[student8607]

Which of the following substances affect the pH of an aqueous NH₃ solution?
a. KOH
b. NH₄NO₃
c. NH₄Br
d. KBr

I got that its the ones with a common ion, but does it have to be exactly the same?
EX: Cant be NH₄, must be NH₃

I'm guessing it's B & C because of the N

[Borek]

So you think adding strong base will not change pH of the solution?

Do you know what buffer solutions are?

[student8607]

So you think adding strong base will not change pH of the solution?

Do you know what buffer solutions are?

I never thought about it like that. Buffers resists major changes in pH.


With the problem before this one: which of the following substances affect the percent dissociation of HNO₂?
NaNO₂ - NaCl - HCl - Ba(NO₂)₂
I just used common ion effect for A - C & D and got it right.
I just assumed this was asking a similar thing?

[student8607]

a. KOH --> strong base
b. NH4NO3 --> NH4 is weak? Not sure what NO3 has to do with it?
c. NH4Br --> NH4 is weak? Not sure what Br has to do with it?
d. KBr --> Not on my chart but since K is 1A: strong acid?

[Astrokel]

I think Borek is asking what composes a buffer solution.

[student8607]

A weak acid and its conjugate base?

[typhoon2028]

NH3 will form NH4+ in water.

There is an equilibrium:  NH3 + H20 <--> NH4+  + OH-
The pH will be >7, because NH3 accepts proton from water forming ammonium hydroxide solution (weak base).

You need to understand that KOH (potassium hydroxide) is a strong base, and will dissociate completely.  This will drive the OH- concentration higher, increasing the pH.  This shifts the equilibrium towards the formation of NH3, depending on the amount of KOH.

NH4NO3 (ammonium nitrate) and NH4Br (ammonium bromide) are weak acids.

KBr forms a neutral salt solution.

[student8607]

NH3 will form NH4+ in water.

There is an equilibrium:  NH3 + H20 <--> NH4+  + OH-
The pH will be >7, because NH3 accepts proton from water forming ammonium hydroxide solution (weak base).

You need to understand that KOH (potassium hydroxide) is a strong base, and will dissociate completely.  This will drive the OH- concentration higher, increasing the pH.  This shifts the equilibrium towards the formation of NH3, depending on the amount of KOH.

NH4NO3 (ammonium nitrate) and NH4Br (ammonium bromide) are weak acids.

KBr forms a neutral salt solution.

OK. I think I am starting to understand.

So weak acids are going to try and lower the pH but wont be very successful.
Neutral salts wont affect the pH at all

[Borek]

So weak acids are going to try and lower the pH but wont be very successful.

In general you are right, but once again - think buffer. In two cases you will have solutions containing both NH₃ and NH₄⁺.

[student8607]

So weak acids are going to try and lower the pH but wont be very successful.

In general you are right, but once again - think buffer. In two cases you will have solutions containing both NH₃ and NH₄⁺.

So does that mean it wont affect it at all?

[Borek]

No. It just will be described by the Henderson-Hasselbalch equation.

[AWK]

Which of the following substances affect the pH of an aqueous NH₃ solution?
a. KOH
b. NH₄NO₃
c. NH₄Br
d. KBr

I got that its the ones with a common ion, but does it have to be exactly the same?
EX: Cant be NH₄, must be NH₃

I'm guessing it's B & C because of the N

All compounds affect pH of ammonia solution.
a) strong base increases pH greatly. Practically, KOH decides upon pH of solution.
b) and c) form ammonia buffers - decreasing of pH is about 1 to 2 units depending on both concentrations
d) KBr through ionic strength change acivities of ions - concentration of KBr up to 1 M pH may increase pH  up to 0,15 unit

Some printing errors corrected

[student8607]

Ok. Thanks.