TPR General Chemistry Question

[MissionStanford]

Which of the following salts will produce a basic solution when added to pure water?

A) KCl
B) NaClO
C) NH4Cl
D) MgBr2

Solution: NaClO (choice B) will dissociate into Na+ and ClO-. Na+ is a Group 1 cation, so it has no effect on the pH. However, ClO-, the hypochlorite ion, is the conjugate base of a weak acid, HClO (hypochlorous acid). Therefore, the solution will be basic. The salt in choices A will have no effect on the pH, and the salts in choice C and D will leave the solution acidic (since NH4+ is the conjugate acid of a weak base, NH3, and Mg2+ will react with water to form the weak base Mg(OH)2).

I thought the answer would be choice B since Na would combine with OH- to form NaOH (strong base) while ClO would combine with H+ to form HClO (weak acid), thus creating a basic solution.

How would you go about solving this question?

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[shaboobly]

figure out which one dissolves in water and has the strongest basic anion.

 

[ebasappa]

Actually( someone correct me if I'm wrong), I believe the Na+ would do nothing once seperated, and the ClO would grab an H from H2O to raise OH concentration. The HClO made does not have an affect as great as the OH created

 

[V5RED]

Which of the following salts will produce a basic solution when added to pure water?

A) KCl
B) NaClO
C) NH4Cl
D) MgBr2

Solution: NaClO (choice B) will dissociate into Na+ and ClO-. Na+ is a Group 1 cation, so it has no effect on the pH. However, ClO-, the hypochlorite ion, is the conjugate base of a weak acid, HClO (hypochlorous acid). Therefore, the solution will be basic. The salt in choices A will have no effect on the pH, and the salts in choice C and D will leave the solution acidic (since NH4+ is the conjugate acid of a weak base, NH3, and Mg2+ will react with water to form the weak base Mg(OH)2).

I thought the answer would be choice B since Na would combine with OH- to form NaOH (strong base) while ClO would combine with H+ to form HClO (weak acid), thus creating a basic solution.

How would you go about solving this question?

Problems with thinking about the Na+ forming NaOH
Adding NaOH to water creates a basic solution because it dissociates very favorably into Na+ and OH- ions. The equilibrium strongly favors the ions. The reverse reaction of Na+ and OH- combining to form NaOH is not only extremely unfavorable, but it would also make to solution more acidic by removing hydroxide from solution, not more basic.
Why the ClO- matters
The ClO- consumes protons present in water to form HClO which makes the solution more basic. Recall that water has an autoionization, so there are always protons and hydroxide ions present in water. (edit: This works because HClO/ClO- is a weak acid/base pair, so an appreciable amount of HClO forms. This would also work if ClO- were strongly basic since a ton of HClO would form.)
Extra information
When the NaClO dissolves, the ClO- binds to the protons and the Na+ does nothing except replace the charge that the protons had in the water which maintains the neutral net charge of the water. When the protons are consumed, the water becomes more basic. The water also dissociates to form more protons and hydroxide(think Le Chatlier's) and the dissolved ClO- will continue to consume the protons until its equilibrium is reached.

 

[ebasappa]

^^ better answer, well put v5red

 

[MissionStanford]

V5Red - Everything you said makes sense, but I'll borrow a quote I found online:

"A salt consists of a cation and anion ionically bonded with each other. If the cation can form a strong base, and the anion can form a strong acid, then the salt is neutral. If the cation forms a strong base and the anion a weak acid, then it's basic. If the cation forms a weak base and the anion a strong acid, then it's acidic. If they both form weak acids/bases, then it depends on something I don't recall.
So for example, NaCl is neutral because NaOH is a strong base and HCl is a strong acid. NaCH3COO is basic because NaOH is strong and CH3COOH is weak. NH3Cl is acidic because NH3 is weak and HCl is strong."

That's the way I always learned how to determine if a salt's acidic, basic or neutral, which was my line of reasoning. Is that all wrong? Because all my chem teachers taught me this way, yet it seems to contrast with what you say, or maybe I'm not understanding properly.

 

[V5RED]

V5Red - Everything you said makes sense, but I'll borrow a quote I found online:

"A salt consists of a cation and anion ionically bonded with each other. If the cation can form a strong base, and the anion can form a strong acid, then the salt is neutral. If the cation forms a strong base and the anion a weak acid, then it's basic. If the cation forms a weak base and the anion a strong acid, then it's acidic. If they both form weak acids/bases, then it depends on something I don't recall.
So for example, NaCl is neutral because NaOH is a strong base and HCl is a strong acid. NaCH3COO is basic because NaOH is strong and CH3COOH is weak. NH3Cl is acidic because NH3 is weak and HCl is strong."

That's the way I always learned how to determine if a salt's acidic, basic or neutral, which was my line of reasoning. Is that all wrong? Because all my chem teachers taught me this way, yet it seems to contrast with what you say, or maybe I'm not understanding properly.

This quote is in line with the things I posted. It just says them in a way that is more about memorizing rules and less about understanding what is happening. One problem I have with teaching things that way is that it can lead to misunderstanding of the rules which is what happened with your reasoning.

 

[MissionStanford]

V5Red - I appreciate all the help. I think maybe I understand now how your reasoning explains that quote. If you don't mind, please correct me if I'm wrong.

If a salt's anion and cation can form a strong acid and base, then the solution is neutral. The reason is that when the salt dissociates in solution, neither the anion nor cation can form the acid or base anymore because they would form strong acids and bases, which would dissociate completely.

If the cation forms a strong base and the anion a weak acid, then it's basic. The reason is that when the salt dissociates, the anion will be in equilibrium with the weak acid and remove protons to form the weak acid, thus making the solution is basic.

If the cation forms a weak base and the anion a strong acid, then it's acidic. The reason is that when the salt dissociates, the cation will be in equilibrium with the weak base and remove hydroxide ions to form the weak base, making the solution more acidic.

If the anion and cation form weak acids and bases, then the solution can be acidic or basic depending on whether the acid or base would be weaker. For example, if the acid is weaker, then the equilibrium between the anion and weak acid will be more significant, so more acid will be created than base (by removing protons), and the solution will become more basic.

Is that right?

 

[V5RED]

V5Red - I appreciate all the help. I think maybe I understand now how your reasoning explains that quote. If you don't mind, please correct me if I'm wrong.

If a salt's anion and cation can form a strong acid and base, then the solution is neutral. The reason is that when the salt dissociates in solution, neither the anion nor cation can form the acid or base anymore because they would form strong acids and bases, which would dissociate completely.

If the cation forms a strong base and the anion a weak acid, then it's basic. The reason is that when the salt dissociates, the anion will be in equilibrium with the weak acid and remove protons to form the weak acid, thus making the solution is basic.

If the cation forms a weak base and the anion a strong acid, then it's acidic. The reason is that when the salt dissociates, the cation will be in equilibrium with the weak base and remove hydroxide ions to form the weak base, making the solution more acidic.

If the anion and cation form weak acids and bases, then the solution can be acidic or basic depending on whether the acid or base would be weaker. For example, if the acid is weaker, then the equilibrium between the anion and weak acid will be more significant, so more acid will be created than base (by removing protons), and the solution will become more basic.

Is that right?

That is mostly right. The exception is the part about the cation that can form a weak base. I don't know of any salts that would work that way. The only cationic weak acid I can think of right now is NH4+ or some ammonium-like weak acid like CH3NH3+ which would make the solution more acidic by dissociating into NH3 and H+ (or CH3NH2 and H+).