Homework questions thread

[soccerboi]

Oh i see, thanks

[ggxoxo]

Guys; I've got a couple of qns about 'fractionating towers'

1- Does the tower have different temperatures along its length- that is, it has different temperatures along it simulaneously? OR do the mechanics set the tower temp. at, let's say, <40 degrees Celsius and collect the refinery gas and then set the temperature at 40-70 and then collect gasoline and, hence, set the different temperature in turn?

2- What does Heinemann mean when they say: 'The vapours over a mixture of liquids will have a higher proportion of the more volatile (low boiling point) component than is in the liquid'?

And let me digress to percentage yield (sorry I have lots of qns!):

Calculate the percentage yield if 5.0 g of ethanol is oxidised to produce 4.8 g of ethanoic acid. The answer is apparently 92% but I don't understand how Heinemann got that answer.

Also is this compound even possible: (CH3)2CHOH; I can't seem to get my head around (CH3)2 part...

Thank you in advance!!!

[Panicmode]

1. The fractionating towers have 'plates' that are kept at certain temperatures and levels withing the tower. When the gas hits these plates, it will either condense into its faction or pass through and rise further up the tower if it has a lower boiling temp.


2. More volatile = lower boiling point. The vapours on top of a liquid mixture of hydrocarbons will contain a higher proportion of the lower factioned hydrocarbons (because these have lower boiling points) than the liquid will.

3. As you continue to heat the mixture of hydrocarbons, more and more of the volatile compounds boil and become part of the vapour mixture. Therefore, the concentration of the volatile (low boiling point) hydrocarbons increases in the vapour and simultaneously decreases in the liquid.

Will answer the other questions when I get a chance to use the scanner

[ggxoxo]

^thank you so much!!!

[Panicmode]

I tried the percentage yield question but I ended up with a weird answer, I'll try again later.

As for the question regarding (CH3)2CHOH, see attached scan .

[Kaille]

hello

simple as question...

Q. 10.0 ml of a 0.0250 M nitric acid solution was added to a 250 ml Standard flask, and the volume was made up to 250.0 mL with distilled water.

The pH of the resultant solution would be?

thanks

[b^3]

n(HNO₃)=cv=0.0100*0.0250
=2.50*10^-4 mol

n(H⁺=n(HNO₃)
=2.50*10^-4 mol
[H⁺ in final sol]=
pH=-log₁₀([H⁺])=-log₁₀(0.001)
=3

[Panicmode]

See attached scan

[nbhindi]

hello

simple as question...

Q. 10.0 ml of a 0.0250 M nitric acid solution was added to a 250 ml Standard flask, and the volume was made up to 250.0 mL with distilled water.

The pH of the resultant solution would be?

thanks

1. Use c₁.v₁ = c₂.v₂ and solve for c₂
2. C(HNO₃) = c(H⁺)
3. Then use pH = -log₁₀(H⁺)

[ggxoxo]

I tried the percentage yield question but I ended up with a weird answer, I'll try again later.

As for the question regarding (CH3)2CHOH, see attached scan .

This is how Heinemann did it:

Calculate the percentage yield if 5.0 g of ethanol is oxidised to produce 4.8 g of ethanoic acid.

          Cr2O72–, H+
CH3CH2OH --> CH3COOH
Theoretical yield (100% conversion of ethanol)
n(CH3COOH)   = n(CH3CH2OH)
         =  5/46 mol
m(CH3COOH)   =  5/46 × 48 g = 5.2 g
% yield   =  actual mass/theoretical mass × 100%
=  4.8/5.2 × 100
         = 92%
 What I don't get though are the steps after and including: 5/46 x 48g (where did the 48 g come from?)!

[Panicmode]

I tried the percentage yield question but I ended up with a weird answer, I'll try again later.

As for the question regarding (CH3)2CHOH, see attached scan .

This is how Heinemann did it:

Calculate the percentage yield if 5.0 g of ethanol is oxidised to produce 4.8 g of ethanoic acid.

          Cr2O72–, H+
CH3CH2OH --> CH3COOH
Theoretical yield (100% conversion of ethanol)
n(CH3COOH)   = n(CH3CH2OH)
         =  5/46 mol
m(CH3COOH)   =  5/46 × 48 g = 5.2 g
% yield   =  actual mass/theoretical mass × 100%
=  4.8/5.2 × 100
         = 92%
 What I don't get though are the steps after and including: 5/46 x 48g (where did the 48 g come from?)!

I converted both amounts to mols then worked directly from there. I think the book has made a mistake in that you shouldn't be multiplying it by 48 but rather 60.05. If you do that, you do end up with the same answer as mine... I'll scan mine and upload it, just gimme a sec.   

[Aurelian]

Could anybody be kind enough to explain to me whether increasing atomic mass increases or decreases the energy required to stretch a bond (e.g. HCl more or less energy than HBr), and also why?

THIS QUESTION RRRRAAAAAGGGGEEEEEEEE

[Aurelian]

THIS QUESTION RRRRAAAAAGGGGEEEEEEEE

Uh oh. What did I do

If you want to find the answer, go to your Heinmann textbook and search with a goddamn magnifying glass for one single extremely obscure sentence hidden on a page surrounded by otherwise useless information. You will find the answer nowhere else. ONE SENTENCE. And then say "fuck you VCAA" for asking students that on last year's midyear.

*eye twitches*

[thushan]

Could anybody be kind enough to explain to me whether increasing atomic mass increases or decreases the energy required to stretch a bond (e.g. HCl more or less energy than HBr), and also why?
So far the only thing I've been able to come up with is the size of the molecules in relation to their charge (like in hydrogen bonding) but the internet isn't giving me any answers.

Much appreciated

Reason for the relationship between mass and energy required to stretch a bond is based on quantum theory, from memory.

[Aurelian]

Reason for the relationship between mass and energy required to stretch a bond is based on quantum theory, from memory.

huh?

Lol. Just ignore that, you don't need to worry about it