Transcription
The experiment for the formula of a hydrated salt is interesting because we're not finding the identity of an unknown because it says right on the bottle. I mean, for example, this one is what is this? Cobalt sulfate. And there's a table, Table 4.1, that you have to fill in before the lab, which gives you the formula of these various possibilities. This one's brown. We got a green one that's kind of pretty. A blue one that's kind of pretty. We got three white ones. Boring. Anyway, um, we'll do a colored one, man. I'll make it amusing for you.
We're trying to find out the formula of this hydrate. Now, hydrates have water molecules that make them stable. Sometimes they actually change color when you drive the water off. So we'll see if this is one of those ones, which makes it useful because you can determine is it still hydrated or not. Regardless, our goal is to find the formula mass of this compound.
Now, if you have not yet covered the concept of the mole in class, we're only going to cover up through line H in the current lab manual. Although I strongly suggest that you obtain from your instructor a line A, new line I, which is the theoretical mass of the hydrate, and then you calculate percent error because you can absolutely certainly do that as a new line J. Let's say.
But if you have covered the concept of the mole in class, then you can figure out the uh exact molar ratio and formula of the compound. If you haven't covered the concept of the mole in class, then all you're going to find is the percent by mass of water in the compound. We will measure it before we heat it. We'll measure it after we heat it. We know that these are all hydrates and that the only thing that's going to disappear upon heating is water. And so if we have the mass before and the mass after, we'll have subtracting the mass of water that we lost, and we'll know how much we started with. And therefore, we can figure out the percent by mass of water that was in it to begin with. So let's carry out our steps, shall we?
So, step one is before the lab, you need to calculate the molar masses of each, or the formula masses, uh, since these are ionic compounds, of each anhydrous, lacking water, salt in Table 4.1. Then we've obtained our unknown. There we go. And we can record its number on line A. This happens to be unknown number four, cobalt sulfate. Uh, we've got the name of it off the bottle, so you don't have to get it from your instructor. Step number three, and you found its formula from Table 4.1, and we record the anhydrous formula on line K.
We got our hot plate and we have cranked the heat. I set it on 375. Let's crank it up to 450. Okay. And what we're going to do is get our 100 milliliter beaker and watch glass that looks like a giant contact lens. And we're going to heat it both, very hot, on this hot plate for about five minutes. I won't make you wait. I'll just pause the recording for that five minutes. But we need to make sure that there's no water stuck to this beaker. We also, anything that does happen to be stuck to this beaker that we didn't clean off gets cooked to a crisp. And it has to stay exactly the same throughout the experiment. Just thinking of things that could go wrong in this experiment, right? If there's something on here at the beginning but not at the end, other than our compound, that will mess up our mass either high or low. I leave it to you to figure out. If there's something on that at the end that wasn't on it at the beginning, that can mess up our mass difference. Again, I'll let you decide if it's high or low and the eventual impact on your percent by mass of water. And then you could also carry it through to figure out if you have covered the mole in class, the effect on the molar mass.
You'll see that we have a different balance this week, and that is because I wanted a really, really, really precise measurement of mass. And you'll note on this one that it reads to four decimal places. The other balance, the cheaper balance, that other balance was only about $200. This one's about $1200. The other balance, uh, only had two decimal places. This one has four. You pay for that. You don't want to move your balance at all, or even really touch it very much. So if we zero our balance, it should stay on zero. Now, it's a little bit close to this hot plate, I gotta admit. For my taste, the the air currents coming off the hot plate and swirling around could affect the mass of this sensitive balance, but it is holding at zero rather nicely.
Okay, so it's been five minutes now. I'm going to take my beaker and watch glass off of the hot plate carefully because it is so hot, using tongs. Now, if I were to put this very, very hot piece of glass onto this very, very cold bench, I could shatter it. So I always, always, always put hot glassware onto wire gauze to cool. Now, we have to wait for this to be at room temperature because if I put it into the balance while it's hot, or even warm, I'm not going to get a stable mass reading. So we have to wait. Waited until our beaker is cool to the touch.
Now, it says in step six to use your tongs to put this into the balance. As long as you're wearing gloves and the gloves are clean, you should be okay to handle this with your hands. Our balance is tared. It's on zero. And so we can put it onto the balance along with the cover. Now, as long as this is at room temperature, we should get a nice stable reading. There shouldn't be any visible vapor or water on the watch glass. If it got hot and there was vapor on it, uh, we would need to wait until that completely evaporated off and then we cooled it to room temperature.
