You have an evacuated container of fixed volume and known mass and introduce a known mass of a gas sample. Measuring the pressure at constant temperature over time, you are surprised to see it slowly dropping. You measure the mass of the gas-filled container and find that the mass is what it should be—gas plus container—and the mass does not change over time, so you do not have a leak. Suggest an explanation for your observations.

Video transcript

Hey everyone in this example, we need to explain for which of the below statements why the pressure of a gas in an isolated container with a fixed volume can increase over time at constant temperature. So we want to recall our formula for the ideal gas equation, which is pressure times volume equal to our moles of gas times the ideal gas constant, r times the temperature in kelvin. And looking at our first statement, it says that the gas decomposes such that there are more gaseous products than react ints. Now we want to make note that in this first statement, we're assuming that temperature and volume are constant. And we would agree that if the gas is decomposing to create more gaseous products, then that would increase our end value, which is our molds of our gas. And so that is only going to happen if temperature and volume are constant. So we can go ahead and put a checkmark by choice a because it does seem to be a true statement or a valid explanation. So, moving on to the next option and B we're told that the average kinetic energy of the gas is going to increase and that that's why the gas can increase in pressure at constant temperature and volume. However, we want to recall that our average kinetic energy of a gas is only affected by temperature. And so this wouldn't really explain our pressure increasing. So we can rule out choice be moving on to choice. See it says that the attractive forces of gas molecules increases and when we think of the attractive forces, we want to think of our inter molecular forces. And so this means that this would be the only factor affecting the attractive forces of our gas molecules. So it wouldn't explain why the pressure of our gas increases. So we can also rule out explanation. See And lastly, we have D. Which says that the gas molecules lose energy upon colliding with each other. And we would recall the fact that for ideal gasses, when we have collisions, they're going to be elastic collisions. And so therefore energy is not lost. And so we would also rule out D. Meaning that we only have a left as the only statement that made sense to explain why the pressure in an isolated container for our gas will increase at constant temperature and volume. So I hope that everything I explained was clear A is our final answer, and I will see everyone in the next practice video.