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Understanding stoichiometry is essential for solving chemical equations and determining the relationship between reactants and products. Stoichiometry involves using the balanced chemical equation to understand the proportions of reactants and products in a chemical reaction. For example, consider the decomposition of potassium chlorate (\text{KClO}_3). The balanced equation is:
\[ 2\text{KClO}_3 (s) \rightarrow 2\text{KCl} (s) + 3\text{O}_2 (g) \]
This equation tells us that \textbf{2 moles} of \text{KClO}_3 produce \textbf{2 moles} of \text{KCl} and \textbf{3 moles} of \text{O}_2.

By using these ratios, we can calculate how much of one substance we need or will create by knowing the quantity of another. In our exercise, we use stoichiometry to find out how many moles of \text{KClO}_3 were used by relating them to the moles of \text{O}_2} produced. By understanding these mole ratios, we can deduce quantitative information about the substances involved in the chemical reaction. This is a powerful tool in chemistry for predicting outcomes and designing reactions.

Molar mass is the mass of one mole of a substance, measured in grams per mole (\text{g/mol}). The molar mass helps us convert between the mass of a substance and the number of moles. To find the molar mass, you sum the atomic masses of all atoms in a molecule.

For example, let's calculate the molar mass of potassium chlorate (\text{KClO}_3):
\[ \text{Molar mass of KClO}_3 = (39.1 \text{ for K}) + (35.5 \text{ for Cl}) + (3 \times 16 \text{ for 3 O atoms}) = 122.6 \text{ g/mol} \]
In our exercise, we calculated the molar mass of \text{O}_2 and \text{KClO}_3 to convert between grams and moles. The molar mass of \text{O}_2 is: \[ \text{Molar mass of O}_2 = 2 \times 16 = 32 \text{ g/mol} \]
Using these molar masses, we can solve the exercise by converting the grams of \text{O}_2 produced to moles, and then using the stoichiometric ratios to find out how many moles of \text{KClO}_3 were decomposed. Finally, we convert the moles of \text{KClO}_3 back to grams using its molar mass.

Mass percent composition is a way of expressing the concentration of a component in a mixture. It measures how much of the total mass is made up by a specific component. It is calculated by dividing the mass of the component by the total mass of the mixture and then multiplying by 100 to get a percentage.

In our exercise, we found the mass percent of potassium chlorate (\text{KClO}_3) in the original mixture by first determining the mass of decomposed \text{KClO}_3.
We found that: \[ \text{Mass of } \text{KClO}_3 = 0.639 \text{ g} \]
Given that the total mass of the mixture was 0.950 g, we calculated the mass percent using: \[ \text{Mass percent of } \text{KClO}_3 = \frac{0.639 \text{ g}}{0.950 \text{ g}} \times 100 = 67.3 \text{%} \]
This means that 67.3% of the original mixture was \text{KClO}_3. The mass percent composition helps us understand the proportion of each substance in mixtures, which is crucial for many chemical applications including reactions and formulations.