Question

 

Experiment 1: Measure the pH of Acids

Lab Results

Record your observations in the table below.

Test Tube #	Bromothymol Blue Color	Methyl Yellow Color	Bromocresol Green Color
1	Goldish Yellow	Dark Red	Goldish Yellow
2	Goldish Yellow	Red	Dark Goldish Yellow
3	Goldish Yellow	Orange	Much Darker Goldish Yellow
4	Goldish Yellow	Dark Yellow	Green
5	Darker Goldish Yellow	Yellow	Blue
6	Green	Yellow	Darker Blue
7	Turquize	Lighter Yellow	Much Darker Blue

Data Analysis

For each test tube, calculate the concentration of H₃O and pH. You can use the following formula to determine the concentration of HCl.
M₁ x V₁ = M₂ x V₂
where M₁ and V₁ are the molarity and volume of the first solution and M₂ and V₂ are the molarity and volume of the second solution.
Given that HCl is a strong acid, the H₃O concentration is equal to the HCl concentration except at very low concentrations (test tube 6 and 7) where the H₃O from the dissociation of water (1.00*10^-7) becomes significant.

Test Tube #	HCl Concentration	[H3O]	pH
1	0.1 M	0.1	1
2	0.01 M	0.01	2
3	0.001 M	0.001	3
4	0.0001 M	0.0001	4
5	0.00001 M	0.00001	5
6	0.000001 M	0.000001	6
7	0.0000001 M	0.0000001	7

Experiment 2: Measure the pH of Bases

Lab Results

Record your observations in the table below.

Test Tube #	Bromothymol Blue Color	Alizarin Yellow Color	Pheonlphtalein Color
1	Dark Blue	Dark Red	Dark Purple
2	Lighter Blue	Redish / Orange	Purple
3	Blue	Orange	Purple
4	Blue	Dark Yellow	Purple
5	Blue	Yellow	Lighter Purple
6	Blue	Yellow	Transparent
7	Turquize	Lighter Yellow	Transparent

Data Analysis

For each test tube, calculate the concentration of NaOH, OH^–, H₃O, and pH. You can use the following formula to determine the concentration of NaOH.
where M₁ and V₁ are the molarity and volume of the first solution and M₂ and V₂ are the molarity and volume of the second solution.
M₁ x V₁ = M₂ x V₂
Given that NaOH is a strong base, the HO^– concentration is equal to the NaOH concentration except at very low concentrations (test tube 6 and 7) where the HO^– from the dissociation of water (1.00*10^-7) becomes significant.

Test Tube #	NaOH Concentration	[HO–]	[H3O]	pH
1
2
3
4
5
6
7

Conclusion

1- Why do you think some of the indicators used in experiment 2 were different than the ones used in experiment 1?

2- Suppose a student mixed his HCl and NaOH solution and decided to use bromothymol blue to figure out which is which. Which test tube in the figure below contains NaOH?

A: Yellow Test Tube

B: Blue Test Tube

Answer 1

Test Tube #	NaOH concentration	[HO-]	[H3O+]	pH
1	0.1 M	0.1 M	1 x 10-13 M	13
2	0.01 M	0.01 M	1 x 10-12 M	12
3	0.001 M	0.001 M	1 x 10-11 M	11
4	0.0001 M	0.0001 M	1 x 10-10 M	10
5	0.00001 M	0.00001 M	1 x 10-9 M	9
6	0.000001 M	0.000001 M	1 x 10-8 M	8
7	0.0000001 M	0.0000001 M	1 x 10-7 M	7

1. Indicators are also a type of acid and base and are useful as color changing agents in a certain pH range only. When we switched from acid to base, the pH range significantly changed from 1 - 7 (overall) to 7 - 14 (overall). Hence the indicators also changed which are effective in high pH range of 7 - 14 pH.

2. Bromothymol blue indicator has a blue color in presence of base. Since NaOH is a strong base, therefore color change with bromothymol blue will be light blue to dark blue depending upon concentration of NaOH. Hence test tube B contains NaOH. Yellow test tube contains HCl.