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!
Originally posted by cubsarecute: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.