In this example, we use standard enthalpies of formation and the standard enthalpy change for a reaction. Our problem reads, "Calculate the standard enthalpy of formation for liquid acetone in kilojoules per mole given the standard enthalpies of formation of water vapor and carbon dioxide gas, and the balanced chemical equation where liquid acetone reacts with oxygen gas to produce carbon dioxide gas and water vapor." We're also given the standard enthalpy change for this reaction. In this problem, we need to use the equation for the standard change in enthalpy for a reaction, which is equal to the summation of the moles of product times the standard enthalpies of formation for the products minus the summation of the moles of reactant times the standard enthalpies of formation for the reactants. The idea behind the equation is that we can find the enthalpy change for a reaction by taking the final minus initial or the change in energy when we form all our products minus the change in energy when we form all our reactants. We use the standard state for consistency. From our balanced chemical equation, our products are 3 moles of CO2 and 3 moles of H2O, and we multiply them by their respective standard enthalpies of formation. Our reactants are 1 mole of acetone and 4 moles of oxygen gas multiplied by their respective standard enthalpies of formation. In the problem, we're given the standard enthalpy of reaction is -1658 kilojoules. We're also given the standard enthalpies of formation for CO2 gas and H2O gas. Notice that the 4 moles O2 gas times the standard enthalpy of formation has dropped out of the equation. This is because the standard enthalpy of formation of oxygen gas is zero. Oxygen gas is already the standard state of oxygen, so there is no energy change to produce oxygen gas from its constituent element, which is oxygen, in its standard state. If we multiply and add what's in the initial brackets, we should get -1905.9 kilojoules. Then we can solve for the standard enthalpy of formation for acetone by rearranging the equation. We should get -247.9 kilojoules per mole.