Question

How much moles of ${{NaOH}}$ required to balance the above equation?
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + NaOH + }}{{{H}}_{{2}}}{{O (l) }} \to {{ NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
(A) 1
(B) 2
(C) 3
(D) 4

Answer 1

In the above question we are asked to find out the number of moles of ${{NaOH}}$ required to balance the above equation. So, we have to first equalise the number of atoms on both sides of the equation. Then the coefficient of ${{NaOH}}$ will be our desired answer.

Complete step by step solution
In the above question, the equation below is given:
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + NaOH + }}{{{H}}_{{2}}}{{O (l) }} \to {{ NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
Now, we will start balancing the equation from the left most atom present.
In the above equation, there are 2 Al in the left hand side and only 1 in the right hand side, so, we will multiply 2 to the right hand side.
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + NaOH + }}{{{H}}_{{2}}}{{O (l) }} \to {{ 2NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
Due to multiplication of 2 in the right hand side the number of Na atoms increased to 2. Hence, we should balance it by multiplying 2 on the left hand side. Hence, we will get:
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + 2NaOH + }}{{{H}}_{{2}}}{{O (l) }} \to {{ 2NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
Now, let us balance H atom:
Since, we have ${{2 + 2}} = {{4H}}$ atom in the left hand side and ${{4 \times 2 = 8H}}$ atom in the right hand side, we can multiply 3 as the coefficient of ${{{H}}_{{2}}}{{O}}$ to balance number of hydrogen atom.
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + 2NaOH + 3}}{{{H}}_{{2}}}{{O (l) }} \to {{ 2NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
Now, let us balance the number of oxygen atoms. There are ${{3 + 2 + 3 = 8O}}$ atoms in the left hand side and ${{4 \times 2 = 8O}}$ atoms in the right hand side.
Hence, the balanced equation is:
${{A}}{{{l}}_{{2}}}{{{O}}_{{3}}}{{ + 2NaOH + 3}}{{{H}}_{{2}}}{{O (l) }} \to {{ 2NaAl(OH}}{{{)}}_{{4}}}{{ (aq)}}$
The coefficient of ${{NaOH}}$ is 2. Hence, 2 moles of ${{NaOH}}$ is required to balance the equation.

Therefore, the correct option is option B.

Note
In these types of questions where we want to find the number of moles of a reactant or product we have to balance the equation in order to get the correct result. These coefficients are called stoichiometric ratios and are useful in calculating the mass of a product formed from a given mass of reactants.