In the laboratory, 3.50 grams of salicylic acid were reacted with 7.20 ml of acetic anhydride (density equal to 1.08 g/ml). Determine the limiting reagent.
Calculate the theoretical yield of the product, acetylsalicylic acid (ASA), produced in the above reaction.
I work these by working two simple stoichiometry problems.
mols SA = grams/molar mass
g AH = volume x density, then
mols AH = grams/molar mass
Convert mols SA to mols product.
Conevrt mols AH to mols product.
The SMALLER number is the correct value of the product to use and the reagent producing that value is the limiting reagent.
To determine the limiting reagent, we need to compare the number of moles of each reactant and see which one is present in a lower amount compared to the stoichiometry of the reaction.
First, let's calculate the number of moles of salicylic acid:
Molar mass of salicylic acid = 138.12 g/mol
Number of moles of salicylic acid = mass / molar mass = 3.50 g / 138.12 g/mol = 0.0253 mol
Next, let's calculate the number of moles of acetic anhydride:
Molar mass of acetic anhydride = 102.09 g/mol
Volume of acetic anhydride = 7.20 ml
Mass of acetic anhydride = volume x density = 7.20 ml x 1.08 g/ml = 7.776 g
Number of moles of acetic anhydride = mass / molar mass = 7.776 g / 102.09 g/mol = 0.0761 mol
Now, let's look at the balanced chemical equation for the reaction between salicylic acid and acetic anhydride to form acetylsalicylic acid:
C7H6O3 + (CH3CO)2O -> C9H8O4 + CH3COOH
From the stoichiometry of the reaction, we can see that the ratio of salicylic acid to acetic anhydride is 1:1. Therefore, the limiting reagent is the one that is present in a smaller amount, which in this case is salicylic acid because it has fewer moles (0.0253 mol) compared to acetic anhydride (0.0761 mol).
Now, let's calculate the theoretical yield of the product, acetylsalicylic acid (ASA):
The molar mass of ASA is 180.16 g/mol.
Since salicylic acid and acetic anhydride have a 1:1 molar ratio in the reaction, the number of moles of ASA produced will be equal to the number of moles of salicylic acid reacted.
The theoretical yield in grams can be calculated as follows:
Theoretical yield = number of moles of ASA * molar mass of ASA
Theoretical yield = 0.0253 mol * 180.16 g/mol
Theoretical yield = 4.56 grams
Therefore, the theoretical yield of acetylsalicylic acid (ASA) produced in the reaction is 4.56 grams.