Hydroxide ion is a strong enough base to deprotonate a certain indicator to give

Question

Hydroxide ion is a strong enough base to deprotonate a certain indicator to give a red colored compound. Phenoxides however, like the phenoxide obtained when acetaminophen reacts with hydroxide ion, are not strong enough bases to deprotonate that same indicator so the indicator does not turn red. A drop of this indicator is added to 2.00 liters ot a solution of acetaminophen. After adding 22.6 mL of 1.20 M NaOH to this solution, the indicator turns red. What is the molarity of the original acetaminophen solution?

Explanation / Answer

[acetaminophen]OH + NaOH -------> [acetaminophen]O- Na+ + H2O

1 mole                         1 mole

Acetaminophen                                                 NaOH

M1 =                                                              M2 = 1.2M

V1 = 2L = 2000ml                                         V2   = 22.6ml

n1   = 1                                                            n2 = 1

                  M1V1/n1     =     M2V2/n2

                   M1             =      M2V2n1/n2V1

                                      =     1.2*22.6*1/1*2000    = 0.01356M

molarity of acetominophen   = 0.01356M

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