Question

INTRODUCTION: Curses! There is a critically important strong acid solution that has lost its label. But wait, it gets worse! There's also a bottle of weak base that has lost its label! Luckily, you are a titration master, and so you can find the identity of the weak base and concentration of that strong acid. You know that the concentrations of the strong acid solution and the weak base solution were about 0.100M, but you don't know them exactly. H H 0 IL C-C-0-H II C-C- II This is Potassium Hydrogen Phthalate, commonly referred to as KHP, a weak solid acid commonly used for titrations. It does not absorb water well, and so remains dry when weighed in air. This reduces contamination by atmospheric water, ensuring accurate measurements for making standardizing solutions. С C - H MATERIALS: pH probe Solid impure NaOH Solid pure potassium hydrogen phthalate (KHP) Sample of unknown strong acid Sample of unknown weak base Appropriate glassware, including a buret and beakers . PROCEDURE: 1. Make 100 mL of approximately 0.100M KHP. Do this by weighing out approximately 2.000 g of KHP. You should know the concentration of this acid exactly but it is not important that it be exactly 0.100M. 2. Make 100 mL of approximately 0.100M NaOH. It is not important that the concentration of this acid be exactly 0.100M, but it should be close. Remember - the NaOH is impure, which means that your concentration will not be very accurate. 3. Titrate exactly 25.00 ml NaOH solution with the KHP solution. Put the NaOH in the beaker and the KHP in the buret. Use the pH meter and a stir bar. Hint: you should know approximately where the equivalence should be. You should perform this titration twice. The first time perform it about 0.5 mL at a time to determine approximately where the equivalence point is. The second time, add KHP drop wise near that point so as to get good data near the equivalence point. Using this information, calculate the concentration of the NaOH solution 4. Titrate the unknown strong acid with the NaOH. Put exactly 25.00 mL of strong acid in the beaker and the NaOH in the buret. Use the pH meter and a stir bar. Hint: you should know approximately where the equivalence should be. You should perform this titration twice. The first time perform it about 0.5 mL at a time to determine approximately where the equivalence point is. The second time, add NaOH drop wise near that point so as to get good data near the equivalence point. Using this information, calculate the concentration of the strong acid solution. 5. Titrate the unknown weak base with the “unknown" strong acid (you know the concentration now!) Put exactly 25.00 mL of the unknown weak base in the beaker and the strong acid in the buret. Use the pH meter and a stir bar. Hint: you should know approximately where the equivalence should be. You should perform this titration twice. The first time perform it 0.5 mL at a time to determine approximately where the equivalence point is. The second time, add the strong acid drop wise near that point so as to get good data near the equivalence point. Using this information, calculate the concentration of the weak base solution. 6. Using the titration curve from part 5, calculate the K, value of the unknown base. Then, determine which of the 10 possible unknown bases you have been assigned by finding the K, value on the table below.

Equivalence Point for Titration #1: 24.96 mL

Equivalence Point for Titration #2: 25.40 mL

Equivalence Point for Titration #3: 25.20 mL

Midpoint pH for Titration #3: 9.80

QUESTIONS:

4) Set up the calculation required to determine the concentration of the NaOH solution via titration of a given amount of KHP. Include all numbers except the given mass of KHP.

5) Set up the calculation required to determine the concentration of the unknown strong acid via titration with a known volume of NaOH of known concentration. Include all numbers except the volume and concentration of NaOH.

6) Set up the calculation required to determine the concentration of the unknown weak base via titration with the unknown strong acid of known concentration and volume. Include all numbers except the volume and concentration of the unknown acid.

7) Error analysis - EXPLAIN what effect each of the following would have on the result listed:

A) The pipet you used to measure out exactly 25.00mL of KHP was rinsed only in distilled water but not rinsed in acid solution. What is the effect on the calculated concentration of NaOH? (remember: concentration is a number - increase, decrease, or no change). Explain your answer.

B) The beaker into which the 25.00mL of NaOH was added was not dry. What is the effect on the calculated concentration of NaOH - increase, decrease, or no change? Explain your answer.

C) An air bubble of some size was present in the liquid in the buret at the end of the first titration but not at the beginning. What is the effect on the calculated concentration of NaOH - increase, decrease, or no change? Explain your answer.

D) After you correctly determine the concentration of NaOH, the NaOH is left open overnight, so that some CO2 dissolved in it. What is the effect on the calculated concentration of strong acid - increase, decrease, or no change? Explain your answer. Include a balanced chemical equation involving NaOH and CO2.

E) Why would it be OK to use phenolphthalein during the second titration, but not the first and the third?

F) Suppose the KHP is contaminated and is only 95% pure. What is the effect on the calculated concentration of the weak base? Explain your answer.

***NOTE: Questions #4 through #6 ask you to set up the calculations but NOT to put in all of the numbers. This was done to help with question #7. So, for questions #4 - #6, don't do the calculations - just set up the work.***