In order to understand what is the limiting reactant is useful to convert the masses of reactants into moles. The molecular masses are:
[tex]C_7H_6O_3 = 138\frac{g}{mol}\\ C_4H_6O_3 = 100\frac{g}{mol}[/tex]
Converting masses into moles:
[tex]70\g \C_7H_6O_3\cdot \frac{1\ mol}{138\ g} = 0.5\ mol\ C_7H_6O_3[/tex]
[tex]80\ g\ C_4H_6O_3\cdot \frac{1\ mol}{100\ g} = 0.8\ mol\ C_4H_6O_3[/tex]
In the reaction, 2 moles of [tex]C_7H_6O_3[/tex] are necessary to react with 1 mol of [tex]C_4H_6O_3[/tex]. Following this ratio:
[tex]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[/tex]
We have only 0.5 mol of [tex]C_7H_6O_3[/tex], i.e, we don't have enough amount of salicylic acid, therefore, this is the limiting reactant.
