# Homework Help: Compute Heat capacity

1. Jan 3, 2012

### Firben

You warm 1.00 kg of water at a constant volume of 1.00 L from 20.0 degrees C to 30.0 degrees C in a kettle. For the same amount of heat, how many kilo grams of 20.0 degree C air would you be able to warm to 30.0 degree C? What volume would this air occupy at 20.0 degrees C and a pressure of 1.00 atm? Make the simplifying assumption that air is 100% N2.

2. Relevant equations

m = 2CvQ/(3RΔTcv)

ρ=m/v

dQ = mcdT

3. The attempt at a solution

Q = (1.00kg)(4190 J/mol*K)(303K-293K) = 41900 J

c(air) = 1.01 * 10^3 J/kg*K
V= 1.00 L
ρ(air) =1.2929 kg/m^-3
m(air) = ρ(air)*V <=>
m(air) = (1.2929 kg/m^-3)(1.00 L) = 1.2929 kg
C(air) = (1.01*10^3 J/kg*K)(1.2929 kg) = 1305.829 J/mol*K

R = 8.314 J/mol*K
cv = 4190 J/mol*K

the answer should be 5.65 kg

Last edited: Jan 3, 2012
2. Jan 3, 2012

### Staff: Mentor

Hint: you ignored V=const part.

3. Jan 3, 2012

### Firben

Is this formula right m = 2CvQ/(3RΔTcv) ?

4. Jan 3, 2012

### Staff: Mentor

Argh, what was I thinking. You used wrong heat capacity value, but not because of the constant volume, you just have your units wrong. Sigh.

Check your units. Is the result in kg?