Some general GC questions

[tisler]

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1. Is Weak acid also a weak base and Strong acid a strong base??
2. this is from kaplan GC practice test about ionization
If the actual freezing point of MgCl2 is 0.0558 celsius and the theoretical freezing point of the solution is -0.0186, what can be said about inization of MgCl2?
Kaplan says that it is completely ionized.
But if it is, then, wouldn't it lower the freezing point since the actual freezing point is higher than theoretical??

 

[yakuza]

Is this exactly how they worded it? A theoretical yield is a reaction that goes to completion therefore it dissociates fully/more therefore the freezing point of the theoretical yield is lower than the actual (look at colligative properties)

 

[tisler]

sorry for the confusion!
here's how they worded
when 0.95g of MgCl2 are dissolved in 1kg of water, the freezing point of the solution is found to be 0.0558 celsius. If the molal freezing-point depression constant for water is 1.86 C/m, what can be said about the inonization of MgCl2 in aqueous solution?
a. MgCl2 does not ionize
b. it is totally ionized
c. it is 1/2 ionized
d. it is ionized to a negligible extent

 

[mintz123]

1. Strong Acid dissociates into a weak conjugate base, however, a weak acid will dissociate into a strong base. Same goes for Strong Base and weak bases.

 

[klutzy1987]

1. Strong Acid dissociates into a weak conjugate base, however, a weak acid will dissociate into a strong base. Same goes for Strong Base and weak bases.

Not true. not all weak bases have strong conjugate acids. Only a select few bases doFor example CN- is a weak base and its conjugate acid is also a weak acid HCN. Also bases do not dissociate into acids, rather they pull hydrogens away from water to form OH-

 

[osimsDDS]

sorry for the confusion!
here's how they worded
when 0.95g of MgCl2 are dissolved in 1kg of water, the freezing point of the solution is found to be 0.0558 celsius. If the molal freezing-point depression constant for water is 1.86 C/m, what can be said about the inonization of MgCl2 in aqueous solution?
a. MgCl2 does not ionize
b. it is totally ionized
c. it is 1/2 ionized
d. it is ionized to a negligible extent

What I would do for this one is actually calculate the decrease in temperature and see if it matches, since you know the vont-hoff factor is 3 (MgCl2 dissociates into Mg and 2Cl-, 1+2=3) Use the freezing point depression formula to calculate the change in temperature...

T= (kb)(molality)(i)
T= (1.86)(.95/95g/mole)(3)
T= .0558

If the depression is equal to the temperature of depression they gave you then you know that it completely dissociated, you could have also went along and found the vont-hoff factor (i) and then compared it to the actual to see if it fully dissociated...

 

[hdu]

I believe there is a lack of communication because of the way the questions and answers are worded.

My two cents:

Let's use these concepts:
1.- an acid is a substance that release H+ and a base a substance that can accept protons
2.- H3O+ is the same as H+ (in fact, H+ does not exist in water, but H3O+; the books use H+ for facilitating notation)
3.- A strong acid is a substance that release easily H+, it is said also, that completely ionize in water forming H3O+

AH + H2O ----> [A-] + [H3O+]

A- can accept protons, it is a base, and it is the conjugate base of AH; additionally we can say that A- is a weak base since it can not hold very strong the H+, and that is why the AH is a strong acid, since H+ is released easily (In general,the conjugate bases of strong acids are weak bases).

4.- A weak acid has a low tendency to release a proton, and dissociate poorly in water,

WH +H20 <-----> [W-] + [H3O+] (observe that the dissociation does not go to completion, but up to an equillibrium)

[W-] can accept protons, it is the conjugate base of WH; it is a strong base, since it "holds" the proton and does not release it easily, that is why WH is a weak acid. In general, the conjugate bases of weak acids are strong conjugate base)

It is kind of simplified, but I hope it helps.

A good site for these issues: http://www.wwnorton.com/college/chemistry/gilbert/overview/ch16.htm

My blog: www.biochemistryquestions.wordpress.com

 

[tisler]

thanks for the great answers