Question

Standard enthalpy of formation of water is \(-286\,\text{kJ mol}^{-1}\). When \( 1800\,\text{mg}\) of water is formed from its constituent elements in their standard states, the amount of energy liberated is
 

• A. \(2.86\,\text{kJ}\)
• B. \(5.72\,\text{kJ}\)
• C. \(57.2\,\text{kJ}\)
• D. \(28.6\,\text{kJ}\)

Hint:

Always convert the given mass into moles before using molar enthalpy values.

Answer 1

The Correct Option is D

Step 1: Convert mass of water into moles.
\[ 1800\,\text{mg} = 1.8\,\text{g} \] \[ \text{Moles of water} = \frac{1.8}{18} = 0.1\,\text{mol} \]
Step 2: Use standard enthalpy of formation.
Standard enthalpy of formation of water \(= -286\,\text{kJ mol}^{-1}\).
Step 3: Calculate energy released.
\[ \Delta H = 0.1 \times 286 = 28.6\,\text{kJ} \]
Step 4: Conclusion.
Hence, the amount of energy liberated is \(28.6\,\text{kJ}\).