What mass of potassium chlorate `(KClO_3)` on heating gives 1.891 g of potassium chloride (KCl) and 0.672 litres of oxygen at NTP?

Explanation

To solve the problem, we need to determine the mass of potassium chlorate (KClO₃) that decomposes to produce a given mass of potassium chloride (KCl) and a certain volume of oxygen gas (O₂).

### Step-by-Step Solution:

1. **Write the Decomposition Reaction:**
The decomposition of potassium chlorate can be represented by the following balanced chemical equation:
\[ 2 \text{KClO}_3 \rightarrow 2 \text{KCl} + 3 \text{O}_2 \]

2. **Calculate Moles of Oxygen Produced:**
We know that at NTP (Normal Temperature and Pressure), 1 mole of gas occupies 22.4 liters. We need to find the number of moles of oxygen produced from the given volume (0.672 liters).
\[ \text{Moles of } O_2 = \frac{0.672 \text{ L}}{22.4 \text{ L/mol}} = 0.03 \text{ moles} \]

3. **Relate Moles of Oxygen to Moles of KClO₃:**
From the balanced equation, we see that 3 moles of O₂ are produced from 2 moles of KClO₃. Therefore, we can set up a ratio to find the moles of KClO₃ that decomposed:
\[ \text{Moles of KClO}_3 = \frac{2}{3} \times \text{Moles of } O_2 = \frac{2}{3} \times 0.03 = 0.02 \text{ moles} \]

4. **Calculate the Mass of KClO₃:**
The molar mass of KClO₃ can be calculated as follows:
- K (Potassium) = 39.1 g/mol
- Cl (Chlorine) = 35.5 g/mol
- O (Oxygen) = 16.0 g/mol × 3 = 48.0 g/mol
\[ \text{Molar mass of KClO}_3 = 39.1 + 35.5 + 48.0 = 122.6 \text{ g/mol} \]
Now, we can find the mass of KClO₃ that decomposed:
\[ \text{Mass of KClO}_3 = \text{Moles of KClO}_3 \times \text{Molar mass of KClO}_3 = 0.02 \text{ moles} \times 122.6 \text{ g/mol} = 2.452 \text{ g} \]

5. **Calculate Total Mass:**
The total mass of KClO₃ that decomposed is the sum of the mass of KCl produced and the mass of O₂ produced.
- Mass of KCl given = 1.891 g
- Mass of O₂ produced (0.03 moles) = 0.03 moles × 32 g/mol = 0.96 g
\[ \text{Total mass of KClO}_3 = \text{Mass of KCl} + \text{Mass of O}_2 = 1.891 \text{ g} + 0.96 \text{ g} = 2.851 \text{ g} \]

### Final Answer:
The mass of potassium chlorate (KClO₃) that decomposes to give 1.891 g of potassium chloride and 0.672 liters of oxygen at NTP is **2.851 g**.