Here it says, consider the following thermochemical reaction. We have 2 moles of magnesium solid reacting with 1 mole of oxygen gas to produce 2 moles of magnesium oxide solid. It gives us an enthalpy of reaction equal to negative 1204 kilojoules. Here we're asked how many grams of magnesium oxide are produced during an enthalpy change of negative 375 kilojoules. So what we have to do here is we have to convert the given quantity into moles of the given.
Alright. They're giving us negative 375 kilojoules of energy, and what we need to do is establish a relationship between magnesium oxide and this value of negative 375 kilojoules. Well, according to my balanced equation for step 2, it says we need to do a mole to mole comparison to convert moles of given into moles of unknown. Here because it's a thermochemical equation, it's going to be moles of given relating to delta H of reaction. So we're going to say for every 2 moles of magnesium oxide, the energy involved, or the enthalpy involved, is negative 1204 kilojoules per mole. So we've just found our moles concerning magnesium oxide.
Step 3 says if necessary, convert the moles into desired units. Here, they want grams not moles, so we're going to do one more step and say for every one mole of magnesium oxide, the mass of magnesium oxide, 1 magnesium is 24.31 grams according to the periodic table. One oxygen is 16 grams. Multiplying and adding those numbers together gives us 40.31 grams. Moles cancel out, and now I'm going to have 12.59 grams of magnesium oxide.
Step 4 isn't needed here because in step 4, if we have to calculate more than one final amount, then we must compare them to determine the theoretical yield. Here we're only given one given amount of negative 375 kilojoules and using that helped us to determine the final answer of 12.59 grams of magnesium oxide.