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Methyl orange is an indicator with a Ka of 3.4 × 10−4 . Its non-ionized form...

Methyl orange is an indicator with a Ka of 3.4 × 10−4 . Its non-ionized form is red, while its ionized form is yellow. At a pH of exactly 4.00, this indicator’s color would best be described as which of the following colors?

1. blue

2. green

3. cyan

4. yellow

5. orange

6. yellow-orange

7. red

8. red-orange

9. purple

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Answer #1

answer : 4. yellow

pKa = -log Ka = -log (3.4 x 10^-4 ) = 3.47

pH = 4.00

HA ---------------------------> A-

red                                    yellow

pH = pKa + log [A- / HA ]

4.00 = 3.47 + log [A- / HA ]

[A- / HA ] = 3.39

[A-] = 3.39 [HA]

ionized form is more than non-ionized form so it should be yellow in color

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