 We're going to do a titration to determine the concentration of an unknown solution of sodium hydroxide. You're going to titrate a sample of potassium hydrogen phthalate, sometimes referred to as KHP, with a solution of sodium hydroxide using a phenothaline indicator. The sodium hydroxide and potassium hydrogen phthalate react in a one-to-one ratio. You'll mass out a sample of potassium hydrogen phthalate, which will give you the moles of potassium hydrogen phthalate. You'll titrate the sample until you reach a pale pink endpoint. This will give you the number of moles of sodium hydroxide. You'll know the volume that you use for the titration, and you'll be able to calculate the molarity of the solution. We've massed a sample of potassium hydrogen phthalate. We're going to add it to our 250 milliliter Erlenmeyer flask. You can rinse your wayboat or your weighing paper with water to make sure that all the sample gets transferred to the flask. We're going to add 50 milliliters of distilled water to the flask. The amount of water that's added is not critical, since you know the mass of the potassium hydrogen phthalate. We're going to gently swirl the flask in order to get the solid to dissolve. We've rinsed the burette with the unknown sodium hydroxide solution. We've filled the burette. We've made sure that the tip of the burette is filled with the sodium hydroxide solution. Once the potassium hydrogen phthalate has dissolved, we're going to place it on a magnetic stirrer. We're going to add three or four drops of phenothaline indicator to the solution. Phenothaline is an acid-based indicator that is colorless in acidic solution and pink in basic solution. We'll gently stir the flask. We want to make sure that we have uniform mixing of the solutions so that the reaction is complete. We'll take an initial burette reading. You should read the initial burette reading to the nearest hundredth of a milliliter. We'll add the sodium hydroxide solution until the solution turns a pale pink color. Periodically during the titration, you should rinse down the walls of the flask. You want to make sure that any sodium hydroxide solution that is splattered on the walls of the flask gets rinsed into the main solution and has a chance to react. As you get closer to the end point, the pink color will stay for a longer period of time. If you stop adding the sodium hydroxide solution, the solution will turn colorless because there's still an excess of potassium hydrogen phthalate. You should rinse down the walls of the flask in to make sure that all the reactants have a chance to react. When you get closer to the end point, you should slow down your addition and add the solution drop-wise. You can actually split a drop by hanging a drop off the tip of the burette and rinsing the drop in with your wash bottle. Once the solution turns pale pink, you need to make sure you rinse down the sides of the flask again to make sure that all the sodium hydroxide and potassium hydrogen phthalate have had a chance to react. The moles of potassium hydrogen phthalate equal the number of moles of sodium hydroxide since they react in a one-to-one ratio. You have the moles of sodium hydroxide. You have the volume of the sodium hydroxide solution used from the burette. You should be able to calculate the molarity of the sodium hydroxide solution.