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
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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