strong acids for freezing point depression

[cc609]

Which of the following produces highest freezing point (with all same moles) when dissolved in water?

a) HClO4
b) HClO3
c) HClO2
d) HClO
e) HCl

So for the equation deltaT=i*Kfreezing*molality, and since it asks for HIGHEST fp, we look for lower i value.

First question: technically, don't they all have an i value of 2? (ex: H+ and Cl-, and H+ and ClO3-)
Second question: We eliminate answers a and e because they are strong acids and dissociate most. So does the formula from Chad's Videos not work for strong acids? As in, i=2 for HCl, but if we had some other compound that had an i value of 4 but was not a strong acid, would I still pick HCl for dissociating most?

---

Members don't see this ad.

 

[Doctorman45342]

The acid strength varies for each one. I think. I havn't watched the acid strength videos yet but in this case it would be the weakest acid.

HCl pKa = -8
HClO pKa = 7.53
HClO2 pKa = 2
pKa HClO3 = -1
pKa HClO4 = -10

 

[DUPH]

I believe the answer would be D; since the i value for each acid is 2 I think you would look for the acid which disassociates the least.
You're right about HCl and HClO4 being strong acids (HClO3 is a SA too) so then you're left with HClO2 and HClO; the rule of oxoacids would be the one with the least amount of oxygen content would be the least acidic. Therefore, answers D?

 

[PocketRocket]

Tariqd is right. It is a matter of acid strength where the strongest acid is going to produce the greatest amount of ions. We know Hcl is a strong acid and that in heteroacids , or whatever you call em, acidity increases with the amount of oxygen atoms so HClO4> HClO in terms of acidity. This is the concept that this question is asking about. For the calculation type of problems it's going to be a lot more straight forward where you can pretty much assume 100% dissociation like for NaCl has i = 2, Ba3(PO4)2 i = 5 (even though Ba3(PO4)2 is insoluble but I hope you get the point). I don't think you'll ever have to worry about the degree to which something dissociates when doing calculations in colligative properties

 

[DAT Destroyer]

You have missed a very important concept. A strong acid will dissociate 100%..... No values are needed here. HCl and HCLO4 are both strong.....thus 2 particles are given off assuming high dilution. Next... HCLO3 and HCLO2 both have more Oxygens than HCLO, thus are stronger than HClO. This leaves HCLO as the weakest acid, What does this imply ? It implies that it will ionize to the LEAST extent.....in other words give off the fewest particles. Consequently, the boiling point would be raised only slightly, and the freezing point lowered only a tad bit. HCLO would therefore have the highest Freezing Point.

Hope this helps.

Dr. Jim Romano

 

[florida21]

why can't you use the distinction that the stronger the intramolecular forces, the higher the BP and the higher the MP