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March 29, 2024, 05:21:25 am

Author Topic: VCE Chemistry Question Thread  (Read 2313325 times)  Share 

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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #240 on: January 27, 2014, 10:28:12 am »
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Just wondering, what happened to the 8H+ on the left and the 4H+ on the right? Did they cancel out leaving only 4H+ on the left for the final balanced equation? o-o

Correct; the H+ ions cancel out, as well as waters and electrons.

T-Infinite

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Re: VCE Chemistry Question Thread
« Reply #241 on: January 27, 2014, 10:32:13 am »
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Correct; the H+ ions cancel out, as well as waters and electrons.
By the way, in the final equation (iii) , the textbook says there is only 1MnO2 ? I'm so confused, in the half equation theres 2MnO2 but in full equation theres only 1?
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #242 on: January 27, 2014, 01:30:15 pm »
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In gravimetric analysis, what's the point of rinsing the precipitate containing the substance being analysed with small amounts of solvent?
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clıppy

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Re: VCE Chemistry Question Thread
« Reply #243 on: January 27, 2014, 01:48:41 pm »
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Let's say that I'm precipitating PbSO4 from an FeSO4 solution. I've precipitated the PbSO4 and I've taken it out of the solution. To make sure there is no extra weight that isn't PbSO4, I'll rinse my precipitate with water several times to wash out everything that I don't want and then I'll dry my precipitate in an oven several times to make sure that all the water evaporates.
It's basically to get rid of anything that isn't the precipitate that could alter your final measured weight
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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #244 on: January 27, 2014, 10:26:00 pm »
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How do I work out how many Cl- ions there are in 13.4g of Nickel Chloride (NiCl2)?

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thushan

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Re: VCE Chemistry Question Thread
« Reply #245 on: January 27, 2014, 10:36:06 pm »
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You're given a mass of NiCl2. The question asks you to count the number of Cl- ions. Since you need to count an amount, yet you're given a mass, the first thing to do is to determine the amount, in mol, of NiCl2.

Once you've got that, you know that for every NiCl2 formula unit you have, you have 2 Cl- ions. Use this fact to determine the amount of Cl- ions, given you know the amount of NiCl2.
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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #246 on: January 27, 2014, 10:48:17 pm »
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You're given a mass of NiCl2. The question asks you to count the number of Cl- ions. Since you need to count an amount, yet you're given a mass, the first thing to do is to determine the amount, in mol, of NiCl2.

Once you've got that, you know that for every NiCl2 formula unit you have, you have 2 Cl- ions. Use this fact to determine the amount of Cl- ions, given you know the amount of NiCl2.

Cheers. Worked it out. Trying to get back into Chemistry is tough.
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TimewaveZero

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Re: VCE Chemistry Question Thread
« Reply #247 on: January 29, 2014, 07:30:00 pm »
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I've probably made a silly mistake here but I just can't see it haha

Serotonin (C10H12N2O molecular mass = 176g mol -1) is a compound that conducts nerve impulses in the brain and muscles. A sample of spinal fluid from a volunteer in a study was found to contain a serotonin concentration of 1.5 ng L-1 (1.5 nano grams per litre)
How many molecules of of serotonin are there in one millilitre of the fluid?

my calculations:

1 millilitre = 0.001 L
=> 1.5 * 0.001 = 0.0015 nanograms in 1 millilitre

= 0.0000000015 grams

n(c10h12n2o) = 0.0000000015 / 176

= 8.52 * 10^-12

n of molecules should equal (8.52 * 10^-12) * (6.02 * 10^23) = 5.13 * 10^12

the MC answers are:

A. 5.13 * 10^9
B. 9.03 * 10^11
C. 5.13 * 10^27
D. 9.03 * 10^29

Why is A) to the power of 9? Did I do something wrong with the dividing?
« Last Edit: January 29, 2014, 07:34:36 pm by TimewaveZero »
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #248 on: January 29, 2014, 07:48:48 pm »
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I've probably made a silly mistake here but I just can't see it haha

Serotonin (C10H12N2O molecular mass = 176g mol -1) is a compound that conducts nerve impulses in the brain and muscles. A sample of spinal fluid from a volunteer in a study was found to contain a serotonin concentration of 1.5 ng L-1 (1.5 nano grams per litre)
How many molecules of of serotonin are there in one millilitre of the fluid?

my calculations:

