00:01
Particular question, we have a chemical reaction that's occurring where we have calcium cyanamide over here, underlying in red, and it's reacting with water, which i'll underline in blue, although there should be a fairly recognizable molecule, h2o, and we're forming calcium carbonate and ammonia.
00:18
So we have several different parts of this question to work through.
00:23
So let's just begin with part a, and then we can just go through the problem as we need to.
00:28
So part a, we are given the amount of calcium cyanamide, 75 grams, and we're told to figure out how much water we would need to react.
00:41
So how many grams of water would we need to react with this amount of calcium cyanomide? so there's a few different ways you could go about this problem, but before we begin these problems, we need to check the stoichiometry component.
00:52
We need to see if everything's balanced.
00:54
Do we have an equal representation of each element in the reactants and the products? so i've already went through and counted everything, but you can just double check my work.
01:02
It's always a good thing to do.
01:04
And if we were to look at each element, the amount that's per molecule and the amount of moles in that reaction, we should see the same amount on throughout the whole equation.
01:13
And between the reactants and products as well.
01:17
So everything looks balanced.
01:19
So now what we need to do is we need to find out the molar masses, meaning when we look at the components of each of these molecules, how much do all of the elements weigh, or what is their mass in totality? and you can do this by looking at a periodic table.
01:34
You can see the amu value below each element.
01:37
For example, carbon, you'll see 12.
01:39
And the units are going to be amu or grams per mole, which are very useful for these sort of problems.
01:47
So if he calculates calcium cyanamide, i'll just draw a little arrow here to show that it's in reference to this value here.
01:55
Per one mole we have 80 grams per mole if we add up the calcium sorry the calcium and then everything else that is what we're going to be working with here so and for water which we're trying to find out i'll just draw this over here to the right so it's a little bit easier to understand this is going to be 18 just per one mole if we calculate and add hydrogen with oxygen but two hydrogen of course that is the reading per one mole.
02:28
So just intuitively, we can assume that we're dealing with less than one mole of this calcium cyanamide because, or just over.
02:37
Sorry, no, i was right.
02:39
Just one mole less because 75 is less than 80, of course.
02:43
So how would we go about solving this then? so first, i think it's, let's see what the ratio is, what factor is, are we dealing with if it's less than one mole.
02:55
So i'm just going to write it out like this.
02:57
You have 75 out of 80.
03:00
And there's a few different ways you can go about this problem.
03:01
This isn't the only way, but this makes a great deal of sense, and it will be very useful as you see here in a second.
03:08
Then we can do cross multiplication, because this is the ratio, 75 out of 80, and then we don't know this amount for water, right, out of 18.
03:21
So we can just cross multiply like this and then solve for x.
03:26
This is a little bit of a hack.
03:30
I guess you could say.
03:31
So if you were to run through everything, you would just have x equals 75 times 18, and then you're going to divide by 80, of course.
03:41
And this is going to give you your x value, which is the amount of water for this problem, but let's not get ahead of ourselves, because this isn't actually the final say in this part of the problem, because there's another end component as well, which we will get into just one second.
04:02
So the actual amount is 16 .875 for x, 16 .875, which makes sense.
04:12
You know, so that's how much we would have for this water.
04:14
But this is not the amount because we need to look at the mold -to -mole ratios.
04:18
Because for every one mole of calcium cyanamide, we have three of water.
04:22
So we need to actually multiply this by three because this is just one mole.
04:26
But we'd have three times the amount of water in it for this reaction to occur.
04:31
So just take that number, multiply it by.
04:33
Three and we have 50 .625 grams to be exact of h2o and that's all there is to it okay so that's part a we're all set so i'll just circle that in blue and then we can move on to the next part of the problem so circle there third times the charm okay so that's part a i guess fred would have made more sense to circle it but no bother we should be fine to keep on going.
05:12
So now we have part b.
05:14
How many grams of nh3, that is ammonia, are produced from 5 .24 grams of cacn2.
05:22
So here's where we can start to apply things a bit more.
05:27
Okay? so we can look at our mold -a -mole ratios.
05:30
And in this one, we, again, we're given a different starting amount of c -a -c -n -2.
05:36
We're given it's now 5 .24.
05:38
5 .24 grams.
05:40
But the beauty of this problem is, even though it seems like we have a completely different thing to work on, which we do, we have useful information from the beginning.
05:51
Because the key here is when we have these reactions, we have what we call limiting reagent, which means that you're going to run out of one of the reagents first, and that's when the reaction will stop because you have like one less thing to or one of the essential ingredients is gone.
06:06
So if you're baking cake, you want to bake as many cakes as you can, and you run out of flour.
06:09
You can't just keep on putting in the baking powder in the oil because you don't have what it takes to assemble a cake anymore.
06:15
Same thing with the limiting reagent, same concept.
06:19
So i think the best way to do this is you could just, again, i would just cross multiply.
06:30
So you could just do this.
06:34
1 .24.
06:36
You don't have to cross multiply, actually.
06:38
Let's do a little bit more of efficient way because efficiency is key.
06:43
Okay.
06:44
So 5 .24 out of 80.
06:46
Right that's how much out of one mole we have so let's just do that division really quickly 5 .24 divided by 80 this is for the calcium cyanamide because that's something that we know so that equals if we look at it in decimal form 0 .065 this is just a ratio this isn't any mass amounts here this is just a ratio so let's just put a little asterisk by this so we know it's a useful piece of information use later...