All right, so we have a nice stable mass. It's not really changing at all. 85.5272, 82. 85.5272 is our mass of our empty beaker and watch five class. Seven two.
Now, in step seven, it says place between 0.6 and 1.0 grams of stuff into the beaker. Okay, we're at 85.52 something. Let's put in what we think is somewhere around 0.6 to 1 gram of stuff. All right, so I'm going to take my bottle here and I'm just going to rotate it back and forth a little bit. We're at 85.5272. Let's see where we're at now. Notice I didn't just reach in there to pour it in because if this powder ends up in this $1200 balance and trashes it, that would be bad. We're at 85.5272. We're at 86.65. We are over one gram. That is more than we need. We do not want to have extra stuff because they will just delay things. And so what do I do? Get rid of some of it. I'm going to put this in its properly labeled waste container later. Right. So I took some out. Make sure there's none on the outside. Definitely don't want that. Right. Nice clean, just the way that it was. 5.5. Now we're at 86. Close the door so no air currents. 86.3138. 86.31 air currents. 86.3125. 86.3125 is our mass of our hydrated salt.
In the next step, we're going to dehydrate. So we've placed it in the beaker. We've recorded the exact mass on our data sheet, line C. It has to be the actual mass, all of the digits, okay? We didn't tear the beaker because we have the mass of the beaker empty. We'll subtract that out later. So we want to heat the beaker strongly again. So I still have this thing set at for five minutes. It's at room temperature. I can touch it now. Just gotta remember when it's hot and when it's not. Onto the hot plate, and we'll let this heat for five minutes. We'll see if we get any change or not.
Now, if we look at this, you can hear it snap, crackle, popping, which is why we have the cover. You can see the water vapor boiling off. Now, it's going to escape out the little spout area over here, so it's fine because there's definitely water vapor on top too. If we wait, have patience, that will disappear also. You might notice it was the sort of brownish reddish color and now it is grapedy purple. That's kind of fun. So it's dehydrating rather rapidly, but we gotta wait for that water to be gone. Otherwise, it hasn't dehydrated. It's dehydrated off of the purple stuff now, but the water hasn't escaped. It's still on the watch glass. So it's pretty much stopped snap, crackle, popping. Now, what we might carefully do is shift this watch glass around to get the water vapor to escape a little bit better, but this is really, really hot. So we've got to be very, very careful. And now we wait.
Just want to point something out here that my tongs that I'm using, someone else has used in a different lab that involves wax. And so when my tongs magnetically stuck to the stirring hot plate, the wax on the tongs melted and dripped onto the hot plate and is now gassing me with all of these fumes, which is annoying because the person who used the tongs should have cleaned them after they used them. But more important and to the point of this lab, you can see there's a wax dripped off and hardened onto the bench top there. Uh, more to the point, I'm using these tongs right to jiggle my flask around to make sure that the solid all gets exposed to the heat. I'm using the tongs to pick up the beaker and put it over onto the wire gauze and into the bench top. And there's wax all over. And this wax, some of it is hitting the hot plate and evaporating and contaminating my lungs. But some is also probably sticking to my beaker. You have to think, what is the impact of me adding mass to my beaker as the experiment went on? Right? I weighed the beaker empty. I weighed the beaker with my stuff in it. And then I'm going to weigh it with my stuff in it again. But the water will have left, except that wax will have been added. So that's the type of thing that happens during an experiment that you need to keep track of. Will that make it look like more water has left or less water has left? Will that give me a higher percent by mass of water or a lower percent by mass of water? If we go all the way to the end, will that give me a higher number of water molecules per formula unit or a lower number of water molecules per formula unit? All of these things are things that you need to consider. Not because you're going to be a chemist, but because you're going to get a job that requires problem solving and problem correcting. The cash register doesn't work, you're just going to tell your boss, hey, it doesn't work. I can't, I can't ring up any customers if you're in retail. No, you're going to problem solve. Is the cash register plugged in? Is it turned on? Is there paper or whatever in it that's causing an error? I don't know. But that sort of problem solving of figuring out cause and effect, that's again, one of those reasons that you're taking this lab.
