i of freezing point depression/melting point depression

[cc609]

The formula for freezing point depression is change in Temperature = i* Kfreezing*molal

For determining i, I know that NaCl for example would be 2, and H2So4 would be 3.
However, do we disregard the i value if there is a strong acid? For example, if there was say, a strong acid such as HCl, technically with an i of 2, and a Na2So4, which has i of 3....the i value is higher for the latter, but HCl is a strong acid so it would technically dissociate more would it not? Would someone help me with this one :'(

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[DAT Destroyer]

The formula for freezing point depression is change in Temperature = i* Kfreezing*molal

For determining i, I know that NaCl for example would be 2, and H2So4 would be 3.
However, do we disregard the i value if there is a strong acid? For example, if there was say, a strong acid such as HCl, technically with an i of 2, and a Na2So4, which has i of 3....the i value is higher for the latter, but HCl is a strong acid so it would technically dissociate more would it not? Would someone help me with this one :'(

Yes,,,,,The i value for HCl would be two particles..... usually we are dissolving solutes into solution and not strong acids.

Dr. Romano

 

[DustFreeEraser]

http://en.wikipedia.org/wiki/Colligative_properties

I.e. the number of particles is what matters.

And also, wouldn't H2SO4 be 2, not 3? Or at least closer to 2 than 3?

 

[cc609]

http://en.wikipedia.org/wiki/Colligative_properties

I.e. the number of particles is what matters.

And also, wouldn't H2SO4 be 2, not 3? Or at least closer to 2 than 3?

Since H2SO4 has 2 protons to give, I thought it would be 2 protons and SO4 -2, hence i=3 . Someone can correct me if I'm mistaken :O

 

[DustFreeEraser]

While H2SO4 is a strong acid, HSO4- isn't, so I presumed i ~ 2;

 

[w4nt2baDDS]

They would have to specify "completely dissociates" or give the Ka Value for HSO4- and then you could calculate the ions generated by: [Ka*[HSO4-]]^1/2 then multiply that number by 2 to figure out how many i values H & SO4-2 are generated. If you say the SO4- concentration is x then I think the i value would be:

(1) for first Hydrogen
(1 - x ) for HSO4
(x) for 2nd Hydrogen
(x) for SO4-2

If you sum this all together, i would equal 2+x

depending on the Ka value this could be either really close to 2 or close to 3 or somewhere in the middle....