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Homework Help: Please Check my working

  1. Apr 10, 2009 #1
    There are 3 questions below with my working and answers, could someone please check that I am right, even if you can only give approval or correction on just one question and nothing else - anything would be appreciated. Please forgive my not following the template, as I didn't think it would work with checking correct answers. Thanks for any responces!

    Q1: A theoretical salt (X2Y3) (with Ksp= 6.26 x 10 -15 ) is in a saturated solution. What is its millimolar concentration (mM) of X?

    A1: Ksp= [X]2 [Y]3 = 6.26 x 10 -15

    therefore concentration of Y = 3/2 concentration of X (or is it 2/3, iam not sure?)

    therefore rewrite Y as [3/2 X]3

    therefore Ksp= [X]2x 27/8 X3
    = 27/8 [X]5

    therefore, with rearranging, [M] = 1.131514468x10-3 mol/L
    = 1.131514468 mM/L


    Q2: A reaction at temperature of 287K gives a change in enthaply of -7KJ and a free energy change of -10kJ, what is the change in entropy for this reaction (in J/K)?

    A2: [tex]\Delta[/tex]G = [tex]\Delta[/tex]H - T[tex]\Delta[/tex]S

    with rearranging: [tex]\Delta[/tex]S = [tex]\Delta[/tex]H - [tex]\Delta[/tex]G / T
    =(-7 - -10)/287 = 0.010452961 J/K

    That doesn't seem right to me.


    Q3: If a 0.5 L solution at a temperature of 294.75 K contains 13.7g of an unknown solute (with a molecular mass of 60.094g), what is its osmotic pressure in atmospheres?

    A3: since moles (n) = concentration (c)/ volume (v) = mass (m)/ molecular weight (M)

    then C = m/Mv = 13.7 / (60.094x0.5) = 0.455952341 mol/L

    Now use the values of C, T and the gas constant (8.314 J/mol K) into the osmotic pressure formula P=CRT

    P= 0.455952341x8.314x294.75 = 1117.334694 Pa (am I right to say that its in pascals, or is it in killapascals [Kpa]?)

    atmospheres = 101.325 Kpa = 101325 Pa

    osmotic pressure (in atmospheres) = 1117.334694/101325 = 0.011027236 atms

    This is something iam really not sure about, the answer seems almost wrong to me but iam not sure where I would have gone wrong in my working, can someone please help? It would be very much appreciated!

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

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 10, 2009 #2


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    Staff: Mentor

    You use too many significant digits every time.

    First looks OK.

    Second and third - approach seems correct (even if in the second your formula misses parantheses), but second opinion won't hurt.
  4. Apr 10, 2009 #3
    Thank you Borek, and I agree, I'd still appreciate at least another person's opinion on my working.
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