LAB 26 FINDING [H+] USING INDICATORS
BACKGROUND:
You now have experience with equilibrium systems, titrations, acids and bases. In this lab you will use indicators to determine the concentration of H+(aq) of a weak acid solution and then determine the equilibrium constant for that weak acid.
Acid-base indicators are compounds (dyes) whose color depends on the [H+]. There are many dyes that will work. Each shows a color change in a range of [H+] concentrations. You have already used one in lab 25, phenolthalein. It will change to a pink color if the concentration of the H+(aq) is less than 10-10M.
In this lab you will use two indicators that change color in the acidic range, methyl orange and
orange IV. You will also use two indicators that change color in the basic range, indigo carmine and alizarin yellow R.
You will have to make up your own known concentrations of acids by diluting a known concentration of H+(aq) of a strong acid .1M HCl. Then you will prepare your own known concentrations of bases by diluting a known concentration of OH-(aq) using a strong base, .1 M NaOH. Both sets of solutions will be referred to as your standards.
Working in pairs, you will each make up one of the standard sets of solutions. These will be labeled so both partners may use them in other parts of the lab.
In the second part of the lab you will be assigned an unknown solution and you will determine the [H+] using litmus paper to decide whether it is an acid or a base. Once that is determined use your standard solutions and proper indicators to determine, colorimetrically, the[H+].
In the third part of the lab you will be given a known concentration of acetic acid, weak electrolyte, and you will determine the [H+].
PART 1: preparation of standard solutions
standard acid solutions: 10-4M < [H+] < 10-1M
1. Place 5 ml of .1 M HCl in a clean 13 x 100 mm tt. Label it .1 M H+. You know this is a strong acid and therefore you can safely assume it is completely ionized and the [H+] = .1 M (10-1M). Prepare another tt the same way.
2. To prepare .01 M HCl you will dilute 1 ml of .1 M HCL with 9 ml of distilled water. You will use your 10.0 ml graduated cylinder. Make sure you mix the diluted acid very well in a clean beaker. Take 1 ml of this solution and place it into the 10 ml graduated cylinder and save for step 3. Split the remaining solution into each of 2 tt and label them .01 M HCl. (10-2M)
3. Add 9 ml of distilled water to the graduated cylinder with 1 ml of .01 M HCl from the previous step, mix well in a clean beaker; you now have made .001 M HCl. Take 1 ml of this solution and place it into the 10 ml graduated cylinder and save for step 4. Split the remaining solution into each of 2 tt and label them .001 M HCl. (10-3M)
4. Finally add 9 ml of distilled water to the graduated cylinder from step 3 which contains 1 ml of
.001 M HCl; you now have made .0001M HCl. Mix it thoroughly in a clean beaker and split the volume equally between 2 tt labeled .0001 M HCl. (10-4M)
5. You now have 2 sets of 4 tt labeled from 10-1 M to 10-4 M HCl. To one set add 2 drops of methyl orange to each tt and the other set add 2 drops of orange IV to each tt, stopper and invert tt to mix. Record the colors of each tt and save for parts 2 and 3.
standard base solutions: 10-4M < [OH+] < 10-1M
Follow the same directions as the acid dilutions, just substitute .1 M NaOH for .1 M HCl. NaOH is also a strong electrolyte and it also will completely ionize, therefore you can safely assume the [OH-] = M of the NaOH solution you make.
1. You now have 2 sets of 4 tt labeled from 10-1 M to 10-4 M NaOH. To one set add 2 drops of indigo carmine to each tt and the other set add 2 drops of alizarin yellow R to each tt stopper and invert to mix. Record the colors of each tt and save for parts 2 and 3.
PART 2: finding [H+] of unknown solution.
1. Your instructor will assign an unknown solution to you and you will obtain about 5 ml of the solution.
2. You must first determine if the solution is an acid or base. To do this, obtain a piece of red and blue litmus paper. If the red paper turns blue, then you have a base. If the blue paper turns red, then you have an acid. This is easy to remember from the following:
blue litmus turns RED in ACID
red litmus turns BLUE in BASE
3. If your unknown is an acid, divide it equally into 2 different tt and add the acid indicators. The color that appears should allow you to determine the concentration of H+(aq). Record this concentration in the data table.
4. If your unknown is a base, divide it equally into 2 different tt and add the base indicators. The color that appears should allow you to determine the concentration of H+(aq). Record this concentration in the data table.
Part 3: finding [H+] in acetic acid (weak electrolyte)
1. If you were assigned an odd number for the unknown solution in part 2 you will use the 0.1 M acetic acid, if you were assigned an even numbered unknown solution in part 2 you will use the 1.0 M acetic acid.
2. Place 2-3 ml of the proper acetic acid solution into each of two 13x100 mm tt. Add the appropriate indicators and compare the colors with the standard solutions. Record the [H+] in your data table.
QUESTIONS AND CALCULATIONS:
1. Calculate the equilibrium constant for acetic acid ionizing according to the following equation:
HCH3COO <===> H+(aq) + CH3COO-(aq)
Use the value for [H+] determined in part 3.
You know the [CH3COO-] don’t you!!!
2. Predict qualitatively the effect on the equilibrium system in question #1 for each of the following:
A. solid sodium acetate is dissolved in the .1 M acetic acid solution. Would the [H+] increase or decrease?
B. Sodium hydroxide solution is added a drop at a time to the .1 M acetic acid solution.
3. The [H+] is 8.0 x 10-3M in a 1.0 M benzoic acid solution. What % of the benzoic acid , C6H5COOH, is ionized?
4. You know the [OH-] in each of the tt for NaOH, now determine the [H+] in each of them.
5. Determine the pH for each tt of NaOH prepared in Part 1.
6. What is the pH of your unknown solution? Don’t forget to also identify the unknown # of the solution!!