<p>Ok, I just took the first Rea test and got a 680 (eh, I made alot of stupid mistakes) and I have some questions.</p>
<h1>1 What is the Rare Earths Group?</h1>
<h1>2 Can someone post/teach me how to do boiling point elevation and depression (I completely forgot how to do this)</h1>
<h1>3 How does solubility affect a chemical reaction (like if something is soluble do you take it out of the net ionic equation or keep it in)</h1>
<h1>1 - lanthanoids mainly</h1>
<h1>2 - final bp/fp = original + Kb or Kf times molality times i (i = number of ions)</h1>
<h1>3 - give me an example. it varies.</h1>
<h1>1 - Group IIIB, lanthanides and actinides. But this question seems a little weird, they shouldn't be testing you on such quirky details.</h1>
<h1>2 You mean BP elevation and FP depression. Impurities cause BP to increase and FP to decrease. In general, the magnitude of the change is proportional to molality (moles of solute per kg of solvent) with a constant Kf (for freezing) and Kb (for boiling). Kf and Kb are specific to that solvent.</h1>
<p>change in BP/MP = (Kb or Kf) times molality</p>
<p>For example - Dissolve oxygen in water (Kf = 5kgK/mol). 0.001 mol of oxygen in 1 L of water. Calculate the FP of water.</p>
<p>=> Calculate molality. 1 L of water = 1 kg, so molality is 0.001 mol/kg.
=> Calculate change in FP = (5)(0.001) = 0.005 K
=> FP of water = -0.005 deg C</p>
<h1>3 If something is soluble keep it out of the net ionic reaction (as long as the reactants are soluble too)</h1>
<p>E.g. Dissolving sodium chloride
NaCl (s) -> Na+ (aq) + Cl- (aq)</p>
<p>E.g. Mixing sodium chloride and silver nitrate solutions, formation of silver chloride ppt.
NaCl (aq) + AgNO3 (aq) -> NaNO3 (aq) + AgCl (aq)
Since NaNO3 is soluble, you can take out Na+ and NO3- from the ionic eqn to get the net of :
Ag+ (aq) + Cl- (aq) -> AgCl</p>
<p>Thanks for the explanation. Also do you know what the constant is for fp depression and bp elevation is? I am fairly sure that one is ~.52 and another is ~1.86 (but i could be wrong).</p>
<p>Also, another question
23. What is the minimum number of moles of
PbSO4 that must be used to prepare 1 liter of
saturated PbSO4 solution at 25°C? (Ksp at
25°C for PbSO4 =1×10−8 )
(A) 1×10−16mol
(B) 1×10−8mol
(C) 1×10−4mol
(D) 1×10−2mol
(E) 1×10−1mol</p>
<p>The constants can be looked up. Anyway, they're not universal - specific to the solvent.</p>
<p>For this, figure out the molar solubility of lead sulfate. Do up an ICE table.
PbSO4(s) -> Pb2+ (aq) + SO42-(aq). You'll get that Ksp = x^2 where x = molar solubility... So it'd be (C)</p>
<p>Thanks. I didn't know how saturated made a difference.</p>
<p>Saturated just means that as much lead sulfate as possible is dissolved. I.e., Q is equal to Ksp.</p>