The pH of Solutions of Weak Acids and Bases 12.65

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Semi Yoon
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Joined: Fri Sep 28, 2018 12:27 am

The pH of Solutions of Weak Acids and Bases 12.65

Postby Semi Yoon » Sat Dec 01, 2018 4:16 pm

How would you determine whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7. If pH<7 or pH>7, write a chemical equation to justify your answer. (a) NH4Br; (b) Na2CO3; (c) KF; (d) KBr; (e) AlCl3; (f) Cu(NO3)2.

armintaheri
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Joined: Fri Sep 28, 2018 12:26 am

Re: The pH of Solutions of Weak Acids and Bases 12.65

Postby armintaheri » Sat Dec 01, 2018 7:09 pm

a) Compound goes into water. It breaks up into Br- and NH4+. Br- doesn't do anything. Some of the NH4+ release a hydrogen and become NH3. Releasing a hydrogen makes the solution acidic. pH falls below 7.

b) Breaks up in water. The sodium does nothing. The carbonate is the conjugate base of carbonic acid. Being a base, it grabs hydrogens from water to become HCO3 or H2CO3. Taking hydrogen from H2O leaves a lot of OH-, which makes the solution basic. So the pH is above 7.

c) Breaks up into fluoride and potassium ions. Potassium does nothing. F- is the conjugate base of HF. So it's going to grab hydrogens from water, making the solution basic. pH is above 7.

d) Bromide is the conjugate base of HBr (Most negative ions are basic). pH is above 7.

e) Cl- is a negative ion, but it's the conjugate base of HCl. Since HCl dissociates completely in water, you know Cl- is not going to grab any hydrogens when you put it in water. So it doesn't do anything. Aluminum is the conjugate acid of Al(OH)3, so it's going to grab OH ions from water and make an acidic solution. pH is below 7.

f) Nitrate doesn't do anything. Copper is the conjugate acid of copper hydroxide, so it's going to create an acidic solution. pH is below 7.

Samantha Hoegl Roy 2C
Posts: 81
Joined: Fri Sep 28, 2018 12:15 am

Re: The pH of Solutions of Weak Acids and Bases 12.65

Postby Samantha Hoegl Roy 2C » Tue Dec 04, 2018 10:55 pm

hi can you clarify why Na, K and Br "do nothing"? What do you mean by that and how did you know<?

Daniel_Frees_1L
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Joined: Fri Sep 28, 2018 12:18 am

Re: The pH of Solutions of Weak Acids and Bases 12.65

Postby Daniel_Frees_1L » Wed Dec 05, 2018 5:53 pm

@Samantha I think what she means is they do nothing to the pH of the solution. For K+ and Na+ the charge isn't high enough to pull charge on an O in H20 and lead to the release of an H+. Since the charge isn't high enough to do that they aren't causing acidity. For Br-, it does nothing because it is not OH-, and as far as I know anions don't do anything to pull H+ off of H20 (Wouldn't be surprised otherwise but at least as far as we covered).

Clarissa Cabil 1I
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Joined: Fri Sep 28, 2018 12:19 am

Re: The pH of Solutions of Weak Acids and Bases 12.65

Postby Clarissa Cabil 1I » Wed Dec 05, 2018 9:40 pm

armintaheri wrote:a) Compound goes into water. It breaks up into Br- and NH4+. Br- doesn't do anything. Some of the NH4+ release a hydrogen and become NH3. Releasing a hydrogen makes the solution acidic. pH falls below 7.

b) Breaks up in water. The sodium does nothing. The carbonate is the conjugate base of carbonic acid. Being a base, it grabs hydrogens from water to become HCO3 or H2CO3. Taking hydrogen from H2O leaves a lot of OH-, which makes the solution basic. So the pH is above 7.

c) Breaks up into fluoride and potassium ions. Potassium does nothing. F- is the conjugate base of HF. So it's going to grab hydrogens from water, making the solution basic. pH is above 7.

d) Bromide is the conjugate base of HBr (Most negative ions are basic). pH is above 7.

e) Cl- is a negative ion, but it's the conjugate base of HCl. Since HCl dissociates completely in water, you know Cl- is not going to grab any hydrogens when you put it in water. So it doesn't do anything. Aluminum is the conjugate acid of Al(OH)3, so it's going to grab OH ions from water and make an acidic solution. pH is below 7.

f) Nitrate doesn't do anything. Copper is the conjugate acid of copper hydroxide, so it's going to create an acidic solution. pH is below 7.


In the solutions manual, it says that for part d, it is actually neutral. Can anyone explain why?

Leela_Mohan3L
Posts: 44
Joined: Fri Sep 28, 2018 12:26 am

Re: The pH of Solutions of Weak Acids and Bases 12.65

Postby Leela_Mohan3L » Wed Dec 05, 2018 10:28 pm

I think its because the bromide ion is the anion of a strong acid, not a weak acid. Thus, it is ineffective at pulling a proton off water. This leaves the solution as neutral? In general, strong acids have weak conjugate bases.

I'm not completely sure though so someone correct me if I'm wrong.


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