Question

Standard enthalpy of formation of water is - 286 kJ mol^-1. When 1800 mg of water is formed from its constituent elements in their standard states the amount of energy liberated is ______.

Options

• 2.86 kJ

• 57.2 kJ

• 5.72 kJ

• 28.6 kJ

Answer 1

Standard enthalpy of formation of water is - 286 kJ mol^-1. When 1800 mg of water is formed from its constituent elements in their standard states the amount of energy liberated is 28.6 kJ.

Explanation:

\[\ce{H_{2(g)} + 1/2 O_{2(g)} -> \underset{18 g}{H2O_{(l)}}}\]

`Delta "H"_"f"^circ = - 286  "kJ"  "mol"^-1`

For 18 g of H₂O, amount of energy liberated = 286 kJ mol^-1

∴ For 1.8 g of H₂O, amount of energy liberated `= (1.8 xx 286)/18` = 28.6 kJ