What was the percent by mass of aspirin in the tablets?
I dissolved four aspirin tablets with a total mass of 1.427 g in water and added 50.00 mL of 0.500 mol/L sodium hydroxide solution. Then I titrated the excess sodium hydroxide to a phenolphthalein end-point with 31.92 mL of 0.289 mol/L hydrochloric acid.
I dissolved four aspirin tablets with a total mass of 1.427 g in water and added 50.00 mL of 0.500 mol/L sodium hydroxide solution. Then I titrated the excess sodium hydroxide to a phenolphthalein end-point with 31.92 mL of 0.289 mol/L hydrochloric acid.
1 Answer
WARNING! Very long answer! The aspirin was 99.6 % pure.
Explanation:
Step 1. Start with the balanced equation.
For simplicity, let's rewrite the equation as
Step 2. Calculate the moles of
Step 3. Calculate the moles of
Step 4. Calculate the moles of excess
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
Step 5. Calculate the moles of
Step 6. Calculate the moles of aspirin.
Step 7. Calculate the mass of aspirin.
Step 8. Finally (whew!), calculate the purity of the aspirin.