Aspirin is a monoprotic acid. Calculate the volume of 0.43 m naoh required to titrate a 0.1527 gsample of aspirin.
Answers
Answer:
WARNING! Very long answer! The aspirin was 99.6 % pure.
Explanation:
Step 1. Start with the balanced equation.
C
9
H
8
O
4
aspirin
+
2OH
-
→
C
7
H
5
O
-
3
salicylate
+
C
2
H
3
O
-
2
acetate
+
H
2
O
For simplicity, let's rewrite the equation as
Asp + 2OH
-
→
Sal
-
+
Ac
-
+
H
2
O
Step 2. Calculate the moles of
NaOH
used.
Moles NaOH
=
0.050 00
L NaOH
×
0.500 mol NaOH
1
L NaOH
=
0.025 00 mol NaOH
Step 3. Calculate the moles of
HCl
used in the back-titration.
Moles of HCl
=
0.031 92
L HCl
×
0.289 mol HCl
1
L HCl
=
0.009 225 mol HCl
Step 4. Calculate the moles of excess
NaOH
The only base remaining after the reaction is the excess base that has not reacted with the aspirin.
You are doing a strong acid-strong base titration.
Once the NaOH is neutralized, the solution also contains the salts of the weak acids salicylic acid and acetic acid.
The salts of weak acids are stronger bases.
Thus phenolphthalein (which changes colour at pH 9) is a good choice for the indicator.
The equation for the titration is
NaOH
+
HCl
→
H
2
O
+
NaCl
Moles of excess NaOH
=
0.009 225
mol HCl
×
1 mol NaOH
1
mol HCl
=
0.009 225 mol NaOH
Step 5. Calculate the moles of
NaOH
used in the reaction.
Original moles = moles reacted + excess moles
Moles reacted
=
original moles - excess moles
=
(0.02500 - 0.009 225 mol) NaOH
=
0.015 775 mol NaOH
Step 6. Calculate the moles of aspirin.
Moles of Asp
=0.015 78 mol NaOH × 1 mol Asp
2 mol NaOH
= 0.007 888 mol Asp
Step 7. Calculate the mass of aspirin.
Mass of Asp
= 0.007 888 mol Asp × 180.16 g Asp
1 mol Asp
= 1.421 g Asp
Step 8. Finally (whew!), calculate the purity of the aspirin.
Percent by mass
= mass of pure Asp
mass of impure Asp
× 100 %
= 1.421
g 1.427
g× 100 %
= 99.6 %
Explanation: