Video instructions and help with filling out and completing Who Form 5495 Limitation

Instructions and Help about Who Form 5495 Limitation

In this video we're going to work through a practice problem on the limiting reactant and excess reactant this problem is pretty straight forward so if you're just starting out learning limiting reactant this is a great place to begin we're going to use this equation to answer the following questions what is the greatest amount of MgO magnesium oxide in moles that can be made with 7.8 moles of Mg magnesium and 4.7 moles of o2 oxygen which is a limiting reactant which reactant is an excess and how many moles of it are left over so this problem is all about moles how many moles of this do we have how many moles of this can we make so I want to put this information about the number of moles that we have I want to put this front and center all right we have 7.8 moles of magnesium and 4.7 moles of oxygen this is what we're starting with we're going to keep referring back to this information through the whole problem okay so we want to find the greatest amount of magnesium oxide that we can make with this amount of magnesium and this amount of oxygen in order to do that we've got to figure out which of these two things is the limiting reactant is it magnesium or is it oxygen to figure out which of these is a limiting reactant we're going to ask a few questions okay first we're going to ask to use all of the magnesium how many moles of oxygen do we need then we'll kind of ask the reverse well flip it around and we'll ask to use all of the oxygen how many moles of magnesium do we need well look at the answers from these two questions and we'll figure out which of these runs out first and the first reactant that we run out of that is the limiting reactant okay so let's go ahead and start with this question you're going to get rid of this paper right here so we just have a little bit more space and we want to find out to use all of the magnesium how many moles of oxygen do we okay so we're starting with a maximum amount of magnesium seven point eight moles of magnesium how much oxygen do we need to react with that well I'm going to multiply this by a conversion factor that shows the relationship in this equation between moles of magnesium and moles of oxygen okay there's a two in front of the magnesium there's nothing in front of the oxygen that means it's really a 1 here or we can write this in to make it a little easier there's it's always a 1 if there's nothing in front of the compound so this equation is telling us that there are two moles of magnesium for every one mole of oxygen in this reaction okay so I want to get rid of moles magnesium so I'm going to put moles of magnesium on the bottom I have two moles to moles magnesium and then I have one mole oxygen on the top okay so moles magnesium on the top moles magnesium on the bottom please cancel out and the math is going to be this times this divided by this which gives me 3.9 moles of oxygen so what's this telling us well it's telling us if we want to put this in words it's telling us that we need 3.9 moles of oxygen to use all of the magnesium okay so we're going to put this right this is how much oxygen we need to use all the magnesium now let's ask the same thing about the oxygen to use all of this oxygen how many moles of magnesium do we need okay so we're starting with 4.7 moles of oxygen and we're going to multiply that by conversion factor that gets us to moles of magnesium so what's the relationship here two moles of magnesium for one mole of oxygen so I'm going to put moles of oxygen on the bottom one mole oxygen two moles of magnesium on the top two moles mg we have moles o2 on the top moles o2 on the bottom this times this divided by this is going to give nine point four moles of magnesium okay and to put this into words just like we did before this is telling us that we need nine point four moles of magnesium to use all of the oxygen okay so now that we've answered these two questions we can figure out which of these two things is the limiting reactant okay which do we run out of first check this out to use all of the magnesium we need 3.9 moles of oxygen is the oxygen going to run out if we do that well it's not because we have 4.7 moles we're starting with we only need 3.9 to use all the magnesium so it doesn't look like oxygen is going to run out in this situation now down here to use all of the oxygen we need nine point four moles of magnesium is that going to work no it's not because we only have seven point eight moles of magnesium that means that we don't have enough magnesium to use all of the oxygen means that magnesium is going to run out it's going to be the first reactant to run out so it is going to be the limiting reactant in other words we could say this we could say that we have enough oxygen to use all of the magnesium right we have the 3.9 moles that we need we have more than those 3.9 moles to use all the magnets we have enough oxygen to use all the magnesium but we don't have enough magnesium to use all of the oxygen we need 9.4 moles we