# AP Chem

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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?

• AP Chem - ,

Oh, I think I got it. Is it only when the gas is collected OVER water?

• AP Chem - ,

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.