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Molarity expresses the relationship between the number of moles of a solute per liters of solution, or the volume of that solution. In formula form, molarity is expressed as: molarity = moles of solute / liters of solution Example problem: What is the molarity of a solution made by dissolving 3.4 g of KMnO4 in 5.2 liters of water? Finding molarity demands that you have the number of moles and the number of liters. If you are not provided with the number of moles but you do have the volume and mass of the solution, you will need to use these two figures to calculate the number of moles before continuing on. Example problem: Mass = 3.4 g KMnO4 Volume = 5.2 L To calculate the number of moles from the mass or grams of solute used, you must first determine the molar mass of the solute. This can be done by adding together the separate molar masses of each element found in the solution. Find the molar mass of each element using the periodic table of elements. Make sure that you count the atoms for each element and calculate the molar mass of each of the atoms. Example problem: Molar mass of K = 39.1 g Molar mass of Mn = 54.9 g Molar mass of O = 16.0 g (The solute contains 4 O atoms, so count the 16g 4 times.) Total molar mass = K + Mn + O + O + O + O = 39.1 + 54.9 + 16 + 16 + 16 + 16 = 158.0 g Now that you have the molar mass of the solute, you need to multiply the number of grams of solute in the solution by a conversion factor of 1 mole over the formula weight (molar mass) of the solute. This will give you the number of moles of the solute for this equation. Example problem: grams of solute * (1/molar mass of solute) = 3.4 g * (1 mol / 158 g) = 0.0215 mol The grams units of the solute and solute mass cancel each other out so you are left with mol units. Now that you have the number of moles, you can divide this value by the number of liters of solution in order to find the molarity. Example problem: molarity = moles of solute / liters of solution = 0.0215 mol / 5.2 L = 0.004134615 You should round off the number of digits after the decimal point to the place requested by your instructor. Usually, this will be two or three places after the decimal point. Additionally, when you write out the answer, abbreviate "molarity" with "M" and state the solute involved. Example problem: 0.004 M KMnO4
Know the basic formula for calculating molarity. Examine the problem. Find the molar mass of the solute. Convert the grams into moles. Divide the number of moles by the number of liters. Write your answer.