4what is the net amount of heat released when two moles of c2h6(g) are formed from its elements at 101.3 kpa and 298 k?

Enthalpy of formation is defined as the heat change that is needed for formation of one mole of substance from its constituent elements in pure state.

In present case, Heat of formation of one mole of C2H6 at 101.3 kpa and 298 k is -84 kJ/mol. Negative sign indicates that, heat is evolved from the system.

Thus, for two mole of ethane, heat of formation will be 2 X (-84) = -168 kJ/mol.