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June 25, 2019, 03:18:42 am

Author Topic: Chemistry Question Thread  (Read 471315 times)  Share 

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sweetcheeks

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Re: Chemistry Question Thread
« Reply #3705 on: February 07, 2019, 06:45:40 pm »
+2
in the answers the answer for the first one is (NH4)3.PO4
but is there not meant to be a water molecule if it is acid base

Not all acid-base reactions result in the formation of water. Ammonia, NH3, has a lone pair of electrons which is able to accept a proton from a donor acid, forming the ammonium cation, NH4+. Many negatively charged compounds (e.g. carboxylate anions) can react with acids, without there being the formation of water.

Hello,
I was wondering what to do for Q6 and 2.1 a). Can anyone please help me out with these questions? Thanks :)

For question 6, you are given the moles of the substance and the mass of the substance. What other information can gain? (hint, think Mw)

For 2.1, what are your difficulties? You are on the right track, I suggest drawing out the structure of the molecule and looking at where the electrons are located.
« Last Edit: February 07, 2019, 06:48:45 pm by sweetcheeks »

david.wang28

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Re: Chemistry Question Thread
« Reply #3706 on: February 07, 2019, 07:33:35 pm »
0
Not all acid-base reactions result in the formation of water. Ammonia, NH3, has a lone pair of electrons which is able to accept a proton from a donor acid, forming the ammonium cation, NH4+. Many negatively charged compounds (e.g. carboxylate anions) can react with acids, without there being the formation of water.

For question 6, you are given the moles of the substance and the mass of the substance. What other information can gain? (hint, think Mw)

For 2.1, what are your difficulties? You are on the right track, I suggest drawing out the structure of the molecule and looking at where the electrons are located.
I know how to draw the structure of the molecule, it's just the fact that I don't know where the electrons are located.
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keltingmeith

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Re: Chemistry Question Thread
« Reply #3707 on: February 07, 2019, 08:29:46 pm »
+2
I know how to draw the structure of the molecule, it's just the fact that I don't know where the electrons are located.

So think about fact that carbon needs four bonds, hydrogen can only have one bond, and go from there. Where can you add electrons to make this possible?
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david.wang28

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Re: Chemistry Question Thread
« Reply #3708 on: February 07, 2019, 09:12:42 pm »
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So think about fact that carbon needs four bonds, hydrogen can only have one bond, and go from there. Where can you add electrons to make this possible?
Ahhh, I get it now. Thanks for the help guys! :)
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david.wang28

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Re: Chemistry Question Thread
« Reply #3709 on: February 08, 2019, 04:41:00 pm »
0
Hello,
I have more problems on chemistry in the links below. Can anyone help me with 2.2 b), al of 2.5, and 2.6 b) please? Thanks :)
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Bri MT

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Re: Chemistry Question Thread
« Reply #3710 on: February 08, 2019, 08:13:20 pm »
+2
Hello,
I have more problems on chemistry in the links below. Can anyone help me with 2.2 b), al of 2.5, and 2.6 b) please? Thanks :)

Remember m/M = n
You can find M using the periodic table, so if you have m you can then find n.
Then, using the equation you wrote out in a, you can use n of each reactant to find m for each reactant


Remember that you don't have to stick to single bonds between the carbons :)


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david.wang28

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Re: Chemistry Question Thread
« Reply #3711 on: February 08, 2019, 08:18:47 pm »
0
Remember m/M = n
You can find M using the periodic table, so if you have m you can then find n.
Then, using the equation you wrote out in a, you can use n of each reactant to find m for each reactant


Remember that you don't have to stick to single bonds between the carbons :)
Can you help me with the other parts please?
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Re: Chemistry Question Thread
« Reply #3712 on: February 09, 2019, 11:25:08 am »
+1
Can you help me with the other parts please?

Sorry! Completely missed the diagram based questions

I've got notes in the VCE chem section that might be easier to understand because they use diagrams but I'll try to explain here. 

First you need to identify the longest chain, eg where you've identified an ethyl functional group that should actually be part of the parent chain.  This means that there are 5 carbons  (pent) in the parent chain and a methyl group on the 3rd carbon

When naming a cyclohexane, you start at the highest priority functional group and number around the ring in the direction that gives the lowest numbers for the functional groups. 

For the bottom right diagram include the double bond in the parent chain & remember to indicate the number with the -ene

Have a shot at it and let us know if you want more help :)
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david.wang28

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Re: Chemistry Question Thread
« Reply #3713 on: February 15, 2019, 04:28:34 pm »
0
Hello,
I'm stuck on a chem question (Q6), and this is my working out below. Can anyone please help me with that question? Thanks :)
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fun_jirachi

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Re: Chemistry Question Thread
« Reply #3714 on: February 15, 2019, 05:10:28 pm »
+1
First off, it's great to see you understand all these concepts and how to calculate pH! But that for the most part is irrelevant to the question.

Just as a side thing, lines given don't mean you have to fill them all out or write twice as much; it's a good indicator of how much you should be writing. The fact that you've gone and written up a whole page of mathematical calculations should ring a few warning bells (ie. it's all well and good, but am I doing something wrong here?). In addition, you're missing the key word in explain. You can calculate the pH like your life depends on it, but if you don't answer the question you could only get a maximum of 1 with maximum leniency. Most times, you'd get zero because you haven't answered the question. The while part of the question is also pretty important as it indicates that you need a comparison of some sort, but that's basically covered in the explain part.