All right, while this is still, uh, continuing to heat, I think it's been heating actually long enough. I don't see any water vapor on the top. I'll show you. No more water vapor visible whatsoever. There are a couple of splotches of red that look like they splattered up there or purple now. Um, that shouldn't matter as long as the number of them splattered out, if they escaped somehow, that we would have to account for. But I think everything is still in there. So, uh, maybe even with a little added wax. So we're going to let this cool for five minutes. I'm not going to turn off my hot plate because there's still more experiment to be done. Just heating it, cooling it, and weighing it once is not good enough. We want to make sure all the water is gone. How do we do that? We're going to heat, cool it, and weigh it. Now, I'm going to heat, cool, and weigh it again and make sure that that mass hasn't changed. If it's changed, if it's gotten lower, that means more water came off. If it didn't get lower, it means that we got all the water out this time. So we'll wait five minutes and we'll see what the mass of this is once it's at five minutes, or at least, at least five minutes, whatever it is until it gets to room temperature. So cool to the touch again. Balance zeroed. Drifting a little bit. Pre-zeroed higher or lower than last time? 85.4950. 89. 85.9953. 85.9953 seems to be a little lower than last time. Okay, we're recording the exact mass on the data sheet, line D. Then we're going to put our beaker and our cover back on the hot plate for another five minutes, and we'll see if it exactly matches after five more minutes of heating.
No snap, crackle, and popping. No visible water vapor. I'm going to carefully jiggle it with my tongs. Smoke again from the wax that somebody else left all over these tongs. Why don't I just switch to tongs? Well, at this point, most of the wax is burned off. This is going to do this very carefully so you don't fling it off of the hot plate or drop the cover, which has been weighed already as part of this. You can't just swap it out for anyone. No water vapor anywhere visible. Heating it strongly for another five minutes. Nothing. Nothing visible. We want to make sure. So we'll let this go for five minutes, take it off, cool it again, and weigh it.
So it is heated strongly again, very high temperature, for another five minutes. So I'm going to very carefully move it onto the wire gauze. I don't see any water vapor anywhere. I do see a little bit of wax that came off the tongs onto the top of the watch glass, and I was lifting it up with the tongs. So I have some mass added to my beaker and cover that was not there when it was empty, and not there when I weighed my compound originally. So there is some extra mass there now, even though I've heated it. So, for example, that is going to make it appear as if less water disappeared than actually did disappear, which would give me a lower percent by mass of water. And if we went all the way to the end, a lower number of hydrates. Is it enough wax to be significant? We'll find out in a few minutes when it is cool to the touch and we can weigh it.
All right, so back to room temperature. We're looking at what? 55.99. 85.99 something. Hmm. 85.98. That is significantly lower than it was the first time, right? The number we wrote down before was 995 something. This is 989 something. Actually, 009 different. We are within the zone, right? Must be within 0.009. Actually, now it's going up a little bit. Balance is settling. We're at 85.9900. So that is within our range. So we were successful in the first heating at driving off all the water. A minimal, minimal, minimal amount came off in that second heating. And if we wanted to be super, super, super precise, we could heat it a third time just to triple check, but I think we're good. We'll record as our final mass as soon as this settles. Maybe it's not exactly at room temperature, or the hot plate again is still on, blowing this way. 85.99. 85.9915 is our final mass. That is our lowest mass. That is the one that we will use. Okay, the other one was slightly higher.
So if you look on the data sheet, it says the last line of D is the, uh, to get the mass of the anhydrous salt minus B, right? So the last line of D. So we did a second heating and we got that mass that we just had a second ago. We didn't need to do a third or a fourth heating. But if we did, it would probably keep going down very, very, very slightly, and the last final lowest one would be the one that we would use as our value for D. So we have two values, line D, the first time, line D, the second time. We will use the second time. The first time was exactly that, a first time, a practice. We didn't, we're not going to use that one for anything.
So from that, I leave it to you and your instructor to discuss the calculations to figure out the percent by mass of water in the compound if you have not covered the mole in class. And if you have, to figure out the precise formula. And your instructor can give you the theoretical so that you can calculate a percent error to your percent by mass of water. And your instructor can give you the exact formula for you to see how far off you are, if at all.