Volumetric Analysis (1 Viewer)

[Aerlinn]

Some things I would like to clear up:
Does the acid ALWAYS go in the burette, even when it's not the standard solution? I've come across the expection where HCl is standardised with the base in the burette because it's safer...
What are some examples of substances that should not be used as primary standards, and why? Curious

Oh, and a puzzling thing. Reading burettes, supposely, with colourless solutions you're supposed to take the reading that matches the bottom of the meniscus, but for coloured solutions, you're supposed to take the reading at the top of that curved shape sitting on the meniscus. Never figured out why...

 

[xiao1985]

acid always go to burette is really for a historic reason... ages ago (i forgot how long) base actually screw up the tap... so acid is safer to go into burette... technologies of burette these days won't let it happen again, but to be on the safe side, yes, acid in burette, always...

primary standards should not be hydroscopic... most strong acid (correct me if i am wrong) are hydroscopic, which is why usually you'd use something mild like NaHCO₃

beats me... never had to read burette of a colored solution before... perhaps the colour can disguise the miniscus... so you use a floating object so you won't be fooled?!

 

[Aerlinn]

Hmm... hydroscopic refers to? Never heard the term before, I dun think...

-Why is solid sodium hydroxide not used as a primary standard when standard solutions of bases are required?

-How would the accuracy of the standard solution be affected if the volumetric flask had been rinsed with de-ionised water before use and droplets of water were left in the flask when the sample of Na2CO3 is added? (the anhydrous sodium carbonate is used for the primary standard). This was from an experiment we did, where we used a funnel to transfer our sample of sodium carbonate into a volumetric flask, and used di-ionised water to wash traces of sodium carbonate in the weighing bottle and funnel into the flask.

When I first saw this question, my first reaction was, it would dilute the standard solution, affecting the accuracy, but then I thought... you add de-ionised water to the Na2CO3 up to that calibration mark regardless, does it really matter if there was any droplets of water there beforehand?? What do you think...?

-Another question goes: Calculate the amount of sodium carbonate, in mol, present in each conical flask containing sodium carbonate solution. We had four flasks, each with a 20ml aliquot of Na2CO3 (which was used as the standard solution). Wouldn't the no. mole, calculated by n=CV, be, er... identical for all 3 flasks, 'cause they all have the same volume?

 

[yoakim]

Another question goes: Calculate the amount of sodium carbonate, in mol, present in each conical flask containing sodium carbonate solution. We had four flasks, each with a 20ml aliquot of Na2CO3 (which was used as the standard solution). Wouldn't the no. mole, calculated by n=CV, be, er... identical for all 3 flasks, 'cause they all have the same volume?

That's what I thought. Volumetric analysis problems confuse me.

 

[airie]

-Why is solid sodium hydroxide not used as a primary standard when standard solutions of bases are required?

It's because NaOH absorbs moisture from the air quite readily, so its mass and concentration would change in the process of titration, which would affect the results.

-How would the accuracy of the standard solution be affected if the volumetric flask had been rinsed with de-ionised water before use and droplets of water were left in the flask when the sample of Na2CO3 is added?

Perhaps it means that leaving droplets of water in the flask defeats the purpose of rinsing? You're trying to wash out the impurities.

-Another question goes: Calculate the amount of sodium carbonate, in mol, present in each conical flask containing sodium carbonate solution. We had four flasks, each with a 20ml aliquot of Na2CO3 (which was used as the standard solution). Wouldn't the no. mole, calculated by n=CV, be, er... identical for all 3 flasks, 'cause they all have the same volume?

Not sure what you're asking there. Do you mean that all four flasks contain identical solutions (as in, same chemical composition, same volume, same concentration, same temperature)?

 

[Aerlinn]

Thanx for that

It's because NaOH absorbs moisture from the air quite readily, so its mass and concentration would change in the process of titration, which would affect the results.

Does the NaOH also react with CO2? Is it unstable? Just wondering.

Perhaps it means that leaving droplets of water in the flask defeats the purpose of rinsing? You're trying to wash out the impurities.

I was thinking how the answer should be that it doesn't affect it at all. Wondered where there was any point to the question.

For the last thing, I was asking, wouldn't the number of mole of Na2CO3 be identical for all 3 flasks, since they have the same volume, 20ml, and concentration? (accurately known 'cause it's acting as the standard solution)
Against, don't get whether there's any point to the question at all, unless they expect the answer for all 3 to be different, would might mean it's related to the 3 titres you get, but I'm pretty sure it's not...
*confuzzled*

 

[xiao1985]

@2nd post
hydroscopic = absorbs water from atmosphere very readily
naoh(s) will be hydroscopic

q2) doesn't affect it at all... (for eg) you added 10 g of na2co3 (you know this because you weighed it ACCURATELY on an electronic balance), and you added 50mL of water (you know this because you added enough water JUST to the graduation mark), anything else doesn't matter

q3) yes that's right


@3rd post

NaOH can react with CO2 i say... it forms NaHCO3... but it reacts with water in the atmosphere so much more readily...

 

[Aerlinn]

First bit:
Ah, thought so... pointless, tricking questions...

Second bit: hmm, might've made a good point