# Can anyone help explain the reasoning behind the answers?

## I kept getting a different solution for part b and c

Jul 23, 2016

As Wikipedia says
"The limiting reagent (or limiting reactant) in a chemical reaction is the substance that is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent, since the reaction cannot continue without it. If one or more other reagents are present in excess of the quantities required to react with the limiting reagent, they are described as excess reagents or excess reactants."

The balanced equation of the reaction

$3 C {H}_{3} C O O H + A l {\left(O H\right)}_{3} \to {\left(C {H}_{3} C O O\right)}_{3} A l + 3 {H}_{2} O$

$\text{Molar mass of } C {H}_{3} C O O H$
$= 2 \cdot 12 + 4 \cdot 1 + 2 \cdot 16 = 60 \frac{g}{\text{mol}}$

$\text{Molar mass of } A l {\left(O H\right)}_{3}$
$= 27 + 3 \cdot 1 + 3 \cdot 16 = 78 \frac{g}{\text{mol}}$

$\text{Molar mass of } {\left(C {H}_{3} C O O\right)}_{3} A l$
$= 27 + 3 \cdot 59 = 204 \frac{g}{\text{mol}}$

a) By the above balanced equation the mole ratio of reactants

$C {H}_{3} C O O H : A l {\left(O H\right)}_{3} = 3 : 1$

So mass ratio should be $3 \times 60 : 1 \times 78 = 30 : 13$

But the ratio taken$C {H}_{3} C O O H : A l {\left(O H\right)}_{3} = 125 : 275 = 5 : 11$

As per balanced reaction 12 5g $C {H}_{3} C O O H$ requires
$\frac{13}{30} \times 125 g \approx 54 g A l {\left(O H\right)}_{3}$

But $A l {\left(O H\right)}_{3}$ taken is 225g. Hence $C {H}_{3} C O O H$ will be fully consumed but $A l {\left(O H\right)}_{3}$ will be excess.

So $C {H}_{3} C O O H \to$ limiting reagent

b) Excess $A l {\left(O H\right)}_{3} \to 225 - 54 = 221 g$

c) By the above balanced equation the mole ratio of reactants

$C {H}_{3} C O O H : {\left(C {H}_{3} C O O\right)}_{3} A l = 3 : 1$
so mass ratio should be $3 \times 60 : 1 \times 204 = 15 : 17$

As per balanced reaction 12 5g $C {H}_{3} C O O H$ produces
$\frac{17}{15} \times 125 g \approx 141 g {\left(C {H}_{3} C O O\right)}_{3} A l$

Decimal place not considered in calculation