looking for grams of acetylsalicylic acid are in the tablet and % w/w of the tablet is acetylsalicylic acid?

Question

A 0.5130g aspirin tablet which was dissolved in 95% ethanol required 27.98mL of 0.1000M NaOH for neutralization. then an additional 42.78mL of 0.1000M NaOH was added and the solution was heated to hydrolyze the acetylsalicylic acid. After the reaction mixture was cooled, the excess base was back-titrated with 14.29mL of 0.1056M Hcl. How many grams of acetylsalicylic acid are in the tablet? What % w/w of the tablet is acetylsalicylic acid?

Answer 1

I am assuming acetylsalicylic is monoprotic, of course.

27.98mL of 0.1000M NaOH is

0.10000 x .2798 = 0.02798 moles NaOH (wow, I didn't even need a calculator!)

Ahh, but I see that later, a back titration was performed with an additional 42.78mL 0.1M NaOH and 14.29mL 0.1056M HCl. You just subtract MOLES HCl from MOLES NaOH (since HCl and NaOH neutralize each other : Na+ + OH- + H+ + Cl- ---> Na+Cl- +H2O)

.4278 moles NaOH and .1429 x 0.1056 moles HCl. Subtract moles HCl from moles NaOH and that will be your total moles NaOH. Add this number to the previous total for NaOH (0.02798 moles) to get your complete total of NaOH.

Since we are talking about strong base with weak, monoprotic acid, there is a 1:1 ratio between moles titrant and moles titrate! Duh, so that means that your total number of moles of NaOH = moles of acetylsalicylic acid.

You better know how to convert from moles to grams. So do that for acetylsalicylic acid. Now that you have grams acetylsalicylic acid, you better know how to make a fraction and multiply by 100 to get % w/w.

Answer 2

Atomic weights: C= 12 H = 1 O = 16 C9H8O4 = 180
Let acetylsalicylic acid be called A. Let the tablet be called T.

27.98mLNaOH x 0.1000molNaOH/1000mLNaOH x 1molA/1molNaOH x 180gA/1molA = (27.98)(0.1000)(180)/(1000) = 0.503gA

0.503gA/0.513gT x 100% = 98%

The 27.98 mL NaOH is given. The first factor comes from the molarity. The mL cancel, leaving moles NaOH. The second factor comes from the balanced equation. The moles NaOH cancel, leaving moles A. The third factor comes from the molecular weight. The moles A cancel, leaving grams A.