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Homework Help: Chemical Eng: How do I get value M from 2 unknown variables.

  1. Feb 22, 2016 #1
    1. The problem statement, all variables and given/known data
    At T1 the root mean square speed (rms) of compound P is 485.2 m s-1 and at T2 the rms speed is 504.1 m s-1. Using this data and the fact that T2 – T1 = 24.0 °C, determine the molar mass of P in g mol-1.

    2. Relevant equations
    1. From problem statement you have: (3·R·T1/M)1/2 = 485.2 and (3·R·T2/M)1/2 = 504.1

    2. You also have that T2 - T1 = 24.0 °C ≡ 24.0 K

    3. Together, (1) and (2) form a system of 3 algebraic equations with 3 unknowns (T1, T2 and M) that can be easily solved

    4. Solve the equation system by first calculating T1 and T2

    5. Once T1 and T2 are known use one of the expression in (1) to calculate M

    3. The attempt at a solution
    I have the 3 equations:

    485.2 = (3RT1/M)^0.5

    504.1 = (3RT2/M)^0.5

    T2 – T1 = 24 K

    I have squared these equations, (rms)^2 = 3RT/M, in order to eliminate the square root to make the equation as simple as possible, however that is all I have so far and do not understand how to calculate M with two three unknown variables T1, T2 and M. Could you please give me some assistance?
  2. jcsd
  3. Feb 22, 2016 #2


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    Staff: Mentor

    Since you have ##T_2 - T_1## in the third equation, try to get a ##T_2 - T_1## from the other (squared) equations.
  4. Feb 22, 2016 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Find the ratio of T2 : T1 by dividing the appropriate equation by the other. All of the other quantities like R and M will cancel.

    Once you know the ratio of T2:T1, you can express T2 in terms of T1, and since you know the difference T2 - T1 = 24, you can solve for T1.
  5. Feb 22, 2016 #4
    Hi DrClaude. I dont know if I have did this correctly, but my logic is this:

    I have rearranged (3RT1/M)^0.5 to become (rms)^2 = 3RT/M

    What I have now done is this: (rms)^2 = 3RT2/M
    T2 = [M.(rms)^2] \ 3R
    __________________ =
    T1 = [M.(rms)^2] \ 3R

    Is this correct?
  6. Feb 22, 2016 #5


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    Staff: Mentor

    Your equation is coming out funny (not sure what the = sing in the middle is doing there). But now that you have equations for T1 and T2, you can either get T2-T1 as I suggested, or T2/T1 as SteamKing suggested.
  7. Feb 22, 2016 #6
    Hi again!

    I think you both have helped me out enough now. I greatly appreciate it.

    What I've did is T2 = [M.(rms)^2] \ 3R] / T1 = [M.(rms)^2] \ 3R]

    (T1+24)/T1 = ratio

    Solve for T1

    Then use that figure to solve for T2.

    I'm hoping this is correct.

    I hope you both will be able to help me in the future - I'm finding some of the course very hard and quite horrible.

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