Question

Calculate % by mass of a $\text{H}_{2}\text{O}_{2}$ solution that is 67.2 by volume.

• A. 13.60% by mass
• B. 20.40% by mass
• C. 22.44% by mass
• D. 17.60% by mass

Hint:

Shortcut Formula: $% \text{ strength} = \frac{17}{56} \times \text{Volume Strength}$.
Here: $\frac{17}{56} \times 67.2 = 17 \times 1.2 = 20.4%$. Faster than finding molarity!

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
"Volume strength" of $H_2O_2$ refers to the volume of oxygen gas (at STP) liberated by 1 unit volume of the solution.

Step 2: Formula Application:
Molarity ($M$) = $\frac{\text{Volume Strength}}{11.2}$
% by mass ($w/v$) = $\text{Molarity} \times \text{Molar mass of } H_2O_2 \times \frac{1}{10}$

Step 3: Explanation:
1. Molarity = $\frac{67.2}{11.2} = 6$ M.
2. Mass of $H_2O_2$ in 1 L = $6 \times 34 = 204$ g.
3. % strength ($w/v$) = $\frac{204}{1000} \times 100 = 20.4%$.

Step 4: Final Answer:
The percentage by mass is 20.40%.