Amy was titrating a pyridine solution using 0.1 M HCI. After careful calculation

Question

Amy was titrating a pyridine solution using 0.1 M HCI. After careful calculation, she found the pH at the equivalent point should be 5.71. Given the pKa and the colors for the acidic and basic forms of the following indicators, please help Amy pick the appropriate indicator and predict the color change occurring at the equivalence point: Bromocresol Green: pKa = 4.66; acidic color = Yellow; basic color = Blue Methyl Red, pKa = 5.00; acidic color = Red; basic color = Yellow Bromocresol Purple: pKa = 6.12; acidic color = Yellow; basic color = Purple Phenol Red: pKa = 7.81; acidic color = Yellow; basic color = Red She can only use methyl red, and the color change is from yellow to red. She can only use bromocresol purple, and the color change is from yellow to purple. She can use either methyl red or bromocresol purple, and the color change is from yellow to red (if she uses methyl red) or from purple to yellow (if she uses bromocresol purple). She can use bromocresol green, the color change is from blue to yellow.

Explanation / Answer

we know that

the color transition range of an indicator is given by

pKa -1 < pH < pKa + 1

in this pH range the color change happens

so

for bromocresol green : 3.66 < pH < 5.66

for mehtyl red :   4 < pH < 6

for bromocresol purple : 5.12 < pH < 7.12

for phenol red : 6.81 < pH < 8.81

now

given the pH is 5.71

so

from the above information

we can see that

methyl red and bromcresol purple can be used

so

the answer is

She can use either methyl red or bromocresol purple , and the color change is from

yellow to red ( if she uses methyl red) or from purple to yellow ( if she uses

bromocresol purple )

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