Question

25.5 g of ${H_2}{O_2}$ solution on decomposition gave 1.68 L of ${O_2}$ at STP. The percentage strength by weight of the solution is :
a.) 30
b.) 10
c.) 20
d.) 25

Answer 1

The percentage strength by weight means the amount of solute present by weight in the solution. For this, we need to know the decomposition reaction of hydrogen peroxide which is given as -
$2{H_2}{O_2} \to 2{H_2}O + {O_2}$

Complete Solution :
First, let us write the decomposition reaction of hydrogen peroxide. The reaction can be written as -
$2{H_2}{O_2} \to 2{H_2}O + {O_2}$

So, the two moles of hydrogen peroxide will give 1 mole of oxygen.
At STP,
1 mol of oxygen has volume = 22.4 L
So, the 1 L volume of oxygen in moles = $\dfrac{1}{{22.4}}$ moles
So, the 1.68 L volume of oxygen has volume = $\dfrac{1}{{22.4}} \times 1.68$
1.68 L volume of oxygen has volume = 0.075 moles of oxygen
As we know that two moles of hydrogen peroxide will give 1 mole of oxygen.
So, the amount of hydrogen peroxide in solution = 0.075 $\times$ 2

Amount of hydrogen peroxide in solution = 0.15 mole of hydrogen peroxide
We know molar mass of hydrogen peroxide = 34 g/mol
Mass of hydrogen peroxide decomposed = 0.15 $\times$ 34
Mass of hydrogen peroxide decomposed = 5.1 g hydrogen peroxide in 25.5 g solution
100 g solution has = $\dfrac{{100}}{{25.5}} \times 5.1$
100 g solution has = 20 %
So, the original solution has 20 % strength by weight solution.
So, the correct answer is “Option C”.

Note: It must be noted that the hydrogen peroxide is so reactive that if the bottle in which it is remained opened then the solution may react with air to produce water and oxygen. So, the bottle without cap will contain water and not hydrogen peroxide.