Enthalpy of formation

Question

A scientist measures the standard enthalpy change for the following reaction to be -220.4 kJ :

2H₂O₂(l)2H₂O(l) + O₂(g)

Based on this value and the standard enthalpies of formation for the other substances, the standard enthalpy of formation of H₂O(l) is  _____ kJ/mol.

Answer 1

A scientist measures the standard enthalpy change for the following reaction to be -220.4 kJ :

2H₂O₂(l)2H₂O(l) + O₂(g)

Somewhere they told you, or should've that ΔH_formation for O₂=0. They must've also told you that ΔH_formation for H₂O₂= -187.6, because otherwise this problem is impossible.

Then:

ΔH_rxn=ΔH_f(products)-ΔH_f(reactants)

ΔH_rxn=(2·H₂O+O₂)-(2H₂O₂)

-220.4=(2x+0)-(2• -187.6)

-220.4=2x - ^-375.2

-220.4=2x+375.2

-595.6=2x

x=-297.8 kJ

answer makes sense because the actual value is -285 kJ.