[katrina007]

Previously posted on the first topic: Chemistry 123 - Homework Help

Question 4
- What is the approximate pH at the equivalence point when a weak base, like NH4OH (aq), is titrated with a strong acid like HCL (aq) ?

a) 7
b) 5
c) 9
d) 2
e) None of the above. It is...

Question 5
- Metals have loosely held electrons. This explains why metals usually...

a) have high ionization energies
b) have low thermal conductivity
c) form anions
d) are oxidized, rather than reduced
e) form few compounds

If anyone can help me with these questions please let me know. Thanks in advance

- Katrina

[AWK]

Q1 Think about salt hydrolysis
Q2 read textbook or wikipedia
http://en.wikipedia.org/wiki/Metallic#Chemical_properties
on properties of metals

[katrina007]

Hi, Borek.
Sorry for directly postingi the questions without any attempt at it first. It wont happen again.

(AWK)
For question one I said answer is (a) because at the equal point the pH is seven; neutral. Is this correct?

so for question two, I said that answer is (d) because metals having loose (or less) electrons that means its going into oxidation (gaining more electrons). And its being reduced less. Is this correct?

[Yggdrasil]

(AWK)
For question one I said answer is (a) because at the equal point the pH is seven; neutral. Is this correct?

Maybe it would help to write out the balanced chemical reaction for the neutralization of NH₄OH with HCl.  The products of the reaction will be water and a salt.  Is the salt acidic, basic, or neutral?

so for question two, I said that answer is (d) because metals having loose (or less) electrons that means its going into oxidation (gaining more electrons). And its being reduced less. Is this correct?

Right, but for the wrong reason.  I find the following mnemonic helpful for remembering oxidation v. reduction:  OILRIG = oxidation is loss, reduction is gain.  Oxidation refers to the loss of electrons while reduction is the gain of electrons.  Another one I've heard is "LEO (Lose Electrons = Oxidation) the lion goes GER (Gain Electrons = Reduction)."

But, if the metal's electrons are loosely held, than it is easy for the metal to lose electrons.  Therefore, oxidation is easier because oxidation is the loss of electrons.

[katrina007]

I read the chapter over again and now I understand these two questions. The salt is acidic therefore its pH should be less than 7.00. And the answer is 5.

Thanks for your help.

[Borek]

The salt is acidic therefore its pH should be less than 7.00. And the answer is 5.

Isn't 5 less then 7.00? Or am I missing something?

NH₄⁺ - is it acid, base, or none?

[katrina007]

I think NH4 is acidic. and in my book it says when the pH is less than 7.00 its acidic solution and since the computer NH4 is acidic its pH has to be lower than 7.00 and thats why I choose five.

Am I wrong? please let me know.

[AWK]

Though NH4+ is a Bronsted acid, it is a very weak acid! Do weak acids can show pH=2 ?

[ARGOS++]

Dear Katrina007,  Dear All,

Sorry, but I think, a very confuse situation has now arrived, as everybody talks a different item!

First: Katrina007,  after your second study the Chapter, ALL the Statements you did are unexceptional correct.
Why: The equivalence point is NOT defined as the equivalent concentration of the Base to their corresponding Acid, and therefore the answer of a pH of 5 is a quite good approximation for the reality, I think.
But why is it only an approximation?: As we don’t know the exact starting concentration and don’t know also the constancy of the titration volume, we  cannot calculate the exact pH value.
The pH of 9, from the List, corresponds in this Example with the value where [NH₄ ⁺ ] and [NH₃] are equivalent, and this value is nearly independent of concentration influences at all.
A Picture for this circumstance you may see on the left side of:
   http://www.p-forster.com/english/themes/Spectroscopy/Spectroscopy_FAQ.htm
(But keep in mind, that the equivalence point is lying,  - for titration with strong acid on the left boarder,  - and for titration with strong base on the right boarder of the graph. )
I think you know in which physical constant both concentrations are relevant.

So the thumb-rule holds, which tells you that the equivalence pH lies always on the side of the stronger titration partner.

Good Luck!
                    ARGOS⁺⁺


P.S.:  In titration with a strong acid, not the week acid is making pH 2, or similar.