[John11]

10.0g of sodium reacts with an excess of water  with ΔH = –79.91 kJ, whereas 20.0g of sodium oxide react with an excess of water with ΔH = –76.76 kJ.
Standard enthalpy of formation of water: ΔHf = −286 kJ mol
Calculate the standard enthalpy change of formation of sodium oxide.
I'm not entirely sure how to set this problem up, could someone please help me. Thank you!

[Orcio_87]

First reaction (reaction of sodium with water) + enthalpy of formation of H₂O gives an information about enthalpy of formation of sodium hydroxide.

Second reaction (sodium oxide + water) + calculated earlier value, gives an information about enthalpy of formation of Na₂O (about -415,6 kJ / mol)

[John11]

Thank you!