Chemistry Question

[USArmyDoc]

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I am doing EK 1001 GEN CHEM and I am up to question 807.

Which of the following salts is (are) basic?
I. NaNO3
II. NaCN
III. Na2CO3

The answer is II and III. The explanation is saying that cyanide is conjugate to hydrocyaynic acid, a weak acid, and is thus a weak base.

* I thought if an acid was strong it had a weak base and vice versa. Am I just getting something confused or is it a typo? Plus, I always thought CN- is a strong base.

 

[medstu2006]

I am doing EK 1001 GEN CHEM and I am up to question 807.

Which of the following salts is (are) basic?
I. NaNO3
II. NaCN
III. Na2CO3

The answer is II and III. The explanation is saying that cyanide is conjugate to hydrocyaynic acid, a weak acid, and is thus a weak base.

* I thought if an acid was strong it had a weak base and vice versa. Am I just getting something confused or is it a typo? Plus, I always thought CN- is a strong base.

Wouldn't NaNO3 be a base as well, since NO3- is a conjugatae base of HNO3 ?

 

[vmp200]

Wouldn't NaNO3 be a base as well, since NO3- is a conjugatae base of HNO3 ?

That's what I had thought as well.

 

[USArmyDoc]

I think what they are looking for are strong bases. The conjugate acid of NaNO3 is strong. Therefore, it would be a weak base. That is their rationale at least.

 

[toxin]

Hydrocyanic acid is a weak acid, what that means is that the equilibrium lies to the left (Weak acids do not completely dissociate). So if you have a salt, that dissociates into a conjugate base of a weak acid it will react with HOH to form the acid, therefore the solution will be slightly basic (Since the conjugate base will react to form the acid we will have OH- left in the solution which will make it basic.

am I making sense?!

 

[BigRedPremed]

I am doing EK 1001 GEN CHEM and I am up to question 807.

Which of the following salts is (are) basic?
I. NaNO3
II. NaCN
III. Na2CO3

The answer is II and III. The explanation is saying that cyanide is conjugate to hydrocyaynic acid, a weak acid, and is thus a weak base.

* I thought if an acid was strong it had a weak base and vice versa. Am I just getting something confused or is it a typo? Plus, I always thought CN- is a strong base.

The salt formed from the reaction between a strong base and a weak acid is basic. NaCN comes from the reaction between NaOH (strong base) and HCN (weak acid) so it is basic. Na2CO3 comes from the reaction between NaOH (strong base) and H2CO3 (weak acid) so it too is basic. NaNO3 comes from the reaction between a strong base (NaOH) and a strong acid (HNO3) so it is neutral.

 

[Twitch]

The easiest way to solve these types of problems is using a simple table.
Next to each salt listed draw a two column table.

Acid Base
--------------------
Salt 1 SA SB => Neutral
Salt 2 WA SB => Basic
Salt 3 SA WB => Acidic

Where
SA means it came from a Strong Acid
SB means it came from a Strong Base
WB means it came from a Weak Base
WA means it came from a Weak Acid

Once you get the hang of it you don't even need the table. You can verify this to be true by writing out the net ionic equations for each case. Consult your CHEM II text for further information / clarification. You can also post questions on the MCAT sub forum devoted for this purpose.

Thanks,
-Y_Marker