[Renegad3]

Hi,

These are two problems from a practice chem midterm that relate to pH that I'm still trying to understand why the answers are the way they are:

1)If the reaction H₃0⁺_(aq) + OH^-_(aq)  2H₂0_(l) is exothermic with  :delta:H = -55.5 kJ/mol, the neutral pH for water at 333.15 K is:

A)0   B) < 7   C) 7    D) > 7   E) 14

Correct answer:  B   

Now, I can easily choose between B and C.  A, D, and E all don't make any sense when it concerns water, but in the question it asked for the NEUTRAL pH of water, which I thought was 7?  How does temperature/enthalpy affect the pH of water?


2)The pH of an aqueous solution of Hydrochloric Acid __________  upon the addition of aqueous Ammonium Chloride and ___________  upon the addition of aqueous Sodium Chloride.

A) Raises, Lowers
B) Lowers, Raises
C) Raises, Stays the same
D) Lowers, Stays the same
E) Stays the same, Lowers

Correct Answer: C

HCl is a strong acid, so NaCl won't affect the pH of it.  Now it's between C and D.  Well, I remember that when Ammonium Chloride dissolves in water, it is very slightly acidic in that it produces NH₄⁺ ions.  Even though this is true, is it because of the very small acidity that it will increase the pH of HCl?

No rushes, finals are in 2 weeks.

[Borek]

First question just asks for use of Le Chatelier's Principle. See also

http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product

No idea what to do with the second question. C is not a correct answer.

Addition of NaCl changes pH due to change of ionic strength of the solution, same happens when you add ammonium chloride, however, addding ammonium chlooride you also add an acid - weak, but in diluted slutions of HCl it will lower pH. So there are two effects that have to be taken into account and final result depends on the concentrations of HCl and added salts.

Could be you are to ignore thermodynamic effect of ionic strength - even if so, C is still wrong.