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March 29, 2024, 08:21:53 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313706 times)  Share 

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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #420 on: March 02, 2014, 05:40:14 pm »
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AN unkown organic compound was analysed to determine its indentity. The compound contained only carbon, hydrogen and oxygen and did not contain any double carbon-carbon bonds. When completely oxideised in air, 8.20g of this compound produced 8.19g of water, 20.2g of carbon dioxide.

Determine the empirical formula of this compound

(Thanks for the help! - you don't need to write out all the working out if it takes too long! I just need the answer.. thanks :))

I think i'm just unsure about the hydrogen.. I found the mole to be 0.455mol and therefore the mass 0.910g.. is this correct?
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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #421 on: March 02, 2014, 05:47:45 pm »
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AN unkown organic compound was analysed to determine its indentity. The compound contained only carbon, hydrogen and oxygen and did not contain any double carbon-carbon bonds. When completely oxideised in air, 8.20g of this compound produced 8.19g of water, 20.2g of carbon dioxide.

Determine the empirical formula of this compound

(Thanks for the help! - you don't need to write out all the working out if it takes too long! I just need the answer.. thanks :))

I think i'm just unsure about the hydrogen.. I found the mole to be 0.455mol and therefore the mass 0.910g.. is this correct?
I got C4H2O3
H2O = 8.19g
n=m/M
n=0.455
n(H2O) x 2 = n(H)
n(H) = 0.91

hope this clears it up, if not i'll post my complete working out!
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #422 on: March 02, 2014, 05:58:06 pm »
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I'm skeptical solely based on the ridiculous number of double bonds that must be present in a molecule like that...

So, the number of moles of water gives the number of moles of hydrogen atoms in the organic compound. Similarly, the number of moles of carbon dioxide is the number of moles of carbon atoms in the original compound.

8.19 g of water has 8.19/18*2 moles of hydrogen atoms => 0.91 mol hydrogen, which is 0.91 g hydrogen (these are hydrogen atoms, not hydrogen molecules!). Why? 8.19/18 is the number of moles of water molecules and each water molecule has two hydrogens.

Number of moles of CO2 is then 20.2/44 mol = 0.459 moles of CO2 and hence carbon atoms. The mass of carbon atoms in the original compound is then 5.51 g. In total, we've accounted for 6.42 g of stuff in the organic compound. This leaves 8.20g - 6.42g = 1.78g of oxygen atoms, which is 1.78/16 = around 0.11 mol oxygen atoms.

Putting this altogether, we have 0.11 mol O, 0.91 mol H and 0.45 mol C. Ratio looks like 1 O to 4 C to 8 H to me i.e. C4H8O, which could be anything from a cyclobutanol (not very stable those things) to tetrahydrofuran (very stable solvent; five membered ring) or a ketone or even an aldehyde. None of these contain carbon-carbon double bonds.
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Jason12

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Re: VCE Chemistry Question Thread
« Reply #423 on: March 02, 2014, 10:08:07 pm »
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in a redox full equation I have 10H+ and 5H2O on the reactants side and 16H+ and 8H20 on the products side.
The answer gives 6H+, no H20 on reactants and 6H+ and 3H20 on products. Does this mean we are allowed to just cancel the h20 and H+ by subtracting the smaller from the larger? please explain
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mad_maxine

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Re: VCE Chemistry Question Thread
« Reply #424 on: March 02, 2014, 10:14:30 pm »
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in a redox full equation I have 10H+ and 5H2O on the reactants side and 16H+ and 8H20 on the products side.
The answer gives 6H+, no H20 on reactants and 6H+ and 3H20 on products. Does this mean we are allowed to just cancel the h20 and H+ by subtracting the smaller from the larger? please explain

Yep, you just subtract the smaller number of H+s and H2Os from the larger amounts, and whatever the difference is is your number.
So 6H+ on the products side, and 3H2O on the products side

 :)

Jason12

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Re: VCE Chemistry Question Thread
« Reply #425 on: March 02, 2014, 10:44:37 pm »
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Yep, you just subtract the smaller number of H+s and H2Os from the larger amounts, and whatever the difference is is your number.
So 6H+ on the products side, and 3H2O on the products side

 :)

thanks and also this question

Fe2O3(s) + 3CO(g) -> 2Fe(l) +3CO2 (g)

during this reaction the oxidation number of iron changes from

a. +3 to 0 and CO is the reductant
b. +6 to 0 and CO is the reductant
c. +3 to 0 and CO is the oxidant
d. +6 to 0 and CO is the oxidant

cant seem to figure out how CO oxidises or reduces
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Re: VCE Chemistry Question Thread
« Reply #426 on: March 02, 2014, 11:20:53 pm »
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Check the oxidation number of the carbon atom in CO and what happens to it when it becomes CO2
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swagsxcboi

