Question

The concentration of a certain sodium hydroxide solution was determined by using the solution to titrate a sample of potassium hydrogen phthalate (abbreviated as KHP). KHP is an acid with one acidic hydrogen and a molar mass of . In the titration, of the sodium hydroxide solution was required to react with KHP. Calculate the molarity of the sodium hydroxide.

Answer 1

Sodium hydroxide is hygroscopic and absorbs water from the air when we place it on the balance for the determination of mass. This water from the air will prevent us from being able to find the exact mass of sodium hydroxide. In order to determine the exact concentration of a sodium hydroxide solution we must standardize it by titrating with a solid acid that is not hygroscopic. Potassium hydrogen phthalate, KHC₈H₄O₄ (abbreviated KHP), is a non-hygroscopic, crystalline, solid that behaves as a monoprotic acid. It is water soluble and available in high purity. Its molecular weight is 204.22 gm/mole. Because of its high purity, we can determine the number of moles of KHP directly from its mass and it is referred to as a primary standard. We will use this primary standard to determine the concentration of a sodium hydroxide solution.

Slowly add the NaOH solution to one of your flasks of KHP while gently swirling the contents of the flask. As the NaOH is added a pink color appears where the drops of the base come in contact with the solution. This coloration disappears with swirling. As the endpoint is approached, the color disappears more slowly, at which time you should begin add drop by drop. Titrate the sample of KHP until the faint, pink endpoint is reached. Rinse down the sides of the flask to make sure that any splattered NaOH get a chance to react. The endpoint has been reached when a faint pink color persists throughout the mixed solution for about 30 seconds. Exercise care to avoid overshooting the endpoint (intense pinkish-red color).

Calculations:

Calculate the moles of KHP, NaOH, and the molarity of the NaOH. Average the molarities from the different trials and calculate the percent error.

Normality is similar to molarity but is defined as the equivalents per liter of solution. For an acid-base reaction, the equivalent is the mass of acid or base that can furnish or accept exactly 1 mole of protons (H⁺ ions). NaOH has 1 equivalent per mole so Normality = Molarity.