# Calculating Ethane Pressure and Mass in a Flask

• WY
In summary, at the end of the problem the pressure in the flask is equal to the initial pressure plus the pressure due to the change in temperature. There is 1.5 grams of ethane in the flask at the end.
WY
Hey - I'm doing this question (see below) but I don't hav a clue where to start!

A flask with a volume of V, provided with a stopcock, contains ethane gas at a temperature of T_0 and atmospheric pressure p_0. The molar mass of ethane is M. The system is warmed to a temperature of T, with the stopcock open to the atmosphere. The stopcock is then closed, and the flask cooled to its original temperature.

What is the final pressure of the ethane in the flask?

and

How many grams of ethane remain in the flask? Use R for the ideal gas constant.

for the first part i tried relating the initial and final pressures and volumes as a ratio... but that didn't work and since I don't have the volume of the actual gas which changes I can't apply the pV=nCRT equation...
Can someone tell me how to start?

Thanks in advance!

WY said:
Hey - I'm doing this question (see below) but I don't hav a clue where to start!

A flask with a volume of V, provided with a stopcock, contains ethane gas at a temperature of T_0 and atmospheric pressure p_0. The molar mass of ethane is M. The system is warmed to a temperature of T, with the stopcock open to the atmosphere. The stopcock is then closed, and the flask cooled to its original temperature.

What is the final pressure of the ethane in the flask?

and

How many grams of ethane remain in the flask? Use R for the ideal gas constant.

for the first part i tried relating the initial and final pressures and volumes as a ratio... but that didn't work and since I don't have the volume of the actual gas which changes I can't apply the pV=nCRT equation...
Can someone tell me how to start?

Thanks in advance!

I don't know about the C in pV=nCRT. The usual representation of the ideal gas law is PV = nRT, where R is a constant. In this problem, the pressure is constant during heating, and the volume of the flask is constant for the whole problem. During heating, PV is constant and you are raising the temperature. What has to change, and how is that change related to the temperature change?

Thanks for replying! Sorry I wrote down the wrong equation -_-
but in this problem doesn't the volume of gas change? So why doesn't it effect the way we caluclate it?

Nope, the problem essentially starts when the cork is closed, after which the volume is constant. I don't know why they included all that information, its just confusing, they should have just stated that the system started at a certain temperature and dropped.

oh I see! thanks for clearing that up now - its all crystal clear now!

## 1. How do you calculate the pressure of ethane in a flask?

To calculate the pressure of ethane in a flask, you need to know the volume of the flask, the number of moles of ethane present, and the temperature of the ethane. Then you can use the ideal gas law (PV = nRT) to calculate the pressure, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature.

## 2. What is the relationship between pressure and volume in a flask of ethane?

The relationship between pressure and volume in a flask of ethane is known as Boyle's Law. It states that at a constant temperature, the pressure of a gas is inversely proportional to its volume. This means that as the volume of the flask decreases, the pressure of ethane inside will increase, and vice versa.

## 3. How do you determine the mass of ethane in a flask?

The mass of ethane in a flask can be determined by multiplying the number of moles of ethane present by its molar mass. The molar mass of ethane is 30.07 g/mol. Therefore, the mass of ethane in a flask can be calculated by multiplying the number of moles by 30.07 g/mol.

## 4. Can the temperature of ethane affect its pressure in a flask?

Yes, the temperature of ethane can affect its pressure in a flask. According to the ideal gas law, as the temperature of a gas increases, its pressure will also increase, assuming all other variables are constant. So, if the temperature of ethane in a flask increases, its pressure will also increase.

## 5. How does the pressure of ethane in a flask change if more ethane is added?

If more ethane is added to a flask, the pressure inside the flask will increase. This is because the number of moles of gas has increased, but the volume of the flask remains constant. According to the ideal gas law, an increase in the number of moles of gas will result in an increase in pressure, assuming all other variables are constant.

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