What mass of potassium chlorate (`KClO_3`) on heating gives 1.491 g of potassium chloride (KCl) and 0.672 litres of oxygen at NTP?

Explanation

To find the mass of potassium chlorate (KClO₃) that decomposes to give 1.491 g of potassium chloride (KCl) and 0.672 liters of oxygen (O₂) at NTP, we can follow these steps:

### Step 1: Write the balanced equation for the decomposition of KClO₃.

The decomposition reaction of potassium chlorate can be represented as:
\[ 2 \text{KClO}_3 \rightarrow 2 \text{KCl} + 3 \text{O}_2 \]

### Step 2: Calculate the number of moles of oxygen produced.

At Normal Temperature and Pressure (NTP), 1 mole of gas occupies 22.4 liters. We can calculate the number of moles of oxygen produced from the given volume:
\[ \text{Number of moles of } O_2 = \frac{0.672 \text{ L}}{22.4 \text{ L/mol}} \approx 0.03 \text{ moles} \]

### Step 3: Determine the moles of KCl produced.

From the balanced equation, we see that 3 moles of O₂ are produced for every 2 moles of KCl. Therefore, we can set up a ratio to find the moles of KCl produced:
\[ \frac{2 \text{ moles KCl}}{3 \text{ moles } O_2} = \frac{x \text{ moles KCl}}{0.03 \text{ moles } O_2} \]
Solving for \( x \):
\[ x = \frac{2}{3} \times 0.03 \approx 0.02 \text{ moles of KCl} \]

### Step 4: Calculate the mass of KCl produced.

The molar mass of KCl is approximately 74.55 g/mol. Thus, the mass of KCl produced can be calculated as:
\[ \text{Mass of KCl} = \text{Number of moles} \times \text{Molar mass} = 0.02 \text{ moles} \times 74.55 \text{ g/mol} \approx 1.491 \text{ g} \]

### Step 5: Calculate the mass of oxygen produced.

The molar mass of O₂ is approximately 32 g/mol, so the mass of oxygen produced is:
\[ \text{Mass of O}_2 = \text{Number of moles} \times \text{Molar mass} = 0.03 \text{ moles} \times 32 \text{ g/mol} \approx 0.96 \text{ g} \]

### Step 6: Calculate the total mass of KClO₃ decomposed.

According to the law of conservation of mass, the total mass of the reactants equals the total mass of the products. Thus, we can find the mass of KClO₃:
\[ \text{Mass of KClO}_3 = \text{Mass of KCl} + \text{Mass of O}_2 \]
\[ \text{Mass of KClO}_3 = 1.491 \text{ g} + 0.96 \text{ g} = 2.451 \text{ g} \]

### Final Answer:
The mass of potassium chlorate (KClO₃) that decomposes to give the specified amounts is approximately **2.451 g**.

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