Got a question
3. Given 325 ml of saturated gas at 760 mmHg and 25°C, what would be its volume if dry at the same pressure and temperature?
I got confused when I read 'if dry' Does that mean I have to subtract something out of 760mmHg?
And would I used the combined gas law and solved for V2?
a teacher answered it but I had a question about their answer.
They said:" There will be less gas when the saturated gas is dried. Subtract the vapor pressure of water at 25 C (23.8 mm Hg) from the 760 mmHg pressure to get the initial dry gas partial pressure. Since the pressure is maintained at 760 mm, that means the volume must be reduced by the fraction (760- initial water vapor pressure)/760 "
How did they get 23.8mmHg?
And I wanted to know if I could subtract 47mmHg from 760mmHg to get the new number component?
thanks
Yes, when the problem says "dry" you subtract the vapor pressure of water AT THAT TEMPERATURE. How do we get that vapor pressure? We look it up in a table. The vapor pressure of water at 25 degrees C is 23.8 mm Hg. So 760-23.8 is the pressure of the dry gas (the other 23.8 is the pressure of the water in the gas). I don't know where you obtained the 47 or what is is. You may use the combined gas laws OR you may use PV = nRT, find the number of mols at the pressure of 760-23.8, then use that n and plug in the new conditions of T and P to find the new volume.
To answer your first question, when it says "if dry," it means that you need to account for the presence of water vapor in the original saturated gas and calculate the volume without it.
To get the volume of the gas if it is dry, you need to subtract the vapor pressure of water at 25°C (temperature given in the question) from the total pressure of the saturated gas.
The value of 23.8 mmHg for the vapor pressure of water at 25°C is known as a standard value and can be looked up in a reference table or obtained from experimental data. It represents the pressure exerted by water vapor when it is in equilibrium with liquid water at that temperature.
Now, let's calculate the volume of the gas if it is dry:
1. Subtract the vapor pressure of water (23.8 mmHg) from the total pressure (760 mmHg) to obtain the initial dry gas partial pressure.
initial dry gas partial pressure = 760 mmHg - 23.8 mmHg = 736.2 mmHg
2. Since the pressure is maintained at 760 mmHg, we can use the fraction (760 - initial water vapor pressure)/760 to calculate the new volume.
If you subtract 47 mmHg (as you suggested) instead of using the correct value of 23.8 mmHg, you will get an incorrect answer. The vapor pressure of water changes with temperature, so it is essential to use the correct value for the specific temperature given.
Hope this explanation helps!