A reaction (C7H6O3 + C4H6O3 --> C9H8O4 + C2H4O2) produced 2.44g of aspirin. The relevant molecular weights are 180.2 grams per mole for aspirin, and 138.2 grams per mole for salicylic acid.
How can you calculate the percent yield?
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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`
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%
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