1. A solution of a weak acid was tested with the indicators used in this experim
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Question
1. A solution of a weak acid was tested with the indicators used in this experiment. The colors observed were as follows:
Methyl Violet Violet
Thymol Blue Yellow
Methyl Yellow Yellow
Congo Red Orange Red
Bromcresol Green Green
What is the approximate pH of the solution?
2. Formic acid, HFor, has a Ka value equal to about 1.8 x 10-4. A student is asked to prepare a buffer having a pH of 3.4 from a solution of formic acid and a solution of sodium formate having the same molarity. How many milliliters of the NaFor solution should she add to 20 mL of the HFor solution to make the buffer?
Explanation / Answer
Answer 1:
Methyl violet gives violet color when pH > 1.6. As solution is violet pH is greater than 1.6.
Thymol blue gives yellow color when pH is 1.2–2.8 and gives blue when pH is 8.0–9.6. So, unknown sloution must be acidic pH.
Methyl yellow gives yellow color at pH > 4. So, unknown sloution pH must be above 4.
Congo red gives red color when pH > 5.2. So, unknown sloution pH must be above that pH.
Bromocresol green gives color change from yellow to blue in the pH range 3.8 -5.4. So, unknown sloution pH must be between 3.8-5.4
As with congo red it shows shows orange red, pH of unknown must be between 5.2-5.4.
So pH must be 5.3
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