Suppose a piece of sodium metal gains 2.05 g of mass after being exposed to air. Assume that this gain can be attributed to its reaction with oxygen. (a) What mass of O2 reacted with the Na? (b) What mass of Na reacted? (c) What mass of Na2O formed?

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the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.