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Re: VCE Chemistry Question Thread
« Reply #427 on: March 03, 2014, 09:46:58 pm »
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Anyone???
For the first question, use C1V1=C2V2
is the answer 240mL?
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Jason12

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Re: VCE Chemistry Question Thread
« Reply #428 on: March 03, 2014, 09:50:25 pm »
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For gravimetric analysis, back titration and redox titration, what are some common errors from these experiments and how does rinsing cause errors in these experiments?
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #429 on: March 04, 2014, 06:21:18 am »
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Hey guys I know this may be simple but it's so important for calculations and I don't completely understand the concept

Can someone please explain the basics of converting ppm to the desired unit?

Like I understand that ppm = mg/L but when changing the units, what do you do? If I wanted this value in grams, could I just multiply the mg amount by 1000?

Can ppm be anything as long as it is one unit to a million? Eg. so 1ppm can be 1g per 106g yet can also be 1ml per 106ml?

How do I convert ppm to M? If the value is in g per L, can I just multiply it by the M?

Sorry if some of that didn't make sense.. this is just how I've understood it :| Thanks for the help :-)
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #430 on: March 04, 2014, 08:43:05 am »
+1
Hey guys I know this may be simple but it's so important for calculations and I don't completely understand the concept

Can someone please explain the basics of converting ppm to the desired unit?

Like I understand that ppm = mg/L but when changing the units, what do you do? If I wanted this value in grams, could I just multiply the mg amount by 1000?

Can ppm be anything as long as it is one unit to a million? Eg. so 1ppm can be 1g per 106g yet can also be 1ml per 106ml?

How do I convert ppm to M? If the value is in g per L, can I just multiply it by the M?

Sorry if some of that didn't make sense.. this is just how I've understood it :| Thanks for the help :-)

Ppm just means one part in a million. As you've said, it can mean 1 gram in a million grams, which is its common meaning. If you have mg/L, you would actually divide by 1000. You always less grams than mg.

Mg/l is only ppm because we take the mass of 1 ml of water to be 1 g. Thus, 1g/million grams = 1g/1000 L = 1 mg/L

To go from mg/L to M, you would convert to g/L then divide by the molar mass in g/mol. Try dividing the units g/L by g/mol. Yes you can do this to check your units.
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Re: VCE Chemistry Question Thread
« Reply #431 on: March 04, 2014, 10:24:52 am »
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Yo team I'm taking an intro Chem subject at uni but since not many people take it I think it's best to ask in the VCE forums.

Homogenous vs heterogenous mixtures, is the different literally defined by what you can see with your own eyes?  That seems like a pretty vague line to me but everything I read seems to define the difference as being whether or not you can 'see' the individual components of the mixture. 
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #432 on: March 04, 2014, 10:00:00 pm »
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In a redox reaction, can the hydrogens in water cancel out hydrogen ions?
So I have Iron3 oxide that converts into metallic iron
the half equation would like something like this:
Fe2O3 + 6H+ + 6e- -------> 2Fe + 3H2O              (ceebs with states)
Could Hydrogens cancel out to:
Fe2O3 + 6e- -------> 2Fe + 3O2-

Has that always been the case? Is it necessary? Need some confirmation

thanks.
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Re: VCE Chemistry Question Thread
« Reply #433 on: March 04, 2014, 10:07:45 pm »
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In a redox reaction, can the hydrogens in water cancel out hydrogen ions?
So I have Iron3 oxide that converts into metallic iron
the half equation would like something like this:
Fe2O3 + 6H+ + 6e- -------> 2Fe + 3H2O              (ceebs with states)
Could Hydrogens cancel out to:
Fe2O3 + 6e- -------> 2Fe + 3O2-

Has that always been the case? Is it necessary? Need some confirmation

thanks.



Ok, so the easiest thing to do is actually break it down to the ionic equation.

The iron ion in iron (III) oxide is Fe3+(aq).

We know that this is converted to metallic iron which is Fe(s) .

Fe3+(aq) + 3e ----> Fe(s)

Answering your question, no, the hydrogen atoms in water don't cancel out H+ ions.             


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Re: VCE Chemistry Question Thread
« Reply #434 on: March 04, 2014, 10:35:24 pm »
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Chapter 5, Q22 a) of the heinamann textbook.
It seems like that's what they've done.
Anyone else confirm that I can cancel the Hydrogens like this?

Cheers
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