Assume you used 0.200g of salicylic acid and 0.480mL of acetic anhydride, and you obtain 0.152g of pure, recrystallized, dry aspirin. Calculate the percent yield.
Write and balance the equation; however I think the ratio is 1:1 for reactants and products.
When amounts are given for both reactants you know it is a limiting reagent problem.
mols salicylic acid = grams/molar mass
Look up the density of acetic anhydride and determine grams acetic anhydride. Then mass = volume x density and mols = grams/molar mass
Convert mols SA to mols product.
Convert mols of AH to mols product.
It is likely that these two values will not be the same which means one of them is wrong; the correct answer in limiting reagent problems is ALWAYS the smaller value.
Then grams = mols x molar mass. This is the theoretical yield. We will call that TY.
%yield = (actual yield/TY)*100 = ?
To calculate the percent yield, you need to compare the actual yield (the amount of pure, recrystallized, dry aspirin obtained) with the theoretical yield (the amount of aspirin you would expect to obtain if the reaction went to completion).
First, let's determine the theoretical yield of aspirin. The balanced chemical equation for the reaction between salicylic acid and acetic anhydride to produce aspirin is as follows:
C7H6O3 (salicylic acid) + (CH3CO)2O (acetic anhydride) → C9H8O4 (aspirin) + CH3COOH (acetic acid)
From the equation, you can see that the molar ratio between salicylic acid and aspirin is 1:1. So, we need to convert the mass of salicylic acid used to moles:
0.200g salicylic acid * (1 mol salicylic acid / molar mass of salicylic acid) = X moles
The molar mass of salicylic acid (C7H6O3) is calculated as follows:
(7 * atomic mass of carbon) + (6 * atomic mass of hydrogen) + (3 * atomic mass of oxygen) = 138.12 g/mol
Now we can convert the volume of acetic anhydride to moles using its density and the molar mass:
0.480 mL acetic anhydride * (1 g acetic anhydride / density of acetic anhydride) * (1 mol acetic anhydride / molar mass of acetic anhydride) = Y moles
The density of acetic anhydride can be found in a reference table, and its molar mass (C4H6O3) is calculated as follows:
(4 * atomic mass of carbon) + (6 * atomic mass of hydrogen) + (3 * atomic mass of oxygen) = 102.09 g/mol
Since the stoichiometry between salicylic acid and aspirin is 1:1, the moles of salicylic acid used should be equal to the moles of aspirin produced. Therefore, the theoretical yield (in grams) of aspirin is:
X moles aspirin * (molar mass of aspirin) = Z g
The molar mass of aspirin (C9H8O4) is calculated as follows:
(9 * atomic mass of carbon) + (8 * atomic mass of hydrogen) + (4 * atomic mass of oxygen) = 180.16 g/mol
Finally, you can calculate the percent yield:
percent yield = (actual yield / theoretical yield) * 100
In this case, the actual yield is 0.152g (given in the problem), and the theoretical yield can be calculated using the steps outlined above.