Question

Salicylic acid (C7H6O3) reacts with acetic anhydride (C4H6O3) to form acetylsalicylic acid (C9H8O4). 2C7H6O3(aq) + C4H6O3(aq) mc002-1.jpg 2C9H8O4(aq) + H2O(l) What is the limiting reactant if 70.0 g of C7H6O3 and 80.0 g of C4H6O3 react?

Answer 1

In order to understand what is the limiting reactant is useful to convert the masses of reactants into moles. The molecular masses are:

$C_7H_6O_3 = 138\frac{g}{mol}\\ C_4H_6O_3 = 100\frac{g}{mol}$

Converting masses into moles:

$70\g \C_7H_6O_3\cdot \frac{1\ mol}{138\ g} = 0.5\ mol\ C_7H_6O_3$

$80\ g\ C_4H_6O_3\cdot \frac{1\ mol}{100\ g} = 0.8\ mol\ C_4H_6O_3$

In the reaction, 2 moles of $C_7H_6O_3$ are necessary to react with 1 mol of $C_4H_6O_3$. Following this ratio:

$0.8\ mol\ C_4H_6O_3\cdot \frac{2\ mol\ C_7H_6O_3}{1\ mol\ C_4H_6O_3} = 1.6\ mol\ C_7H_6O_3$

We have only 0.5 mol of $C_7H_6O_3$, i.e, we don't have enough amount of salicylic acid, therefore, this is the limiting reactant.

Answer 2

Answer: B) Salicylic acid

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