/0's Chem Questions

[/0]

Thanks! Any help with the second question?

[ryley]

I attached my working for that question, don't have their solutions with me at the moment. I wasn't sure about this one either, could someone please check my working?

[/0]

Thanks ryley!

The solutions were too abrupt so I didn't really understand them, but now I do

[/0]

This one is from the VCAA 2008 Unit 3 Exam.

Question 1 d)

In the solutions it says "Marks were awarded for drawing a graph that levelled off when the V(BaCl2) added was slightly less than 45 mL, and the m(BaSO4) was 2.00 g."

Why does this need to be slightly less?

[ryley]

If you look at the original graph they provided, it levels off slightly, but noticeably, before 90mL, so they must have expected people to take this into account when drawing their graphs.

[/0]

thanks ryley

also, in the report it said has a polar end that bonds with water and a non-polar end that bonds with petrol. Is this 'bond' just dispersion forces? Also for petrol do we just assume ?

[lacoste]

For petrol I'd assume its CxHy but you could say octane: C8 H18

[Mao]

thanks ryley

also, in the report it said has a polar end that bonds with water and a non-polar end that bonds with petrol. Is this 'bond' just dispersion forces? Also for petrol do we just assume ?

It's a bit incorrect to say the non-polar end 'bonds' with the petrol.

Polar molecules have dipoles, and the molecules have intermolecular attraction. When immersed in a non-polar solvent, the attraction between polar molecules is much greater than attraction between polar and non-polar (i.e. the polar would much rather be around other polar molecules because attraction is greater). The result is the two are immiscible.

For ethanol however, it has the polar hydroxy, and a non-polar ethyl. In non-polar solvents, the attraction between ethanol molecules isn't strong enough to maintain their proximity, hence the molecules are dispersed in the solvent. the two are miscible.

In polar solvents, polar liquids are miscible because attraction between the solvent/solute is comparable to attraction between solvent/solvent. Hence there is no particular 'preference' for the solvent, and no separation occurs.

Non-polar liquids are not soluble in polar solvents because the attraction between solvent/solvent is greater than solvent/solute.

[/0]

thanks :D:D::D

[/0]

Correct prodcedures for a titration include which of the following?
I. Draining a pipette by touching the tip to the side of the conical flask used for the titration.
II. Rinsing the burette with distilled water just before filling it with the liquid to be used in the titration.
III. Swirling the solution in the conical flask frequently during the titration.

[dekoyl]

I, III
Is that correct?

[Toothpaste]

Correct prodcedures for a titration include which of the following?
I. Draining a pipette by touching the tip to the side of the conical flask used for the titration.
II. Rinsing the burette with distilled water just before filling it with the liquid to be used in the titration.
III. Swirling the solution in the conical flask frequently during the titration.

I and III?

[/0]

Yep, but I've never heard of I before, why do you need to touch the tip to the side?

[dekoyl]

I can't go too deep.. but it's about surface tension. The bit of solution in the pipette will have a stronger "attraction" with the silica(glass?) of the conical flask and hence get "sucked" out of the pipette. The "___" words are not accurate but I hope you get the point.

[Toothpaste]

Yep, but I've never heard of I before, why do you need to touch the tip to the side?

The pipette is calibrated to dispense a certain amount of substance that way. That extra bit of stuff on the pipette's end is part of the measurement.

EDIT: ^ read that (post above), lol. I think I just stated the obvious.


EDIT EDIT: Wikipedia says
When dispensing the pipette should be held at a 45 degree angle, and the tip placed against the side of the receiving vessel. Glass vessels are preferred; the surface tension of the glass provides additional torsion that results in complete evacuation of the tip.
http://en.wikipedia.org/wiki/Pipette