Why Does Gas Volume Decrease with Temperature Increase in My Calculations?

In summary, the conversation discusses using the ideal gas law to calculate the volume of a fixed mass of gas at different temperatures. The first attempt used Celsius, but the correct method is to use Kelvin and consider the units and variables carefully. The conversation also mentions the universal gas constant and the equation for calculating the number of moles.
  • #1
CathyLou
173
1
As part of my AS Physics homework a week or so ago, I was set the question:

3. A fixed mass of gas has a volume of 144 cm3 at 15ºC. Calculate its volume at

(a) 33ºC
(b) 0ºC, and
(c) -67ºC (the pressure being constant).

Yesterday the whole class was asked to re-attempt questiosn 3 and 4 using temperatures in Kelvin instead of Celcius.

When I was trying to figure out the question the first time around I tried using K but found that the answers didn't make logical sense as I thought that as temperature increases so does the volume of gas.

Originally, I used V = kT and calculated k to be 9.6. I the got the three answers to equal 317, 0 and -643 cm^3. Obviously, I now know that this is wrong.

However, whenever I try to calculate the answer using a temp. in Kelvin (by adding 273 degrees to the celcius temp), I get k to equal 0.5 and this figure makes the volume decrease as temp. increases.

Can anyone please tell me what I'm doing wrong?

Cathy
 
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  • #2
Start with PV=nRT=NkT, which is the ideal gas law. Note there are three variables, P, V and T. P and T must be used in terms of their absolute values. P can be expressed as absolute, gage or differential pressure, and T can be expressed in terms of absolute scales (Kelvin or Rankine) or corresponding relative scales, Celsius and Fahrenheit.

In addition, the units must be consistent, MKS (SI) or cgs, or British.

See http://hyperphysics.phy-astr.gsu.edu/Hbase/kinetic/idegas.html for a discussion of the ideal gas law.

A reasonable discussion of temperature - http://en.wikipedia.org/wiki/Thermodynamic_temperature
 
  • #3
Thank you for replying.

As far as PV = nRT is concerned, how do I work out the number of moles so that I only have one unknown in the equation?
 
  • #4
Last edited:

Related to Why Does Gas Volume Decrease with Temperature Increase in My Calculations?

What are the gas laws and how do they work?

The gas laws, also known as the ideal gas laws, are a set of mathematical relationships that describe the behavior of gases under various conditions. These laws include Boyle's law, Charles's law, Gay-Lussac's law, and Avogadro's law. They explain how pressure, volume, temperature, and number of particles are related in an ideal gas.

What are some common gas law problems and how can I solve them?

Common gas law problems involve finding the value of one variable (pressure, volume, temperature, or number of particles) when the others are known. To solve these problems, you can use the appropriate gas law equation and plug in the given values. It is important to convert all units to the standard units (atm, L, K, and mol) before solving the equation.

What is the ideal gas law and how is it used?

The ideal gas law is a combination of all four gas laws and is represented by the equation PV=nRT. It relates the pressure, volume, temperature, and number of particles of an ideal gas. It is often used to calculate the value of a missing variable or to compare the behavior of different gases under the same conditions.

What is the difference between an ideal gas and a real gas?

An ideal gas is a theoretical gas that obeys all of the gas laws perfectly. It has no intermolecular forces and its particles take up no volume. A real gas, on the other hand, deviates from the ideal behavior due to intermolecular interactions and non-zero particle volume. At low pressures and high temperatures, real gases behave more like ideal gases.

How do I convert between different units of pressure, volume, temperature, and number of particles?

To convert between different units, you can use conversion factors or unit conversion formulas. For example, to convert between Celsius and Kelvin temperature scales, you can use the formula K = °C + 273.15. It is important to pay attention to units and always convert to the standard units (atm, L, K, and mol) when using the gas laws.

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