Homework questions thread

[Tonychet2]

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Which of these compounds have the highest boiling point?

• Butanol
• Butene
• Butane
• 1-Chlorobutane

highest butanol followed by chlorobutane then butane then butene

[Shenz0r]

Guys, this is from Insight '12, Q19 in MC. Why does ethanol vaporise earlier than ethyl propanoate? I'd have thought that the hydrogen bonds that ethanol forms would require much more energy to break than the dispersion forces holding ethyl propanoate together?

[charmanderp]

That'd be to do with the relative sizes of the molecules. Ethanol may have strong hydrogen bonds between molecules but the dispersion forces in ethyl propanoate are collectively quite strong, seeing as you have five carbon atoms in total. Additionally you have dipole-dipole interactions between ethyl propanoate molecules which are reasonably strong intermolecular forces, so the boiling point for the substance would be reasonably high.

[sahil26]

Boiling point isn't affected by intramolecular forces (or at least not to my knowledge). Butene actually has a lower boiling point than butane because the double bond interferes with the intermolecular forces (dispersion forces) between molecules, similarly to how methyl branching has the same affect, as the molecules can't pack as tightly together. Think about why unsaturated fats are liquids at room temperature, whilst saturated fats are solids?

Butanol would have the highest boiling point, as the hydrogen bonds between molecules are the strongest of the intermolecular forces present between any of those four molecules.

Thank you... Can you please also tell me the order of strength of the types of bonding?

[destain]

A particular barnd on fertiliser contains ammonium sulfate, (NH4)2 SO4, as its only source of nitrogen.
When you find out the number of mol of NH4+ that has reacted, to find out the mass of nitrogen in the fertiliser, why don't you multiply it by two?
what does the little 2 do in (NH4)2 SO4??

[charmanderp]

The 2 means that you have twice the amount of NH4+ as you do (NH4)2SO4.

So that's why you don't multiply, as you already know how much NH4+ is present from the titration or w/e. If you wanted to find out the amount of (NH4)2SO4, you'd divide the amount of NH4+ by two.

[destain]

omg yeah ok im fried thanks LOL

and this is in insight 2009, Section B Q1, and i think i saw something similar in 2011 vcaa

The questions has a part where it says 'approximately 20ml of 1M sulfuric acid is added to three separate 20.00ml samples of tis solution' (iron is in the samples)
and iron undergoes oxidation and when it asks to calculate the amount of Fe2+ in the sample, what do you do, what does the 20ml of 1M sulfuric acid do to the sample? so when you calculate concentration or something, is the volume 40ml? But the answers just do it as per normal, like there was no such step as adding sulfuric acid in

[charmanderp]

The sulfuric acid is added to acidify the mixture. Redox titrations (at least those which happen in the VCAA course) can only occur in an acidic environment, where the acid acts as a catalyst. Otherwise the reaction would take ages.

That's why you have H+ ions when you write balanced equations. The acid adds the H+ ions which are required for the reaction to take place.

[destain]

so in calculation do you ignore the 20ml of acid that has been added completely?

[charmanderp]

I believe so. It's a catalyst, which means it's not actually consumed in the reaction, so therefore has no effect on calculations (yield, etc).

[destain]

One sample of potato, of mass 0.45g, was dissolved in acid, filtered and then made up to 10.0ml. The concentration of Cd2+ (aq) in the solution was found to be 3.5ppb.

 Determine the concentration, in ppb, of cadmium in the sample of potato
Can you please explain how that works...I can't get my head around it

[Tonychet2]

if the concentration of the solution is 3.5 ppb then the concentration of the potato is also 3.5ppb

is this the answer? did u read the graph properly?

[destain]

Reading the graph was another question but the concentration was given in the question. and nope the asnwer is 77.78 ppb

[Tonychet2]

so is the answer 3.5ppb?

since they give u the concentration and it says its 3.5ppb then the potatos concentration should also be 3.5ppb

[destain]

apparently not LOL...but yeah the answer is 77.78ppb and that is the right answer because other people got it