Acetaminophen, shown below, is a weak acid used pharmaceutically as an aspirin a
ID: 945900 • Letter: A
Question
Acetaminophen, shown below, is a weak acid used pharmaceutically as an aspirin
alternative. The molecular formula of acetaminophen is HOC6H4NHCOCH3. The
terminal hydrogen (on the OH) is removed during neutralization. The Ka value of
acetaminophen is 3.2 × 1010
.
A sample containing acetaminophen was dissolved in water to make a 50.00-mL solution.
The solution was titrated with 0.5065 M NaOH, and the equivalence point was reached
when 26.10 mL NaOH solution was added.
Calculate the pH of the sample solution before any NaOH was added.
Explanation / Answer
Moles of NaOH used= 0.5065*26.1/1000=0.01322 moles
Acetaminophen]OH + NaOH [Acetaminophen]O- Na+
moles of Acetaninophen = 0.01322
Molarity= 0.01322*1000/50=0.2643M
Let Acetaminophen be HA
HA----> H+ + A-
A- is conjugate base
Ka= [H+] [A-]/[HA]
let x= drop in concentration of HA to reach equilibrium
3.2*10-10= x2/(0.2643-x)
asuming that x is smaller than 0.2643
hence
3.2*10-10 =x2/0.2643
x2= 8.5*10-11
x= 9.2*10-6 (<<<0.2643)
pH= -log(9.2*10-6)=5.03