The hydrogen gas formed in a chemical reaction is collected over water at 30.0 °C at a total pressure of 732 mmHg. What is the partial pressure of the hydrogen gas collected in this way? If the total volume of gas collected is 722 mL, what mass of hydrogen gas is collected?

Video transcript

mhm. Hey, welcome back. Alright, so hydrogen gas produced from a chemical reaction was collected over water at 40°C with a total pressure of 715 mm of mercury. So, we're given the total pressure here. And we also have the water pressure which is 55.3 mm of mercury at that same temperature. Right? So notice that the total pressure here given to us is going to contain the partial pressure of hydrogen gas and partial pressure of water. And actually the first question that we have here is what would be the partial pressure of just the hydrogen gas that's collected. So let's go ahead and answer that. So again, total pressure is going to equal to the partial pressure of H two gas plus the partial pressure of H two gas from H +20 gas or water vapor. Right, So we have the total, we have the pressure of water. Let's go ahead and just solve for the pressure of H two. So we're just going to subtract the water of pressure, the pressure of water from the total. So 7 15 millimeters of mercury -55.3 mm of Mercury. And we're just going to go ahead and leave it in the Si units for now, since they are the same. So that is going to give us 659. mm of Mercury as the partial pressure of H two. Right, So let's go ahead and highlight that, that is the answer to part A. Now, if we take a look at part B, what is the mass of hydrogen gas collected of? The total volume of the gas collector is 763 millimeters millimeters. Alright, so here we're actually going to first, it needs to solve for the moles using ideal gas law PV equals NRT. So look at this, we actually have the pressure, we just found the pressure. We have the volume that's given to us. Um We need to find moles are is always given to us because it's a constant and T. S. Temperature which we have. We just need to make sure that everything is incorrect units. So pressure here has to be an atmosphere. So we have that in millimeters of mercury. So let's go ahead and actually convert this into atmospheres. Okay, so one atmosphere Is going to equal to 760 millimeters of mercury. Alright, so that will cancel out the millimeters of mercury and give us point 868 atmospheres. So both of these numbers are correct answers. They're just in different units since they didn't really specify um what unit they wanted? The partial pressure of hydrogen gas. We can just leave it in 6 59.7, but we do need it in atmospheres in order to do part B. Okay, so okay, so we have that. So let's go ahead and actually rearrange this equation so that we can solve for n. We're going to bible sides by R. T. So we have P. V over R. T. Alright, so what can we plug in now so we can plug in the pressure which is .868 atmospheres. The volume has to be in leaders and it isn't milliliters. So just divide that by 1000. And that will give us 10000.763 liters Divided by R. Which is the gas constant and it is . 206 L. Times atmospheres over mold, times, Calvin, move this up a little bit. Alright. And then times temperature temperature here has to be in Calvin because we want all of the units to agree and we have Calvin within the gas constant. So the temperature given to us is 40°C. Right? So let's go ahead and add to 73 0.15 to that in order to convert from degrees Celsius to Calvin. And that will give us 3 13.15 Calvin. Alright, so that's the temperature. We're gonna use 3 13.15 Calvin. All right. So now if we take a look, we can go ahead and figure out what units are going to cancel out. So we have atmospheres with atmospheres Calvin canceling out leaders canceling out and we're going to have moles leftover because we're solving for number of moles. Okay, So and here is going to equal to let's go ahead and multiply the top numbers multiply the bottom numbers and then divide them and we're going to get 0.2 Moles of H two right of hydrogen gas. So there we go. Now we have the moles of gas. Now we can actually go ahead and figure out the mass of that. Right? So mass and I'm going to move this down here. Alright, so we have moles. Now we can just go ahead and convert to grams. So in one mole of H two, how many grams are there? If hydrogen weighs 1.1 g per one mole? There are two of them here, right to hydrogen. So in one mole of H two, there's 2.2 g of H two. So moles cancel out and we're going to have gramps. So the final answer for part B is going to be a .05-1 g of H two. Alright, there we are folks, we are all done. So again, anytime you have a gas um being collected over water, realized that the pressure that they give, you will always be total pressure of that particular gas plus water if you're collecting it over water. So you always have to figure out what the pressure of water at that given temperature is. Sometimes they don't give you the water pressure of water. Water pressure adds a certain temperature. So you might have to look it up either in your book or online, but it should always be used to find the partial pressure of just that one gas. And for the second part we just needed to use ideal gas law equation to find animals. Everything else we had, we had already found the pressure in part a we just needed to convert into correct units and of course everything else we needed to convert, like the volume and the temperature. If you don't remember all of the units that you need to use, at least remember the units of the gas constant, because they're going to tell you exactly what the units you need to use for the other variables. Thanks so much for watching.