# Homework Help: Heating a gas at constant pressure

1. May 17, 2006

### Amith2006

Sir,
A Volume – Temperature diagram was obtained when a gas was heated at a constant pressure. During the heating process from state 1 to state 2 how does its mass vary?
Sometimes the diagram may not clear, so I will try to describe how it looks like. It is a straight line graph with an inclination of around 30 degrees. It has a positive slope. The volume is along the Y axis and the temperature is along the X axis.
I solved it the following way:
From the graph it is clear that at any point on the graph V/T < 1
But according to gas law,
PV = wRT/m
Where w = mass of gas and m = Molecular weight of gas
i.e. V/T proportional to w ( Since P is constant)
i.e. w < 1
Hence the mass of the gas decreases. Though it agrees with the answer in my book, I don’t know whether it is right. Please say whether its right or not?

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• ###### V T diag..JPG
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2. May 18, 2006

### Andrew Mason

Not quite. The graph is a straight line. What does that tell you about how V varies with T?

Now apply the ideal gas law: PV=nRT or V = nRT/P

If P is constant and the graph of V vs. T is linear, can n vary at all?

AM

3. May 18, 2006

### Amith2006

Sir,
So do you mean that as V/T is a constant and moreover pressure being constant, mass of gas is constant?

4. May 18, 2006

You got it.

AM