of the solvent, in kilograms, to determine the molality. Molality isn't that important in analytical chemistry, when you want to find the concentration of a certain solution. It's useful, however, in physical chemistry, when you care less about a given molarity and more about how that number will change when you alter the temperature or the mixture.