MediumChemistryHigh School (11–12)States of MatterNumericalBoard Exam AP A-Level IB

Potassium chlorate decomposes on heating: 2KClO₃ → 2KCl + 3O₂…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Potassium chlorate decomposes on heating: $2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$ . The oxygen is collected over water at $30\ ^\circ\text{C}$ . The gas jar holds $255\ \text{mL}$ at a total pressure of $101.3\ \text{kPa}$ . The vapour pressure of water at $30\ ^\circ\text{C}$ is $4.25\ \text{kPa}$ . Find the pressure of the dry oxygen, the amount of oxygen, and the mass of potassium chlorate that decomposed, with $M = 122.55\ \text{g/mol}$ .

Need a reminder? The key ideaShow

For gases that do not react, $P_{\text{total}} = \sum P_i$ and $P_i = x_iP_{\text{total}}$ . Over water, $P_{\text{gas}} = P_{\text{atm}} - P_{\text{water}}$ . Graham's law at the same $T$ and $P$ : $\dfrac{r_1}{r_2} = \sqrt{\dfrac{M_2}{M_1}}$ , and for equal amounts $\dfrac{t_1}{t_2} = \sqrt{\dfrac{M_1}{M_2}}$ .

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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