What you need to be doing is writing two key equations:
HCl(aq)→H+(aq)+Cl-(aq)
CH3COOH(aq)⇌CH3COO-(aq)+H+(aq)

Note that the second equation uses an equilibrium arrow, while the first doesn't. Here you can also throw in your Ka values if you a) want to be super fancy or b) somehow remember them or c) both, but it's not completely necessary. Relate these two things if you have them, otherwise relate the equations to the strength of dissociation in water. Say that HCl is a strong acid, and (for our purposes) dissolves completely in water (ie. 1 mol L-1 of HCl will produce an equal amount of hydrogen ions, and then by subbing into the pH equation, gives you that pH of zero.) In contrast, acetic acid doesn't dissociate completely as a weak acid, and will not produce 1 mol of hydrogen ions, thus resulting in a corresponding higher pH. The hole here is the 2.6, but you can do the math if you really want to and prove it to them, but it's totally unnecessary, and in an exam situation, a total waste of time given time pressure.

Hope this helps :)

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david.wang28

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Re: Chemistry Question Thread
« Reply #3715 on: February 15, 2019, 05:38:42 pm »
0
First off, it's great to see you understand all these concepts and how to calculate pH! But that for the most part is irrelevant to the question.

Just as a side thing, lines given don't mean you have to fill them all out or write twice as much; it's a good indicator of how much you should be writing. The fact that you've gone and written up a whole page of mathematical calculations should ring a few warning bells (ie. it's all well and good, but am I doing something wrong here?). In addition, you're missing the key word in explain. You can calculate the pH like your life depends on it, but if you don't answer the question you could only get a maximum of 1 with maximum leniency. Most times, you'd get zero because you haven't answered the question. The while part of the question is also pretty important as it indicates that you need a comparison of some sort, but that's basically covered in the explain part.

What you need to be doing is writing two key equations:
HCl(aq)→H+(aq)+Cl-(aq)
CH3COOH(aq)⇌CH3COO-(aq)+H+(aq)

Note that the second equation uses an equilibrium arrow, while the first doesn't. Here you can also throw in your Ka values if you a) want to be super fancy or b) somehow remember them or c) both, but it's not completely necessary. Relate these two things if you have them, otherwise relate the equations to the strength of dissociation in water. Say that HCl is a strong acid, and (for our purposes) dissolves completely in water (ie. 1 mol L-1 of HCl will produce an equal amount of hydrogen ions, and then by subbing into the pH equation, gives you that pH of zero.) In contrast, acetic acid doesn't dissociate completely as a weak acid, and will not produce 1 mol of hydrogen ions, thus resulting in a corresponding higher pH. The hole here is the 2.6, but you can do the math if you really want to and prove it to them, but it's totally unnecessary, and in an exam situation, a total waste of time given time pressure.

Hope this helps :)
So what exactly do I do for the acetic acid part again?
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sav07

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Re: Chemistry Question Thread
« Reply #3716 on: February 23, 2019, 06:14:20 pm »
0
HI GUYS ITS ME AGAIN!
How would dilution affect the equilibrium in a system? And what would be explanation be in terms of the collusion theory?

carlasilvia

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Re: Chemistry Question Thread
« Reply #3717 on: February 26, 2019, 12:29:25 pm »
0
Explain why the enthalpy of neutralisation differs when using solid reactants to using aqueous reactants.

Jefferson

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Re: Chemistry Question Thread
« Reply #3718 on: March 01, 2019, 06:08:40 pm »
0
Why is the answer (B) in this question (from Nesa).

r1ckworthy

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Re: Chemistry Question Thread
« Reply #3719 on: March 01, 2019, 06:46:36 pm »
+1
HI GUYS ITS ME AGAIN!
How would dilution affect the equilibrium in a system? And what would be explanation be in terms of the collusion theory?

Dilution simply means you are decreasing the molarity/ concentration of a solution. So, if you think about an equilibrium system as a whole, the concentration of all reactants and products decreases when it is diluted, as more water is added. This would decrease the pressure, resulting in the system favouring the reaction with the most number of moles, in order to increase the pressure and minimise the change (le Chatelier's principle).

Now let's analyse this in terms of the collision theory. The collision theory states that particles must be colliding with sufficient kinetic energy and at correct orientation in order for a reaction to occur. When you dilute a solution, the particles will spread out as there is more volume. The pressure will obviously decrease, as there are less particles colliding as they have spread out. Le Chatelier's principle states that a system will try to minimise this change, so the system will favour the reaction will the most number of moles (as this will ensure the formation of the most number of particles), in order to increase the pressure/ amount of particles colliding with eachother. This is where equilibrium will shift.

I think the best way for you to understand would be through an example. Let's go through this pretend equation:

2A + 3B --> 4C + 3D ( the letters represent the molecules- I just made them up)

The solution is at equilibrium. We dilute the solution (add more water to increase volume), thereby decreasing the concentration of all reactants and products (as they have now spread out). The pressure of the system has decreased. The system, due to Le Chatelier's principle, will favour the reaction with the most number of moles, which is the forward reaction. Equilibrium will shift to the right, as more product particles will be made to increase the pressure).

Hopefully that made sense. If not, reply on what part confuses you the most, and I or someone else will try to help out.
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