1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Heat required at constant pressure

  1. Dec 24, 2006 #1
    1. The problem statement, all variables and given/known data
    1) How much heat energy must be added to the gaseous mixture consisting of 1 gm of Hydrogen and 1 gm of Helium to raise its temperature from 0 degree C to 100 degree C
    at constant pressure? Given that g1=1.41, g2=1.67

    2. Relevant equations

    Heat required = nC(p)dT

    3. The attempt at a solution

    Heat energy in Joules:

    Let C(p1), C(p2) and C(p) be the specific heat of Hydrogen, Helium and the mixture at constant pressure respectively. Let g1,g2 and g be the ratio of the specific heats at constant volume and pressure for Hydrogen, Helium and the mixture respectively. Here I have taken the value of the universal Gas constant[R] = 8.314 Joule/mole-K
    C(p1) = (g1 x R)/(g1 -1)
    = 28.59 J/mole-K
    C(p2) = (g2 x R)/(g2-1)
    = 20 .72 J/mole-K
    C(p) = {(n1 x C(p1)) + (n2 x C(p2))}/(n1 + n2)
    = 709.7 J/mole-K
    Let n1,n2 and n be the number of moles of Hydrogen, Helium and the mixture respectively.
    n1 = 0.5 moles
    n2 = 0.25 moles
    n = 0.75 moles
    Heat required = nC(p)dT
    = 53200 Joules = 12666 Calories

    Heat energy in Calories:

    Here I have taken the value of the universal Gas constant[R] = 1.99 Calorie/mole-K
    Heat required = 466 Calories
    Why do I get 2 different answers just because of change in units of R?
  2. jcsd
  3. Dec 24, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    Because your C(p) = 709.7 can't possibly be right. If the components have Cp of 28.59 and 20.72, the mixture must have Cp in between those values.

    The equation looks OK, so I guess you hit the wrong buttons on your calculator.
  4. Dec 24, 2006 #3
    U r right. What am I doing??Thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook