Standardization of a NaOH Solution with Potassium Hydrogen Phthlate (KHP)
1. A 0.8234-g sample of "KHP" required 38.76 mL of NaOH for titration to the phenolphthalein endpoint. What is the exact molarity of the NaOH solution?
2. A 25.00-mL aliquot of an unstandardized HCl solution is titrated with the previously standardized NaOH solution from #1 above. If 32.55 mL of NaOH titrant is required to reach the endpoint, what is the exact molarity of the HCl solution?
3. How accurately can you estimate the buret reading on a 50-mL buret that has calibration markings every 0.1mL? (Circle the correct answer)
a) 1 mL
b) 0.1 mL
c) 0.01 mL
d) 0.001 mL
4. Why should the sample size be such that not more than 50 mL of titrant is required to reach the endpoint?
5. What mass (in grams) of "KHP" should be used for the standardization of a NaOH solution that is approximately 0.14 M NaOH if the volume of NaOH titrant used is to be about 45 mL? (Approximate calculation - 2 sig. figs is adequate)
Standardization of a NaOH Solution with Potassium Hydrogen Phthalate (KHP)
You will determine the concentration (standardize) of an unknown solution of NaOH using the primary standard, potassium hydrogen phthalate.
Sodium hydroxide is hygroscopic and absorbs water from the air when you place it on the balance for massing. This water will prevent you from being able to find the exact mass of sodium hydroxide. In order to determine the exact concentration of a sodium hydroxide solution you 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. Because of its high purity, you can determine the number of moles of KHP directly from its mass and it is referred to as a primary standard. You will use this primary standard to determine the concentration of a sodium hydroxide solution. The structure of KHP is shown below:
When KHP and a base a reacted, a neutralization reaction occurs that is represented by the following equation: KHC8H4O4 (aq) + NaOH(aq) KNaC8H4O4 (aq) + H2O(l)
The net ionic equation is:
HC8H4O4-1(aq) + OH-(aq) C8H4O4-2 (aq) + H2O(l)
The reaction can be considered to proceed completely to the right. If exactly equivalent amounts of acid or base are used so that neither reactant is present in excess the solution is said to be at the equivalence point. If only monoprotic acids and bases are used (those that furnish or react with one H+ per molecule), then at the equivalence point the number of moles of acid equal the number of moles of base (moles acid = moles base). An acid-base titration is a procedure used to compare the amount (moles) of acid in one sample with the amount (moles) of base in another. In this laboratory exercise you will carry out such a titration to standardize (determine the exact concentration of) a NaOH solution by measuring accurately how many milliliters of it are required to exactly neutralize a known amount of acid. A buret filled with the titrant (NaOH solution) is used to measure the volume of NaOH solution added to the known amount of acid in a flask. An indicator is added to signal the endpoint has been reached. The indicator is a weak acid (or base) itself and reacts with some of the titrant to produce the color change The endpoint occurs when all the acid has been neutralized and the solution composition changes suddenly from excess acid to excess base. When the endpoint is reached the addition of titrant should be stopped. Since the indicator reacts with some of the titrant and the indicator may not change at the exact pH of the equivalence point, a small error in introduced in the titration. This...
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