Q1. When calculating
change in temperature of a solution due to addition of solutes, the
formula is
(change in
temp) = K(constant of solvent) x molality.
Then the new
b.p./f.p. = b.p./f.p. of pure solvent + the change in temp (as
calculated above)
My question is: If
ethanol and water is mixed, how do we determine which is the pure
solvent, and which is 'solute', since both are liquid?
(So that we can
choose the correct K value to substitute into the above
equation...)
Is it based on the
no. of moles of ethanol and water? Larger no. of moles is
the pure solvent... What if both are in equal
amount?
Q2. When we say "solubility of
Mg(OH)2 is 9 mg/L in water", does the "9 mg/L"
refer to the cations ([Mg2+]) only, or does it actually
refer to the whole compound Ksp =
[Mg2+][OH-] = 9 mg/L?
Thank you!
Q1. By definition solvent is the majority species, and solute is
the minority species. However, the assumption required to
use the formula you quoted, is that the solute is
non-volatile, which is not true for alcohols. For more info, see :
http://en.wikipedia.org/wiki/Boiling-point_elevation
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Q2. Mass solubility refers to the sample mass of the ionic
species (ie. Mg(OH)2, not just Mg2+) that can dissolve in one dm3
or L of solution. Mass solubility is not Ksp (which is the
solubility product). To convert mass solubility to solubility
product, you need to first obtain the molar solubility.
Eg. If the mass solubility of Mg(OH)2 is 9mg/dm3, the molar
solubility is 1.544 x 10^-4 mol/dm3, and the solubility product is
(1.544 x 10^-4) ( 2 (1.544 x 10^-4) )^2 = 1.472 x 10^-11
(mol/dm3)^3
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Lastly, based on your Qns above (ie. content of Q1 and
units used in Q2), you're probably doing a Chemistry-based
course in the Uni, correct?
In future (ie. when you've more Uni level questions you
wanna seek help on), Chemistry questions beyond JC 'A'
levels should best be posted in the Chemical
Forums, rather than SgForums.