DAT Destroyer question 59 GC 2016

[Likkriue]

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Which of the following will not produce a buffered solution?

100 ml of 0.1M Na2CO3 and 50 ml of 0.1 M HBr
100 ml of 0.1M NaHCO3 and 25 ml of 0.2 M HCl
50 ml of 0.2 M Na2CO3 and 5 ml of 1.0 M
HCl
10 ml of 0.25 M NH3 and 20 ml of 0.25 M HCl
20 ml of 0.25 M NH4Cl and 10 ml of 0.25 M KOH

The answer key says it is D because the ammonia (weak base) is being titrated with a strong acid HCL. And the corresponding moles of strong acid is in excess making it not a buffer. However I don't think sodium carbonate or sodium hydrocarbonate are strong bases either since they don't disassociate completely in water as they aren't part of the strong base chart.

I'm terrible with multiplying small decimals and I'm trying to make sure I get the concept right. So as long as the moles of a weak base is bigger than the moles of strong acid it would be a buffer solution?

Also choice E is a salt and a strong base. Can someone explain how salts play into buffering solutions?

 

[sharkteethdentist]

Buffer should have weak acid or base and its salt. But if you see the choice D, you have high concentration of hydrochloric acid which is to be eliminated.

 

[DAT Destroyer]

Which of the following will not produce a buffered solution?

100 ml of 0.1M Na2CO3 and 50 ml of 0.1 M HBr
100 ml of 0.1M NaHCO3 and 25 ml of 0.2 M HCl
50 ml of 0.2 M Na2CO3 and 5 ml of 1.0 M
HCl
10 ml of 0.25 M NH3 and 20 ml of 0.25 M HCl
20 ml of 0.25 M NH4Cl and 10 ml of 0.25 M KOH

The answer key says it is D because the ammonia (weak base) is being titrated with a strong acid HCL. And the corresponding moles of strong acid is in excess making it not a buffer. However I don't think sodium carbonate or sodium hydrocarbonate are strong bases either since they don't disassociate completely in water as they aren't part of the strong base chart.

I'm terrible with multiplying small decimals and I'm trying to make sure I get the concept right. So as long as the moles of a weak base is bigger than the moles of strong acid it would be a buffer solution?

Also choice E is a salt and a strong base. Can someone explain how salts play into buffering solutions?

In this problem, choice D has more moles of strong acid than moles of weak base . This type of a solution cannot be a buffer.
The best buffer solution is when a weak acid is mixed with a weak base, ideally one to one concentration.
If strong acid is used in a buffer, then a weak base must be at least twice as much in concentration as a strong acid.

Hope this helps.

 

[Likkriue]

Yes that does help, thank you!