Question

Heat of formation of H\(_2\)O is = -270 kJ mol\(^{-1}\). How much water can be decomposed by 675 kJ of heat?

• A. 45 g
• B. 40 g
• C. 27 g
• D. 33 g

Hint:

Use the heat of formation and molar mass to calculate the mass of a substance that can be decomposed or formed based on the given heat.

Answer 1

The Correct Option is A

Step 1: Understanding the problem.
The given heat of formation for H\(_2\)O is -270 kJ mol\(^{-1}\), which represents the heat released when 1 mole of H\(_2\)O is formed. We need to calculate the amount of water that can be decomposed by 675 kJ of heat.
Step 2: Calculation.
First, calculate the moles of H\(_2\)O that can be decomposed: \[ \text{Moles of H}_2\text{O} = \frac{675 \, \text{kJ}}{270 \, \text{kJ/mol}} = 2.5 \, \text{mol} \] Now, use the molar mass of water (18 g/mol) to find the mass of water: \[ \text{Mass of H}_2\text{O} = 2.5 \, \text{mol} \times 18 \, \text{g/mol} = 45 \, \text{g} \] Step 3: Conclusion.
The correct answer is (A) 45 g, as 675 kJ of heat will decompose 45 g of water.