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Homework Help: Van der waals equation question

  1. Mar 12, 2010 #1
    Hi everyone,

    Struggling with the following question - maths and physics never been my strongest subjects. Thanks in advance

    1. An equation of state that has been used to model the behaviour of a fixed amount of a real gas is:
    In the equation Z, a, b are constants, and P represents pressure, V = volume, T = temp. The constant Z depends on the amount of gas.
    Consider what the dimensions of a and b are in terms of the fundamental quantities of mass (M), length (l), and time (t).

    2. Q1: The dimensions of b are:
    a) L3
    b) L6
    c) M.L-1.T-2
    d) M-1.L.T2

    Q2: The dimensions of a are:
    a) L6
    b) M.L5.T-2
    c) M.L-1.T-2
    d) M.L-5.T-1

    Q3: The value of the constant Z could be expressed in terms of the unit:
    a) W
    b) W.N.s-1
    c) N.J.K-1
    d) J.K-1

    3. According to std van der Waal's equantion Z is equivalent of R, therefore Q3 = D, not sure about the calculations for Q1 or Q2

    Any help would be greatly appreciated.

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Mar 12, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I'll help you with this one since it is the easiest. Without just giving you the answer I will explain by analogy. The portion of the equation that contains b is (v-b), right? You are subtracting b from the volume.

    Now here is the hint/analogy... Can you subtract gallons from miles? Gallons from degrees C? No. What must the units of b... err be?
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