Here in this example question it says, how many grams of H2O are produced when 12.3 grams of H2 reacts? Alright. So we know that they're giving us information on one compound in a balanced equation and asking for information on another. We know that this is the definition of stoichiometry. So we're going to have to utilize the stoichiometric chart in some way to solve this problem. Now, if we follow the steps, it says step 1, map out the portion of the stoichiometric chart you will use. From the question, they're giving us 12.3 grams of H2. Since that's a value they're giving to you, that represents our grams of given. So we're gonna start at grams of given, which is just grams of H2, and we're gonna convert those grams into moles of given, so moles of H2. Now in the same question they're asking us to find the grams of H2O. Since we don't know they're asking us to find it, this represents our grams of unknown. So that tells me I have to go for moles of given and find a way to get to grams of unknown. Now at this point, to go for moles of given, I have to go to moles of unknown and that is where it's required to do the job. So we're gonna go from moles of given to moles of unknown. And then finally, we go from moles of unknown to grams of unknown. This is the path that we're going to take to answer this question. So let's go to step 2. It says convert the given quantity into moles of given. And if a compound is set to be in excess, then just ignore it. In this question, they don't say anything as being in excess, so we don't have to worry about this, next line under step 2. Later on, we'll come into situations where we're told something is an excess. Excess, and in that case, we just simply ignore it. Alright. So we're going to take our given quantity, which is 12.3 grams H2, and we're going to convert it into moles of given. Grams of H2 go on the bottom, and 1 mole of H2 goes on top. H2 has in it 2 hydrogens, and according to the periodic table, each one weighs 1.008 grams, so that's 2.016 grams. Grams here cancel out and I'll have moles of H2 which comes out to 6.1012 moles of H2. So I've just gone from grams of H2 to moles of H2. At this point, we have to do the jump. So going to step 3, it says to do a mole to mole conversion in order to..."
convert moles of given into moles of unknown. So we take that 6.1012 moles of H2. To get rid of moles of H2, I put them on the bottom. What am I looking for? I'm looking for my unknown. My unknown is water, so I need to find moles of H2O. Now remember, to go from moles of given to moles of unknown, that's called the jump, and we do a mole to mole comparison. The equation says that for every 2 moles of H2, I have 2 moles of H2O. So for every 2 moles of H2, I have 2 moles of H2O. So here, moles of H2 cancel out, and now I have moles of H2O. So that's 6.1012 moles of H2O. Now finally, if necessary, convert the moles of unknown into the final desired units. Sometimes they may ask us to just find the moles of our unknown, and in that case, we'd stop. For this particular question though, they're not asking us to find moles of water, they're asking us to find grams of water. So an additional step is required. So 6.1012 moles of H2O. Gotta get rid of moles of H2O, so one mole of H2O on the bottom. How many grams of H2O do we have on top? H2O is composed of 2 hydrogens and one oxygen. Hydrogen, according to the periodic table, is 1.008 grams. Oxygen is 16 grams. This is 2.016 grams, and this is 16 grams, so the complete mass of H2O is 18.016 grams. So we plug that here. Moles of water cancel out, and now I'm finally gonna have grams of water. So here we're going to multiply those out, so that's 109.9 grams of H2O. Now, technically, within this question, 12.3 has 3 significant figures. So technically we should write this in 3 significant figures as well. So in scientific notation that will come out to 1.10 times 10 to the 2 grams of H2O. So this would be our final answer for this stoichiometric question. So remember, if we are given the amount of a compound within a balanced equation and asked to find another, we're dealing with stoichiometry, which means you have to utilize the stoichiometric chart in order to find your final answer.