HNO3 & CH3COONa: A Buffer Solution That Doesn't Follow the Rules

[zorro]

I read somewhere that HNO₃ and CH₃COONa can act as a buffer solution. I know that the following solutions can act as a buffer-

1) A solution of weak acid and its salt with a strong base.
2) A solution of strong base and its salt with a strong acid.
3) A solution of a salt of weak acid and a weak base.

The above two compounds don't fit in any of these criteria.
How can they act as a buffer?

 

[espen180]

Try to think about it. What compounds/ions are in solution? What are the equilibrium reactions?

 

[zorro]

HNO₃ + CH₃COONa -------> NaNO₃ + CH₃COOH

Here we have a mixture of salt of a strong acid and strong base and a weak acid on the product side.
So do you mean CH₃COOH and CH₃COONa (at equilibrium) are responsible for buffer here? What about NaNO₃ and HNO₃? Do they play any role? Give reason for your answer.

 

[Borek]

1) A solution of weak acid and its salt with a strong base.

Agreed - that will be mixture of acetic acid and sodium acetate.

2) A solution of strong base and its salt with a strong acid.

Like mixture of NaON and NaCl? It is not a classic buffer. Relatively concentrated solutions of strong bases or strong acids do resist pH changes, so in a way they are similar to buffers, but they don't require presence of salts.

3) A solution of a salt of weak acid and a weak base.

Like solution of ammonium acetate? Buffering capacity close to zero, much lower than in both earlier cases.

How can they act as a buffer?

What happens if you mix - say - 2 moles of sodium acetate, with 1 mole of nitric acid?

Edit: I see you were on the right track already. Think about composition of the solution after the reaction. Does it contain nitric acid?

 

[zorro]

What happens if you mix - say - 2 moles of sodium acetate, with 1 mole of nitric acid?

Edit: I see you were on the right track already. Think about composition of the solution after the reaction. Does it contain nitric acid?

HNO₃ would be completely consumed and 1 mole of sodium acetate would remain as a reactant.
1 mole each of NaNO₃ and CH₃COOH are present in product side.

So here CH₃COOH and CH₃COONa form a buffer solution.
But we still have 1 mole of NaNO₃. Will it not affect the buffer?

 

[espen180]

What kind of acid is nitric acid? Will this kind of acid make a buffer?

 

[Borek]

But we still have 1 mole of NaNO₃. Will it not affect the buffer?

No, it just changes ionic strength of the solution.

 

[zorro]

Thanks Mr. PH