Math Is Fun Forum

tigerfan

Member

Registered: 2007-03-09

Posts: 8

Math problems!!!

Need help with this stuff.

A house has well insulated walls. It contains a volume of 100 cm3 of air at 300 K.  Calculate the energy required to increase the Temperature of this diatomic gas by 2 deg C.  Assume it is heating at a constant pressure and use Cp=7R/2

I know that 2deg C is 275.15 kelvin

Last edited by tigerfan (2007-03-09 19:52:55)

John E. Franklin

Member

Registered: 2005-08-29

Posts: 3,588

Re: Math problems!!!

Can you tell us the terminology of the Cp thing?
Also, why is the house so small?  4 cm x 5cm x 5cm is a small house.
And another comment I have is that raising by 2 degrees Celcius is the same as
raising by 2 degrees Kelvin.  There is no need for the 273.15 plus 2. 
The temp is 300Kelvin.

Last edited by John E. Franklin (2007-03-09 10:34:47)

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igloo myrtilles fourmis

93BMW

Member

Registered: 2007-03-08

Posts: 7

Re: Math problems!!!

The C is heat capacity.  Not sure what the sub p is

ryos

Member

Registered: 2005-08-04

Posts: 394

Re: Math problems!!!

Another heat problem that I have to wait until I can look stuff up tomorrow to do.

C sub p is the symbol for heat capacity. I'm not sure about the r though; I'm guessing it's short for rho, or ρ, meaning density. But then the units wouldn't make sense, so it must be something else.

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El que pega primero pega dos veces.

tigerfan

Member

Registered: 2007-03-09

Posts: 8

Re: Math problems!!!

I think the r is supposed to be R if that makes any difference.

ryos

Member

Registered: 2005-08-04

Posts: 394

Re: Math problems!!!

Actually, it does. R is the ideal gas constant, which can be expressed in units of energy/ temperature*mole, which is also the units of heat capacity.

They've given you the heat capacity like it's a constant, which it isn't. Heat capacity is a function of temperature. Also, they've asked you to assume heating at a constant pressure, which you can't because the pressure is going to change when you heat the gas in a closed volume.

What class is this for? I ask because I don't know how far to go with the solution. It doesn't seem to be the type of problem that you'd get in a math class, but other classes (chemistry, engineering) don't usually make such gross oversimplifications so I'm wondering if this is really the whole problem.

Anyway, we'll take them at their word. Since we don't have Cp as a function of T, we can just multiply it by δT to get the specific internal energy change δÛ, which has units of energy/mol. Then just multiply that by the moles of gas, found as n = PV / RT = 0.1 (L*atm) / R (L*atm/K*mol) * 300K .

That should get you the very approximate solution. If you need the real solution I can get that for you too. (This is what I get for studying Chemical Engineering. )

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El que pega primero pega dos veces.