Question

250 ml aq. solution H2O2 produces just sufficient amount of 0.2 at STP which burns 25g impure sample of calcium having 60 percent purity. Calculate volume strength of H2O2 solution.

Answer 1

Answer on Question #44471 – Chemistry – Other

Question

250 ml aq. solution H₂O₂ produces just sufficient amount of O₂ at STP which burns 25 g impure sample of calcium having 60 percent purity. Calculate volume strength of H₂O₂ solution.

Solution

Mass of pure calcium in the sample of impure calcium:

$$m_{Ca} = \frac{m_{Ca}^{impure} \cdot 60\%}{100\%} = \frac{25\,g \cdot 60\%}{100\%} = 15\,g$$

Chemical equation of burning of calcium:

$$\begin{array}{c c} 15\,g & n_{O_2}, \text{mol} \end{array}$$ $$2\,\mathrm{Ca} + \mathrm{O}_2 \rightarrow 2\,\mathrm{CaO}$$

2·40 g/mol 1 mol

Based on the chemical equation number of moles of O₂ sufficient to burn the given amount of calcium can be calculated:

$$n_{O_2} = \frac{m_{Ca} \cdot 1\,\text{mol}}{2\,\text{mol} \cdot M_{Ca}} = \frac{15\,g \cdot 1\,\text{mol}}{2\,\text{mol} \cdot 40\,\frac{g}{\text{mol}}} = 0.19\,\text{mol}$$

Chemical equation of H₂O₂ decomposition to give O₂ is as follows:

$$\begin{array}{c c} m_{\mathrm{H_2O_2}}, \, \text{g} & 0.19\,\text{mol} \end{array}$$ $$2\,\mathrm{H_2O_2} \rightarrow 2\,\mathrm{H_2O} + \mathrm{O_2}$$

2·34 g/mol 1 mol

Based on the chemical equation, mass of absolute H₂O₂ that can produce given amount of O₂ can be calculated:

$$m_{H_2O_2} = \frac{2\,\text{mol} \cdot M_{H_2O_2} \cdot n_{O_2}}{1\,\text{mol}} = \frac{2\,\text{mol} \cdot 34\,\frac{g}{\text{mol}} \cdot 0.19\,\text{mol}}{1\,\text{mol}} = 12.8\,g$$

Density of absolute H₂O₂ is 1.45 g/ml (reference datum). Volume of absolute H₂O₂ in the solution:

$$V_{H_2O_2} = \frac{m_{H_2O_2}}{\rho_{H_2O_2}} = \frac{12.8\,g}{1.45\,\frac{g}{\text{ml}}} = 8.8\,\text{ml}$$

Volume strength of the H₂O₂ solution:

$$\%_{H_2O_2} = \frac{V_{H_2O_2}}{V_{\text{solution}}} 100\% = \frac{8.8\,\text{ml} \cdot 100\%}{250\,\text{ml}} = 3.53\%\,\text{by vol.}$$

Answer: 3.53% by vol.

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