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Know the basic formula for calculating molarity. Examine the problem. Divide the number of moles by the number of liters. Write your answer.
Molarity is equal to the number of moles of a solute divided by the volume of the solution in liters. As such, it is written as: molarity = moles of solute / liters of solution Example problem: What is the molarity of a solution containing 0.75 mol NaCl in 4.2 liters? Finding molarity demands that you have the number of moles and the number of liters. If the problem provides each of these figures, no preliminary calculations are needed. Example problem:  Moles = 0.75 mol NaCl Volume = 4.2 L The resulting quotient will give you the number of moles per liter of solution, otherwise known as molarity. Example problem: molarity = moles of solute / liters of solution = 0.75 mol / 4.2 L = 0.17857142 Round off the number of digits after the decimal point to two or three, depending on your instructor's preference. When you write out the answer, abbreviate "molarity" with "M" and state the chemical abbreviation of the solute involved. Example problem: 0.179 M NaCl