A reaction (C7H6O3 + C4H6O3 --> C9H8O4 + C2H4O2) produced 2.44g of aspirin. The relevant molecular weights are 180.2 grams per mole for aspirin, and...

Percent yield =
(actual yield)/(theoretical yield) x 100%

The actual yield is what was
produced, 2.44 grams.

The theoretical yield can be calculated from the amount
of reactants that you started with. You didn't give this information, but I can walk you through
the process. 

To produce aspirin, salicylic acid (C7H6O3) is reacted with an
excess of acetic anhydride (C4H6O3). This is how we know that the salicylic acid is the limiting
reactant and is therefore used to determine theoretical yield.

You take the
grams of salicylic acid that you started with, and:

1. Divide it by the molar
mass of salicylic acid to get moles

2. Multiply moles obtained by the mole
ratio of acetylsalicylic acid (aspirin) to salicylic acid from the balanced equation to get
moles of acetylsalicylic acid (aspirin) produced

3. Multiply moles of
acetylsalicylic acid  (aspirin) by its molar mass.

The balanced equation
is:

`C_7H_6O_3 + C_4H_6O_3 -> C_9H_8O_4 + C_2H_4O_2`

Since the equation is balanced as written the the mole ratio of `C_9H_8O_4`     (acetyl
salicylic acid or aspirin) to `C_H_6O_3`

is 1:1.

For the
sake of example, I'm going to show the calculation as if you started with 2.00g of salicylic
acid. You should use the amount you actually started with in this calculation:

2.00g C7H6O3 x 1/138.2g x 1mole C9H8O4/1 mole C7H6O3 x 180.2g/1mol

=
(2.00g)(180.2)/(138.2) = 2.61 g = theoretical yield

% yield = (2.44g)/(2.61g)
x 100% = 93.5%