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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Aeden on October 10, 2010, 08:05:35 PM

Title: Looking for which gas is the one in question
Post by: Aeden on October 10, 2010, 08:05:35 PM

"In 3625, archeologists discover a tank of gas that belonged to an ancient civilization. In order to determine what kind of gas is in the tank, they perform a short experiment on site. They open the intact tank valve and draw off 100 mL of gas. They weigh the sample and obtain a mass of 0.128g at 25 degrees C. They know that a mole of gas has a volume of 25L at 25 degrees C. Which one of the following gases is the one that is in the tank?"

Ethane C2H6
Hydrogen H2
Acetylene C2H2
Oxygen O2
Hydrogen sulphide H2S

An explanation to the answer would be appreciated but I'd be just as thankful if someone could get me started and I can figure it out from there. Just never seen a question like this one. Any help appreciated

Title: Re: Looking for which gas is the one in question
Post by: opti384 on October 11, 2010, 02:56:46 AM

Start from findng the mol of the given gas.

Title: Re: Looking for which gas is the one in question
Post by: Aeden on October 11, 2010, 09:43:35 AM

Ethane = 30g/mol
Hydrogen = 2g/mol
Acetylene = 28g/mol
Oxygen= 32g/mol
Hydrogen sulphide= 34.1g/mol

Where to next?

Title: Re: Looking for which gas is the one in question
Post by: Borek on October 11, 2010, 10:24:22 AM

Find number of moles of gas.

Title: Re: Looking for which gas is the one in question
Post by: Aeden on October 11, 2010, 03:49:37 PM

Ok I think I figured it out

"In 3625, archeologists discover a tank of gas that belonged to an ancient civilization. In order to determine what kind of gas is in the tank, they perform a short experiment on site. They open the intact tank valve and draw off 100 mL of gas. They weigh the sample and obtain a mass of 0.128g at 25 degrees C. They know that a mole of gas has a volume of 25L at 25 degrees C. Which one of the following gases is the one that is in the tank?"


N= PV/RT

0.1 L x 101.3 kPA/ 8.31 kPa x 298k

273 kelvin base + 25 degrees C = why 298 exists

So 8.31 x 298= 2476.38

So once again

0.1L x 101.3 kPa/ 2476.38 = 0.004 mol

We have 0.128g of mass. 1 x .128/ 0.004 = 32 = oxygen. Any confirmation would be great but I feel confident about this..

Title: Re: Looking for which gas is the one in question
Post by: Borek on October 11, 2010, 04:19:39 PM

Looks OK to me.

Title: Re: Looking for which gas is the one in question
Post by: DrCMS on October 12, 2010, 04:41:11 AM

Quote from: Aeden on October 11, 2010, 03:49:37 PM

Ok I think I figured it out

"In 3625, archeologists discover a tank of gas that belonged to an ancient civilization. In order to determine what kind of gas is in the tank, they perform a short experiment on site. They open the intact tank valve and draw off 100 mL of gas. They weigh the sample and obtain a mass of 0.128g at 25 degrees C. They know that a mole of gas has a volume of 25L at 25 degrees C. Which one of the following gases is the one that is in the tank?"


N= PV/RT

0.1 L x 101.3 kPA/ 8.31 kPa x 298k

273 kelvin base + 25 degrees C = why 298 exists

So 8.31 x 298= 2476.38

So once again

0.1L x 101.3 kPa/ 2476.38 = 0.004 mol

We have 0.128g of mass. 1 x .128/ 0.004 = 32 = oxygen. Any confirmation would be great but I feel confident about this..

Thats the full way to work it out.  A quicker way with the info given would have been

0.128/(0.1/25) = 32