A solution of a weak acid was tested with the indicators used in this experiment

Question

A solution of a weak acid was tested with the indicators used in this experiment. The colors observed were as follows: Methyl violet violet Congo red violet Thymol blue yellow Bromcresol green yellow Methyl yellow orange What is the approximate pH of the solution? The pH of a 0.10 M HCN solution is 5.2. What is [H^+] in that solution? What is (CN^-]? What is [HCN]? (Where do the H^+ and CN^- ions come from?) What is the value of K_a for HCN? (Eq. 4.) Formic acid, HFor, has a K_a value equal to about 1.8 times 10^-4. A student is asked to prepare a buffer having a pH of 3.40 from a 0.10 M HFor and a 0.10 M NaFor solution. How many milliliters of the NaFor solution should she add to 20 mL of the 0.10 M HFor to make the buffer? When 5 drops of 0.10 M NaOH were added to 20 mL of the buffer in problem 3, the pH went from 3.40 to 3.43. Write a net ionic equation to explain why the pH didn't go up to about 10, as it would have if that amount of NaOH were added 10 distilled water or to 20 mL 0.00040 M HCl, which also would have a pH of 3.40.

Explanation / Answer

1.

Indicators change of color when acid/base are added. They react due to their affinity to H+; when accepting H+

ions, the molecules change (i.e. bend or contract) causing a change in light path, therefore, color change.

Now, we must first, we must know all indicator's pH ranges before analysis

then

Metyhl violet range:

yellow green 0.0 - 2.5 violet

Thymol blue range:

red 1.2 - 2.8 yellow

Methyl yellow range

red 2.9 - 4 yellow

Congo red

3.0 blue - 5.2 red

Bromcresol green

3.8 yellow - 5.4 blue

Now,

analysis

Metyhl violet range:

yellow green 0.0 - 2.5 violet; therefore, it must have a pH higher than 2.5

Thymol blue range:

red 1.2 - 2.8 yellow; therefore, it must have a pH higher than 2.8

Methyl yellow range

red 2.9 - 4 yellow; since it is oraange, it is amix between red and yellow, pH must be between 2.9 and 4

Congo red

3.0 blue - 5.2 red; since it is violet, the mix must be between 3 and 5.2... Note that we already stated 4 in the previous example so the new range is 3 < pH < 4

Bromcresol green

3.8 yellow - 5.4 blue

note that we state then

3.0 < pH < 3.8

that will be a formal range.. Since an approximate value is required, choose either the averate 3.4 or a common rounding such as 3.5

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