A student reacts 4.84 g of salicylic acid with an excess of acetic anhydride.......

Question

in the presence of a few drops of phosphoric acid to make acetylsalicylic acid (aspirin), which the student then purifies. Calculate the mass of the theoretical yield of product.

Answer 1

Salicylic acid + Acetic anhydride ------> Asprin + CH3COOH

C7H6O3 + C4H6O3 ------------ C9H8O4 + C2H4O2

From the above balanced equation, 1 mole of salicylic acid reacts to give 1 mole of asprin,

This means the maximum theoretical yield of asprin will be the same as the number of moles of salicylic acid that you started with (since you are told acetic anhydride is in excess).

First, work out number of moles of salicylic acid you started with.

moles = mass / molecular weight

molecular weight salicylic acid = (7 x 12.01) + (6 x 1.008) + (3 x 16.00)

= 138.118 g/mol

So moles = 4.84 g / 138.118 g/mol

= 0.0350 moles of saliclyic acid.

Therefore the maximum yield possible of asprin will be 0.0350 mol of asprin (1 mol salicylic acid reacts to give 1 mol asprin)

Mass of 0.035 mol asprin:

molecular weight asprin = (9 x 12.01) + (8 x 1.008) + (4 x 16.00)

= 180.154 g/mol

mole = mass / molecular weight

0.0350 mol = mass / 180.154 g/mol

therefore mass = 0.0350 mol x 180.154 g/mol

= 6.31 g