Titration errors

[jaydee]

Determining ammonia concentration of a cleaner by titrating against HCl of known concentration  can be affected by many errors. Explain why the concentration of ammonia is higher, lower or no change and explain
1) Water was added to the mark in the volumetric flask before addition of cleaner
2) Pipette used to deliver cleaner to the volumetric flask had an air bubble in it
3) Conical flask used in titration was rinsed with water
4) Burette was only rinsed with water before being filled with HCl used in titration
5) End point was overshot by one drop

And also, are these types of questions likely to show up on an exam? 

[HERculina]

This is what I got
1) not sure... but maybe no change?
2)the pipette would deliver less than the amount of ammonia required to the conical flask. sooo number of moles would decrease and so would concentration.
3) no change. the number of moles of ammonia cleaner wouldn't change (just more water)as c = n/v with known volume and unchanged moles, neither would the NH3 concentration.
4) higher concentration of ammonia. a larger titre from the burrete would be needed to neutralize the moles in the conical flask of ammonia. higher v = higher n. this n from titre is used to calculate the concentration of ammonia in conical flask (mole ratio). therefore the higher number of moles results in higher concentration found for ammonia.
5) End point (change of colour) occured with a larger titre. so i think the concentration in the flask would be higher. but not completely suree :/

[Studyinghard]

1) possibly higher because the actual volume is higher than you calculated

[Panicmode]

Determining ammonia concentration of a cleaner by titrating against HCl of known concentration  can be affected by many errors. Explain why the concentration of ammonia is higher, lower or no change and explain
1) Water was added to the mark in the volumetric flask before addition of cleaner
2) Pipette used to deliver cleaner to the volumetric flask had an air bubble in it
3) Conical flask used in titration was rinsed with water
4) Burette was only rinsed with water before being filled with HCl used in titration
5) End point was overshot by one drop

And also, are these types of questions likely to show up on an exam? 

1. No change. As the amount of cleaner being added is measured with a pipette, the amount of ammonia added to the flask will be the same regardless of whether or not it has water already in it.

2. Lower. An air-bubble means less cleaner (and therefore less ammonia) is used. This in turn leads to less HCl being used and so the concentration will be lower.

3. No change. See question 1.

4. Higher. The solution of HCl will be diluted by the excess water in the burette. Therefore, more will need to be used and so the calculated amount of ammonia will be higher.

5. Higher. More HCl is used and therefore a higher concentration is calculated. It should be noted however that titrations are usually repeated and only concordant titres used so the effect this error would have on results would be negligible.

[jaydee]

thanks guys! makes more sense now! Basically conical flask and volumetric flask will have no concentration change regardless and error using pipette will ALWAYS cause a LOWER concentration? Errors with burette will ALWAYS cause HIGHER concentration?

[Panicmode]

thanks guys! makes more sense now! Basically conical flask and volumetric flask will have no concentration change regardless and error using pipette will ALWAYS cause a LOWER concentration? Errors with burette will ALWAYS cause HIGHER concentration?

Not necessarily, I don't think it's safe to think in absolutes. A better way of thinking about it is in terms of how much ammonia is used.

If more ammonia than stated is used, it leads to a higher concentration being calculated.
If less ammonia than stated is used, it leads to a lower concentration being calculated.

There could be an error that causes more ammonia to be added than stated. This could be something like the pipette being filled past the mark.

Similarly, there are some errors with the burette that could cause a lower concentration than stated. For example, if the burette was not read correctly from the bottom of the meniscus, than it could lead to someone recording less HCl being used than actually was.

[Shenz0r]

I've got a prac question which I've tried to answer but I'm not completely sure if I'm right.

What would be the effect on the calculated concentration of the hydrochloric acid if the sidum carbonate had not been completely dried and still contained some water of crystallisation?

My answer: If the sodium carbonate still contained traces of water, the weighed mass of the compound will be higher than the actual mass of sodium carbonate. Therefore, the calculated number of mol would have been higher than expected and hence the the calculated concentration of sodium carbonate would also increase. During the titration, the number of mol of sodium carbonate in the aliquot will be lower than its calculated number of mol. This means that less volume of Hydrochloric acid will be required to neutralise the compound, resulting in a higher calculated concentration of Hydrochloric acid than expected.

My reasoning: If you use less volume to neutralise a solution then the concentration has to be greater right? It's like how a weak character has to hit multiple times to kill a boss whereas a more powerful character can finish them in just a few hits.

[HERculina]

Yep that should be right   you're a pretty good explainer. HCl's concentration is unknown but in the burette yea?

[Shenz0r]

Ahaha, cheers mate

Yea, the standard solution of sodium carbonate was put in the conical flask and the unknown solution of hydrochloric acid was put in the burette.