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March 26, 2017

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A 500ml saturated solution of MgCO3 (M=84) is reduced to 120 ml by evaporation. What mass of solid MgCO3 is formed?

The answer is 0.20g, I just need a detailed explanation.
Thanks!

  • AP Chemistry - ,

    Do you have a solubility or a Ksp to use. There is a good bit of disagreement on the net about the correct value for Ksp for MgCO3.

  • AP Chemistry - ,

    Oh sorry the Ksp=4x10^(-5)

  • AP Chemistry - ,

    OK. Thanks.
    MgCO3 ==> Mg^+2 + CO3^-2
    ..x........x.......x
    Ksp = (Mg^+2)(CO3^-2) - (x)(x)
    x = sqrt(4E-5) = 0.00632 moles/L or 0.00632 x 84 = 0.531 g/L
    In 500 mL there will be 1/2 that or .265 grams.

    If we evaporate water until the volume is 120 mL, then 0.531 g/L becomes
    0.531 g/L x (120 mL/1000 mL) = 0.0637 g.
    We started with 0.265 g in 500
    ............less 0.0637 in 120 mL =
    .................-------
    solid ppting.....0.201 g which rounds to 0.2 g to 1 significant figures (from the Ksp value).

  • AP Chemistry - ,

    Thanks! But im confused on why you would subtract the .0637g in 120mL from .265g in 500mL. Why don't you do (grams from 500mL-grams from 380mL)?

  • AP Chemistry - ,

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    Select the single most important reason below to justify your choice should have the higher melting point

    - Both molecules are nonpolar but the higher melting includes hydrogen bonding
    - Both molecules are nonpolar but the higher melting is more polarizable
    - The higher melting molecule is polar and the other is not
    - The lower melting molecule is polar and the other is not
    - The higher melting molecule has smaller Van der Waals forces than the lower melting
    - The higher melting molecule has greater Van der Waals forces than the lower melting
    - The higher melting molecule is ionic and the other is not
    - The lower melting molecule is ionic and the other is not
    - The higher melting molecule has hydrogen bonds and the other does not
    - The lower melting molecule has hydrogen bonds and the other does not
    - Both molecules are ionic but the higher melting has a larger cohesive energy
    - Both molecules are ionic but the lower melting has a larger cohesive energy
    - The higher melting molecule is ionic and the other is not.
    - Ionic bonds are stronger than hydrogen bonds
    - The lower melting molecule is ionic and the other is not.
    - onic bonds are weaker than hydrogen bonds
    - The higher boiling molecule is ionic and the other is not.
    - Ionic bonds are stronger than Van der Waals forces
    - The lower boiling molecule is ionic and the other is not.
    - Ionic bonds are weaker than Van der Waals forces

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