When dealing with gasses, do you subtract the pressure of water vapor every time it is in the equation? I'm confused on when you subtract it. Such as this problem...
NH4NO2(s) -> 2H2O(g) + N2(g)
How many grams of ammonium nitrate must have reacted if 3.75 dm^3 of nitrogen gas was collected over water at 26 degrees C and 97.8 kPa?
I did subtract the water pressure and got 9.09 g NH4NO2
but in the previous problem i posted
When sodium hydrogen carbonate is heated, it decomposes to sodium carbonate, carbon dixoide, and water vapor. What volume in liters of carbon dioxide gas at 77 degrees C and 756 mmHg will be produced from 26.8 g of sodium hydrogen carbonate?
I didn't subtract it and got 4.62 L
Is there a certain wording that shows when you subtract it and when you dont?
Oh, I think I got it. Is it only when the gas is collected OVER water?
You're right. The secret words are OVER WATER. Perhaps we should have subtracted the vapor pressure of water in the NaHCO3 problem, too, but I didn't when I worked that problem. I suppose, when we have a problem like the NaHCO3 problem, when it asks for the volume of CO2 produced at 756 mm Hg pressure, we just assume that is the partial pressure of the CO2 and not the total pressure of the system (because the water hasn't been brought to our attention). If it's the total pressure, then we should have subtracted the vapor pressure of water at 77 C and used the new pressure to calculate the volume occupied by the DRY CO2. Technically, I think our assumption was false; however, I think the intent of the problem was to work it as we did.
When dealing with gases, especially when they are collected over water, it's important to consider the pressure of water vapor in the system. The presence of water vapor affects the total pressure, and therefore, it needs to be taken into account.
In the first problem you mentioned, you correctly subtracted the water vapor pressure. This is because the nitrogen gas was collected over water. When a gas is collected over water, the pressure you measure includes the partial pressure of water vapor. To determine the partial pressure of the gas you are interested in (in this case, nitrogen gas), you need to subtract the vapor pressure of water at the given temperature from the total pressure.
In the second problem, since the question does not specifically mention the collection of carbon dioxide over water, it implies that the gas is being collected independently, not over water. In this case, you don't need to subtract the pressure of water vapor. When a gas is collected directly, its pressure is measured independently without the inclusion of water vapor.
To summarize, if the problem states that a gas is collected over water, you need to subtract the vapor pressure of water to find the partial pressure of the gas you are interested in. But if the problem doesn't mention the gas being collected over water, you can assume that it is collected directly, and there is no need to subtract the pressure of water vapor.
Always pay close attention to the wording and specific conditions mentioned in the problem to determine whether or not the pressure of water vapor needs to be subtracted.