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Chemistry

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Which aqueous solution below is expected to have the lowest freezing point? Assume 100% dissociation for ionic species.

1)0.25m MgBr2
2)0.30m Na2SO4
3) 0.50m KCl
4) 1.0m sucrose
5) 0.40m Cr(NO3)3

I think that the answer is 0.30NaSO4 but I am not positive. Is this correct?

  • Chemistry -

    delta T = i*Kf*m
    You want the largest delta T (which will give the lowest freezing point).
    Kf is constant so we can forget that. The two that matter are i and m
    So multily i*m for the 5 and the largest number wins. Remember i is the van't Hoff factor which is the number of particles produced when the materials are placed in solution. sucrose is l. KCl is 2, etc.

  • Chemistry -

    I understand everything except the van't Hoff factor part. I do not understand how you figure that out.

  • Chemistry -

    That how the material ionizes.
    KCl ==> K^+ + Cl^- and i = 2
    MgCl2 ==> Mg&2+ + 2Cl^- and i = 3
    Na2SO4 == 2Na^+ + SO4^2- and i = 3
    Cr(SO4)3 ==> Cr^3+ + 3SO4^2- and i = 4
    Sugar doesn't ionize so it is just 1 particle
    Sugar ==> sugar(aq) and i = 1

  • Chemistry -

    Ok so then I just have to find the mass of each solution

    so KCl = 74.55 X 2
    MgBr2 = 184.11 X 3
    Na2SO4 = 94.05 X 3
    Cr(SO4)3 = 340.21 X 4

    So Cr(SO4)3 would have the largest delta T, so the lowest freezing point.
    Is this correct?

  • Chemistry -

    Why do you need the molar mass?
    delta T = i*Kf*m
    You know i from my previous response and you know m from the problem. i*m that gives the largest number will be the one that gives the largest delta T.

  • Chemistry -

    Ok so do you mean 0.25MgBr2 X 3 = 0.75 ?

  • Chemistry -

    i equals 3 for MgBr2 correct?

  • Chemistry -

    So I think that 0.40m Cr(NO3)3 has the largest delta T. Would you agree?

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