Chemistry question

Question

If you started with exactly 2.000 g of salicylic acid and 5.00 mL of acetic anhydride, calculate the theoretical yield of aspirin, in grams. The density of acetic anhydride is 1.08 g/mL.

If a team obtains 1.85 g of aspirin product from the reactants, what was their percent yield?

Answer 1

First, let's work out which of the two is the limiting reagent. To do that we'll pick one of them and work out how much of the other one would be consumed by reacting it. To save a lot of typing and keep things concise, I'll call acetic anhydride AcAn and salicylic acid salic.

5.00mL AcAn x 1.08g AcAn/mL x 1 mol AcAn/102g AcAn x 1 mol asprin/1 mol AcAn x 180g asprin/1mol salic

The stages here are:

1. Given 5.00mL of acetic anhydride
2. Using density to convert to grams of acetic anhydride
3. Using molar mass to convert to moles of acetic anhydride
4. Using reaction to convert moles of acetic anhydride to salicylic acid
5. Using molar mass to convert moles of aspirin to grams.

Doing the math gives 9.53g aspirin.

Now we do that again with salicylic acid and if that's less than 9.53g then salicylic acid is the limiting reagent. The form will be similar but we can start right off with grams, so there's no density conversion.

2.000g salic x 1mol salic/138g salic x 1 mol aspriin/1mol salic x 180g aspirin/1 mol aspirin

Working that out gives 2.61g aspirin. Three significant figures since I used three in the molar masses, and I only used three because at the end we're going to compare it to 1.85g which only has three.

That's less than reacting all the acetic anhydride, so the salicylic acid is the limiting reagent and our theoretical yield is 2.61g.

The lab team got 1.85g of aspirin, so we just need to work out what percent that is of 2.61g.

% Yield = 1.85g aspirin/2.61g aspirin x 100%

% Yield = 70.9%