The laboratory setup shown can be used for the gravimetric determination of the empirical formula of potassium oxide, beginning with potassium metal. Which of the following is a false statement? A) The heating of the covered crucible should continue until the potassium is fully reacted. B) The initial piece of K, potassium, should be freshly cut and free from oil. C) The lid should be tightly sealed to exclude oxygen. D) Before measuring their masses, the crucible, the lid, and the contents should be cooled to room temperature. Or E) when potassium appears to be fully reacted, the crucible lid should be partially removed and the heating continued.
In the laboratory setup, we have a clamped crucible. A crucible is simply a heat-resistant container. And on top of the crucible, there’s a lid. And the crucible and its contents are being heated using a Bunsen burner. The question tells us that we’re doing gravimetric analysis using this apparatus.
The word “gravimetric” derives from gravity, which is what gives objects with mass their weight. When we put something on a scale, we’re measuring its weight and then determining its mass from that. So how do we use gravimetric analysis to determine an empirical formula, in this case of potassium oxide?
Well, in this example, we’d start with a lump of metal. We put the piece of metal on a balance and then record its mass. In this case, we’re working out the empirical formula of a metal oxide. So we take our metal and react it with oxygen. That’s what the laboratory setup here is for. It’s for reacting the metal, which we put in the crucible, with oxygen in the surrounding air.
After getting our pure metal oxide, in this case potassium oxide, we would weigh it. We’d then be able to work out the mass of oxygen in our compound by the change in mass, the difference in mass between the metal oxide and the metal. We’d then be able to convert the masses of oxygen and the masses of potassium into their amounts in moles. And by comparing the amount of potassium in our original sample and the amount of oxygen it reacts with, we’d be able to deduce the empirical formula.
Now that we know what this laboratory setup is doing, let’s have a look at the statements. We’re looking for a false statement. So we’ll likely find four true statements and one false statement. Statement A says that the heating of the crucible should continue until the potassium is fully reacted. Oxygen in the air has to get into the crucible to react with the potassium. If we continually heat the potassium, all of it will convert into potassium oxide. If, however, we stop heating, we might be left with some potassium buried under layers of oxide that won’t have reacted. If we do heat until the potassium is fully reacted, we’ll get an accurate measure of the mass of oxygen that can react with our metal sample. And so we’ll end up with an accurate determination of the empirical formula. So this is actually a good thing to do. And so it’s not a false statement.
The second statement is that the initial piece of potassium should be freshly cut and free from oil. Well, you should remember that potassium is one of the most reactive metals. It will react with oxygen in the air quite rapidly. So it’s stored under oil. So we both want a piece of potassium that hasn’t reacted with oxygen much yet and we don’t want any oil left behind. So we’ll take a sample out of the oil, cut it with a knife, and dab off any oil remaining. Doing this will help reduce the error in our measurement. So statement B is also a good thing to do, and it’s not a false statement.
The next statement suggests that the lid should be tightly sealed to exclude oxygen. Excluding oxygen would stop the reaction very quickly. The potassium would quickly consume any oxygen inside the crucible. And we’d be left with mostly potassium metal. So actually, we shouldn’t tightly seal the lid. And this is a false statement.
But then you might ask, why do we have the lid at all? Well, the first reason is safety. As we’re heating up potassium metal, it’ll melt. And some of the vapor might react with oxygen, producing fumes, which would be quite dangerous to inhale. The other reason is to stop anything else getting into our experiment and contaminating the results. So we have good reasons to have the lid in the first place. But if we seal it completely, it completely spoils the process. We have our answer, but let’s have a look at the other two anyway.
The next statement suggests that, before we measure the mass of the crucible, the lid, and the contents, they should all be cooled to room temperature. The first reason you might want to do this is that the crucible, the lid, and the contents are going to be a big hazard when they’re hot. Nobody wants to bump into someone who’s carrying a crucible that’s 200 degrees Celsius. But it’s actually more accurate as well. When you measure the mass of the crucible and the lid without the contents, it will probably be cold. And there might be surface moisture that makes it a little bit heavier. So when you’re measuring the mass with the crucible, the lid, and the contents, you should also do it cold. So you’re comparing like for like.
The last statement suggests that when the potassium appears to be fully reacted, the crucible lid should be partially removed and the heating continued. What this means is, maybe we’ve been heating for a long time. And using tongs, we’ve peaked under the lid to see all we’ve got is white powder. The statement suggests we should partially open the lid, perhaps setting it off centre. And this helps oxygen enter the pile of powder and make sure that all the potassium has reacted. This shouldn’t cause any problems, and it should make our results more reliable. This sounds like a good idea to me, so it’s not a correct answer.
So given the laboratory setup, if we were using it for gravimetric determination of the empirical formula of potassium oxide. Starting with potassium metal, it will be a very bad idea to seal the lid shut so that we couldn’t get any oxygen into our reactant. So the false statement is, the lid should be tightly sealed to exclude oxygen.