VCE Chemistry Question Thread

[cosine]

I have a few questions about volumetric analysis:

1). Why would there be different titres, some with 0.001mL and some with 0.1mL difference, what is causing this?
2). Why is the final percentage of concentration never the same as on the commercial packaging?
3). Can someone please list me some experimental errors that may have occurred during a titration/back titration?

For gavimetric analysis:

1). Why is the filtrate dried multiple times until a constant weight is weighed?

Thanks

[cosine]

I have a few questions about volumetric analysis:

1). Why would there be different titres, some with 0.001mL and some with 0.1mL difference, what is causing this?
2). Why is the final percentage of concentration never the same as on the commercial packaging?
3). Can someone please list me some experimental errors that may have occurred during a titration/back titration?

For gavimetric analysis:

1). Why is the filtrate dried multiple times until a constant weight is weighed?

Thanks

Anyone please?

[keltingmeith]

I have a few questions about volumetric analysis:

1). Why would there be different titres, some with 0.001mL and some with 0.1mL difference, what is causing this?

This could be for any number of reasons depending on the prac. For example - if the acid you're titrating against is a gas, it could be that some of the gas has escaped, and so you're actually titrating to a smaller amount of acid.

2). Why is the final percentage of concentration never the same as on the commercial packaging?

I wouldn't say never - but it also depends on what the concentration is of. In general, Vitamin C concentrations are reported much lower than they actually are, because once a company got sued for under-reporting, so now a lot of companies intentionally over-shoot the mark.

3). Can someone please list me some experimental errors that may have occurred during a titration/back titration?

Preparation of the acid/base for the titration may not have been done to completion (eg, burning Magnesium to form MgO, then reducing the MgO to MgOH, but only 50% of the original Magnesium reacted), see 1. Anything, really - it depends on the prac.

1). Why is the filtrate dried multiple times until a constant weight is weighed?

To ensure there's no more solvent (generally water) in the sample.

For most of these questions, there are no "be all and end all true dinks always works" solutions - it just comes down to really understanding the theory, and knowing your chemistry.

[Champ101]

Hey Guys,

Can someone please tell me whether this organic is defined as an acid? Yes, no and why.

Thanks 

[Macka598]

Hey Guys,

Can someone please tell me whether this organic is defined as an acid? Yes, no and why.

Thanks 

Its not an acid as it has no COOH functional group and hence not an acid.

[xeon88]

For a titration, how would the calculated result of the unknown substance be affected (under/over-estimated or same) if the unknown solution was in the burette (which was only rinsed with water)?

And how would the calculated result be affected (under/over-estimated or same) if the unknown solution was in the burette (appropriately rinsed) and the pipette was only rinsed with water?

Explanations would be appreciated! Thanks

[lzxnl]

Its not an acid as it has no COOH functional group and hence not an acid.

For the purposes of VCE, it's not an acid.

However, if you proceed on with chemistry, you'll find that this is considered acidic; compared to other organic compounds, alcohols are relatively acidic. There are lots of strong bases used in organic chemistry to deprotonate these acids, like LDA

For a titration, how would the calculated result of the unknown substance be affected (under/over-estimated or same) if the unknown solution was in the burette (which was only rinsed with water)?

And how would the calculated result be affected (under/over-estimated or same) if the unknown solution was in the burette (appropriately rinsed) and the pipette was only rinsed with water?

Explanations would be appreciated! Thanks

Burette solution rinsed with water -> actual burette solution concentration drops -> calculated concentration drops
Pipette rinsed with water -> actual aliquot has less analyte -> require lower volume of burette solution
HOWEVER calculations still use the original amount of pipetted solution -> lower volume of burette solution is compared to an unchanged number of moles of burette solution -> calculated concentration increases

[xeon88]

Burette solution rinsed with water -> actual burette solution concentration drops -> calculated concentration drops
Pipette rinsed with water -> actual aliquot has less analyte -> require lower volume of burette solution
HOWEVER calculations still use the original amount of pipette -> lower volume of burette solution is compared to an unchanged number of moles of burette solution -> calculated concentration increases

The effect on the calculated mole for the unknown substance would still be the same as before, because of the standard solution in the flask?

[lzxnl]

The effect on the calculated mole for the unknown substance would still be the same as before, because of the standard solution in the flask?

For which one? The effects on the calculated concentrations are opposites here

[knightrider]

How would you write the electronic configuration of ?

[lzxnl]

This could be for any number of reasons depending on the prac. For example - if the acid you're titrating against is a gas, it could be that some of the gas has escaped, and so you're actually titrating to a smaller amount of acid.

I wouldn't say never - but it also depends on what the concentration is of. In general, Vitamin C concentrations are reported much lower than they actually are, because once a company got sued for under-reporting, so now a lot of companies intentionally over-shoot the mark.

Preparation of the acid/base for the titration may not have been done to completion (eg, burning Magnesium to form MgO, then reducing the MgO to MgOH, but only 50% of the original Magnesium reacted), see 1. Anything, really - it depends on the prac.

To ensure there's no more solvent (generally water) in the sample.

For most of these questions, there are no "be all and end all true dinks always works" solutions - it just comes down to really understanding the theory, and knowing your chemistry.

MgO to Mg (OH) 2 isn't redox. It's acid-base

How would you write the electronic configuration of ?

From memory iron is s2 d6; after manganese which can be oxidised to +7 oxidation state
Anyway
The corresponding noble gas is Argon
You could call it [Ar] 4s2 3d6
Or 1s2 2s2 2p6 3s2 3p6 4s2 3d6 if you wanted

That's for elemental iron. Now remove two electrons. The 4s energy level is slightly lower in energy so rewrite the above without the two 4s electrons

[keltingmeith]

MgO to Mg (OH) 2 isn't redox. It's acid-base

Except I sent it to MgOH.

[lzxnl]

Except I sent it to MgOH.

Can you prove the existence of this species?

[keltingmeith]

Can you prove the existence of this species?

http://pubs.acs.org/doi/pdf/10.1021/ja00208a002

Here's an article with experimentation of MgOH, but I feel this is enough digressing.

[Callum@1373]

Can dispersion forces exist in polar molecules? And if so, would that include molecules with hydrogen bonding existing as well?