general chem. salts.

[BrownieDDD]

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WHICH SALT WILL FORM A SOLUTION WITH A pH LESS THAN 7?

A. KBr
B. NH4NO3
c. NaF
d. KCN
e. Na2CO3

it says remove the spectator ions first which includes K,Br,NO3, F, Na ....how do we know that they are spectator ions?

This questions has been asked many times on sdn, but could someone show me a logical explanation and methodic way to do this problem??

for example
KBr in water--> K+ and Br in water---> KOH and HBr?

im so lost.

 

[Mr. Thirsty]

WHICH SALT WILL FORM A SOLUTION WITH A pH LESS THAN 7?

A. KBr
B. NH4NO3
c. NaF
d. KCN
e. Na2CO3

it says remove the spectator ions first which includes K,Br,NO3, F, Na ....how do we know that they are spectator ions?

This questions has been asked many times on sdn, but could someone show me a logical explanation and methodic way to do this problem??

for example
KBr in water--> K+ and Br in water---> KOH and HBr?

im so lost.

The best way to approach these problems is to draw the ions of each if they were to dissociate. (spectator ions are usually just typical salt cations i.e. Na or K)
a.) KBr -> K+ + Br(-)
b.) NH4NO3 -> NH4+ + NO3(-)
c.) NaF -> Na+ + F(-)
d.) KCN -> K+ + CN(-)
e.) Na2CO3 - > 2Na+ + CO3(2-)

Ok now that we can see all of the ions we need to see what the question is asking. In order to form a solution with a pH of less than 7 (High proton concentration / acidic) the ions at hand must be somewhat acidic.

a.) Br(-) -> not going to be acidic because it is the conjugate base of a STRONG ACID (HBr) which are always negligible!
b.) NO3(-) -> Same as the Br(-) situation.
NH4+ -> acidic! Could be the ion that makes a solution with a ph <7
c.) F -> F(-) is the conjugate base of a weak acid. ph >7
d.) CN -> CN(-) is a weak base. (Ph > 7)
e.) CO3 -> CO3 (2-) is a weak base. (Ph > 7)

So as you can see, NH4+ is the only molecule that could make this solution acidic, being that it is the only acid present upon dissociation (even though it is weak, that still means ph < 7!)

Hope that helps.

EDIT: Sorry I guess I really did not answer your question.

 

[SmilingTooth]

All Group 1 and Group 2 metal (cations) as well as anions such as Cl- Br- I- No3- ClO4- and ClO3- are spectator or neutral. This is because Group 1 and Group 2 forms very strong acids, whereas the previously mentioned anions form very strong bases. And therefore, the conjugate acids and bases they form are very weak that they can be considered spectator.

Just memorize all of the 7 strong acids and 9 strong bases, which helps me to spot the spectators!

 

[BrownieDDD]

As in, why is Na+ unreactive with water (making it a spectator ion)? Is it because if Na+ were to react with water yielding NaOH, the NaOH (since it is a strong base) would dissociate completely 100%, shifting the equilibrium in favor of Na+ and H20?

 

[SmilingTooth]

Hi BrownieDDD, for the sake of this question, you are assuming that the salts will be dissolved in water. There are other solutions that doesn't have water as solvent. For these type of questions, just compare the salts among themselves.