A 30 % solution of \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] is marketed as ‘100 volume’ hydrogen peroxide.
Reason: 1L of 30 % \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] will give 100 mL of oxygen at STP.
A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct and reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer
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Hint:
The percentage of any solution is generally expressed in three ways: as ‘percentage by mass’ or as ‘percentage by volume’ or as ‘mass by volume’.
Percentage by mass is weight/ weight or w/ w and percentage by volume is weight/ volume or w/ v.

Complete step by step answer:
The concentration of a solution represents the amount or quantity of the solute which is dissolved in a known amount of the solvent or solution. The concentration of any solution can be usually expressed in various common ways.
One of these common ways to express concentration of a solution is the percentage of a solution.
In case of a solid dissolved in a liquid, the percentage by mass denotes the mass of the solute in grams which is present in 100 g of the solution. This means it is weight/ weight or w/ w. For example, if the concentration of sodium carbonate is expressed as a 10 % solution of sodium carbonate by mass, then it means that 10 grams of sodium carbonate are present in 100 grams of the solution.
In case of a solid dissolved in a liquid, the percentage by volume denotes the mass of the solute in grams which is present in 100 mL of the solution. This means it is weight/ volume or w/ v. For example, if the concentration of sodium carbonate is expressed as 10 % solution of sodium carbonate by volume, then it means that 10 grams of sodium carbonate are present in 100 millilitres of the solution.
In case of a liquid dissolved in a liquid, the percentage by mass has the same meaning but the percentage by volume denotes the volume of the liquid solute in ${\text{c}}{{\text{m}}^{\text{3}}}$ present in 100 ${\text{c}}{{\text{m}}^{\text{3}}}$ of the solution. This means it is volume/ volume or v/ v.
Therefore, the statement ‘30 % solution of \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] is marketed as ‘100 volume’ hydrogen peroxide’ is correct as 30 % solution of \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] means 30 grams of hydrogen peroxide present in 100 mL of the solution. It is not because 1L of 30 % \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] will give 100 mL of oxygen at STP. Also, 1 millilitre of 30 % \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\] will give 100 mL of oxygen at STP.
So, the assertion is correct and reason is incorrect and so the correct option is C.

Note:
Concentration of a solution can also be expressed in terms of molarity and normality.
Molarity represents the number of moles of the solute which is dissolved per litre of the solution while normality represents the number of gram equivalents of the solute which is dissolved per litre of the solution.