1 millilitre = 0.001 L
=> 1.5 * 0.001 = 0.0015 nanograms in 1 millilitre

= 0.0000000015 grams

n(c10h12n2o) = 0.0000000015 / 176

= 8.52 * 10^-12

n of molecules should equal (8.52 * 10^-12) * (6.02 * 10^23) = 5.13 * 10^12

the MC answers are:

A. 5.13 * 10^9
B. 9.03 * 10^11
C. 5.13 * 10^27
D. 9.03 * 10^29

Why is A) to the power of 9? Did I do something wrong with the dividing?
I think you got that bold step wrong, 0.0015 nanogram = 0.0015 x 10 ^-9 g= 1.5 x 10^-12.
Hence, if you fix that number and sub it into the other steps, everything would be fine!
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TimewaveZero

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Re: VCE Chemistry Question Thread
« Reply #249 on: January 29, 2014, 07:57:06 pm »
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I think you got that bold step wrong, 0.0015 nanogram = 0.0015 x 10 ^-9 g= 1.5 x 10^-12.
Hence, if you fix that number and sub it into the other steps, everything would be fine!

Ah yes, that is right. Although, I subbed the new value in, and am now presented with 5.13 * 10^24. ???? hahahaha
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darklight

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Re: VCE Chemistry Question Thread
« Reply #250 on: January 29, 2014, 07:59:53 pm »
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Ah yes, that is right. Although, I subbed the new value in, and am now presented with 5.13 * 10^24. ???? hahahaha

I just did it with that value and it did come to A, so maybe try again.

Also: why does one need a H+ catalyst when reacting an alkene with water? Thanks!
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TimewaveZero

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Re: VCE Chemistry Question Thread
« Reply #251 on: January 29, 2014, 08:05:35 pm »
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Ahh yes I did too. I mistakingly left out the power of -15 in the final calculation. My error.
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jgoudie

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Re: VCE Chemistry Question Thread
« Reply #252 on: January 29, 2014, 08:17:33 pm »
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VCE don't really go into the reasons for the catalyst, only which one is needed for each reaction.  Have a look at this page for the reason for the H+ in addition reaction.  From what i can gather it has the ability to break the double bond thus allow the reaction to proceed.  H+ is left over from the breaking of water into the OH- function group attached to the hydrocarbon.  So it is a catalyst in that it is present at the start and finish of the reaction, however it is actually involved in the reaction.

Hope this clears it up a little.

http://chemistry2.csudh.edu/rpendarvis/AlkeneAddn.html#elect


I just did it with that value and it did come to A, so maybe try again.

Also: why does one need a H+ catalyst when reacting an alkene with water? Thanks!
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Re: VCE Chemistry Question Thread
« Reply #253 on: January 29, 2014, 11:33:34 pm »
+1
VCE don't really go into the reasons for the catalyst, only which one is needed for each reaction.  Have a look at this page for the reason for the H+ in addition reaction.  From what i can gather it has the ability to break the double bond thus allow the reaction to proceed.  H+ is left over from the breaking of water into the OH- function group attached to the hydrocarbon.  So it is a catalyst in that it is present at the start and finish of the reaction, however it is actually involved in the reaction.

Hope this clears it up a little.

http://chemistry2.csudh.edu/rpendarvis/AlkeneAddn.html#elect

Here's my take on it. I haven't read the link so I may well be repeating what the link says.
In an alkene, the carbon-carbon double bond contains a rather large amount of electrons (relative to other bonds). Therefore, anything with a positive charge would be attracted to this double bond. The H+ ion, a proton, is well suited to attacking the double bond as it is small. The proton then drags some electron density from the double bond, forming a carbon-hydrogen single bond. This creates a positive charge on the other carbon. Now, as this carbon has a positive charge, it attracts the negative oxygen in water much more readily and the reaction proceeds faster.

In contrast, without the acid catalyst, you're trying to react a water molecule with an alkene. Now, the double bond has lots of electrons, which repels the water molecule. This increases the activation energy of the reaction and thus slows it down.
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #254 on: January 31, 2014, 07:54:42 am »
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Can someone please clarify that i've understood this correctly:

For the equation:
Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g),
we know that n(Mg)=0.0551 and n(HCl)=0.0600.

Since the Mg is in excess, the amount reacting would follow the mole ratio. This means, 0.03mol of Mg would react with 0.0600 mol of HCl. This would produce 0.09mol of MgCl2 and H2 gas.
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