What is the pH of 0.15 M NH4Cl?

  • Thread starter d.tran103
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In summary, the pH of 0.15 M NH4Cl is 5.04. The Kb of NH4 does not exist because it is an acid, not a base. The equation KaKb=Kw can be used to find both Ka and Kb for an acid/conjugate base pair. To find the pH, you would take -log(x) once you solve for it, and if using Kb, you would do 14.00 + log(x) to get pH.
  • #1
d.tran103
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What is the pH of 0.15 M NH4Cl?
Okay, I set up my ICE table,

NH4 + H2O <---> H3O+ + NH3-
I 0.15 M --- --- ---
Δ -x --- +x +x
E 0.15-x --- x x

KbNH3-=1.8x10-5
So Kw=1.0e-14=Ka*Kb, so KaNH4=5.56e-10

So, Ka=[H3O+][A-]/[HA]

5.56e-10=x^2/0.15-x
[H+]=[H3O+]=[x]=9.13e-6

pH=-log[H+]
pH=5.04

I understand the work, but my question is why can't I take the -log[x] from kb=[OH-][HA]/? Is it because I have the Kb of NH3 and not the Kb of NH4? Thanks!
 
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  • #2
d.tran103 said:
my question is why can't I take the -log[x] from kb=[OH-][HA]/


Huh? -log[Kb] is pKb. I suppose that's not what you mean, but what you wrote makes no other sense to me.
 
  • #3
Oh sorry, I meant to say -log(x) as in taking the -log(H3O+) (from the original ice table)
 
  • #4
I still have no idea what you mean by "-log(H3O+) (from the original ice table)".

H3O+ in your ICE table is either 0 (initial value) - you can't take log of zero, or x - which is an unknown, so you can't calculate logarithm of its value before solving the problem.
 
  • #5
NH4 has no Kb because it is an acid, not a base. I honestly have no idea what your question is even asking.
 
  • #6
I'm still not sure what you're asking. You take the -log(x) once you solve for it to get the pH. If you used the Kb, then you would do 14.00 + log(x) because x in that case would equal the hydroxide concentration, so to get pH you must subtract the pOH from 14.00.
 
  • #7
binomial said:
NH4 has no Kb because it is an acid, not a base

Not that you are wrong, but KaKb=Kw, so you can give both for any acid/conjugate base pair. People often abbreviate it and say Kb of an acid or Ka of a base.

That beiung said, I doubt that's what the OP meant.

binomial said:
I'm still not sure what you're asking. You take the -log(x) once you solve for it to get the pH. If you used the Kb, then you would do 14.00 + log(x) because x in that case would equal the hydroxide concentration, so to get pH you must subtract the pOH from 14.00.

You answered a spammer quoting a random part of the first post :tongue2:
 

1. What is the definition of acid/base equilibrium?

Acid/base equilibrium refers to the balance between the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution. In a neutral solution, the concentration of H+ and OH- is equal, resulting in a pH of 7. When the concentration of H+ is higher than OH-, the solution is considered acidic and has a pH below 7. In contrast, when the concentration of OH- is higher than H+, the solution is basic and has a pH above 7.

2. How is pH related to acid/base equilibrium?

pH is a measure of the concentration of H+ ions in a solution. In an acid/base equilibrium, the concentration of H+ ions determines the pH of a solution. A higher concentration of H+ ions results in a lower pH (more acidic solution), while a lower concentration of H+ ions results in a higher pH (more basic solution).

3. What is the difference between strong and weak acids/bases in an acid/base equilibrium?

Strong acids and bases completely dissociate in water, meaning they fully break apart into their constituent ions. This results in a higher concentration of H+ or OH- ions, respectively, and a lower pH for strong acids and a higher pH for strong bases. Weak acids and bases only partially dissociate in water, resulting in a lower concentration of H+ or OH- ions and a less significant effect on pH.

4. How do you calculate the pH of a solution in an acid/base equilibrium?

The pH of a solution can be calculated using the concentration of H+ ions in the solution. The equation for pH is pH = -log[H+], where [H+] represents the concentration of H+ ions in moles per liter (M). Alternatively, if the concentration of H+ ions is not given, the pH can be calculated using the concentration of a strong acid or base and the equilibrium constant (Ka or Kb) for the acid or base.

5. How do you determine the direction of an acid/base equilibrium reaction?

The direction of an acid/base equilibrium reaction can be determined using Le Chatelier's principle. According to this principle, if a stress is applied to a system in equilibrium (such as a change in temperature, pressure, or concentration), the system will shift to counteract the stress. For example, if the concentration of H+ ions is increased, the equilibrium will shift towards the products (more H+ ions will react to form the products and decrease the concentration). Similarly, if the concentration of OH- ions is increased, the equilibrium will shift towards the reactants (more OH- ions will react with H+ ions to form water and decrease the concentration of OH-).

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