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March 29, 2024, 09:53:13 am

Author Topic: HSC Chemistry Question Thread  (Read 1040672 times)  Share 

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Maz

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Re: Chemistry Question Thread
« Reply #240 on: March 31, 2016, 07:12:04 pm »
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hi
can you please help me with this one- it's a titration...
water from a domestic bore that sprayed onto a nearby fence was observed to leave orange brown deposits on it's surface. this indicated that the water contained iron compounds. the water was analysed for iron by titration in the following way: Aisr was bubbled through a 250 mL sample of water for several hours to convert all the dissolved iron into Fe3+ then boiled to precipitate all the calcium ions as calciu carbonate. the solid was removed by filtration then 10mL of standard 0.103mol L-1 NaOH was added to the bore water which precipitated all of the iron in the sample and left  some excess NaOH in the solution.
After the mixture was filtered it required 27.34mL of standard 0.0277 mol L-1 HCl acid for neutralisation. if the density of the bore water was 1.01g mL-1 calculate the concentration of iron in the bore water measured in parts per million...

so far i have just converted the HCl to moles giving: 0.000757318 moles...
i really don't know what to do after that...please can you help me?
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jakesilove

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Re: Chemistry Question Thread
« Reply #241 on: March 31, 2016, 07:37:07 pm »
+1
hi
can you please help me with this one- it's a titration...
water from a domestic bore that sprayed onto a nearby fence was observed to leave orange brown deposits on it's surface. this indicated that the water contained iron compounds. the water was analysed for iron by titration in the following way: Aisr was bubbled through a 250 mL sample of water for several hours to convert all the dissolved iron into Fe3+ then boiled to precipitate all the calcium ions as calciu carbonate. the solid was removed by filtration then 10mL of standard 0.103mol L-1 NaOH was added to the bore water which precipitated all of the iron in the sample and left  some excess NaOH in the solution.
After the mixture was filtered it required 27.34mL of standard 0.0277 mol L-1 HCl acid for neutralisation. if the density of the bore water was 1.01g mL-1 calculate the concentration of iron in the bore water measured in parts per million...

so far i have just converted the HCl to moles giving: 0.000757318 moles...
i really don't know what to do after that...please can you help me?

Hey!

Unfortunately, I don't have time right now to do a full solution (although can write one up later, if no-one else has), however I can quickly explain the general method.

Knowing the moles of HCl, we can write out the reaction between HCL and NaOH to figure out the number of moles of NaOH that was in excess in solution (knowing that it is a 1:1 ratio).

Now that we have the EXCESS NaOH, we can figure out the amount of NaOH that reacted with Iron (figure out the total moles of NaOH added, and then subtract the excess).

We can write the reaction between Iron and NaOH. Knowing the number of moles of NaOH (from above) that reacts, we can figure out the number of moles of Iron!

Sorry that I can't go into much more detail right now.

Jake
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g98

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Re: Chemistry Question Thread
« Reply #242 on: March 31, 2016, 08:09:34 pm »
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Hi I know this is the Chem thread but I'm hoping someone out there knows something about Bio! Should you know the scientific names of animals and plants you use in examples? Thank You!!!

Rd2487

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Re: Chemistry Question Thread
« Reply #243 on: April 01, 2016, 07:06:10 pm »
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Hi Jake
I just had a question about classifying salts ]
When you explain the salts nature how would you structure your response like lets say NH4CL?
So would you start saying that Cl is a weak conjugate anion of the strong acid HCl and hence it would not further hydrolyze.
Whereas the ammonium is a strong conjugate cation of the weak base ammonia hence it would further hydrolyze.
Now i get confused on this step after providing the reaction of NH4 with water. Would you say that because H30+ ions are produced in solution, it would make the salt acidic ??
Can you please provide a way to structure this response
Thanks

RuiAce

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Re: Chemistry Question Thread
« Reply #244 on: April 01, 2016, 07:55:16 pm »
+1
Hi Jake
I just had a question about classifying salts ]
When you explain the salts nature how would you structure your response like lets say NH4CL?
So would you start saying that Cl is a weak conjugate anion of the strong acid HCl and hence it would not further hydrolyze.
Whereas the ammonium is a strong conjugate cation of the weak base ammonia hence it would further hydrolyze.
Now i get confused on this step after providing the reaction of NH4 with water. Would you say that because H30+ ions are produced in solution, it would make the salt acidic ??
Can you please provide a way to structure this response
Thanks
As I have previously explained somewhere in this thread (not too sure where exactly).

The conjugate of something strong (HCl, NaOH, HNO3) is something extremely weak (respectively, Cl-, Na+, NO3-).
Whereas the conjugate of something weak (CH3COOH, NH3) is something that's also weak, but definitely not extremely weak (respectively CH3COOH-, NH4+)

However, when it comes to classifying salts as acidic/basic/neutral, one may actually safely assume substances such as Cl- and Na+ are neutral when writing exam responses. The reason for this is that the impact of these ions in the overall result is so negligible, it's almost non-existent.

Such a question would be about 3 marks at most (some may be lenient to give 2). Here is what I would say (not the BEST answer - I'm not currently doing the HSC)

The Cl- ion is a neutral anion. It's presence will not affect the acidity of the salt.
However, NH4+ is an acidic cation as it is able to react with water to form it's conjugate base:
NH4+ + H2O(l) ⇌ H3O+ + NH3(aq)
The acidic nature of ammonium chloride therefore occurs as a result of only the acidic ammonium ion impacting, as that of the chloride ion is negligible.

browny2409

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Re: Chemistry Question Thread
« Reply #245 on: April 02, 2016, 01:49:05 pm »
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Hi,

When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brøsted-Lowry) there was one HSC question which asked how each developed from the other

I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former

Thanks
Nathan

jakesilove

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Re: Chemistry Question Thread
« Reply #246 on: April 02, 2016, 02:18:49 pm »
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Hi,

When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brøsted-Lowry) there was one HSC question which asked how each developed from the other

I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former

Thanks
Nathan

Hey Nathan!

Essentially, all you need to know is that each subsequent theory was developed as a result of an unexplained finding! For instance, Lavoisier believed that acids contained Oxygen molecules (as you know). However, Davy noticed that HCl acted as an acid, and therefore obviously Lavoisier's theory was inadequate! As such, Davy developed his own theory.

Each development has similar examples that you can use, however I wouldn't bother. The only question you could really get either a multiple choice, asking you to determine which theory is outlined in the question, or a 'describe Arrhenius's theory of Acids and Bases' etc.

How many marks was that question you found? I would say you just need to outline the general aspects of each theory, and saying that they built on each other as new scientific information came to light. You're not expected to know examples of each: merely that they become increasingly complex,and increasingly accurate as they tend to encapsulate more and more actual acids.

Jake
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RuiAce

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Re: Chemistry Question Thread
« Reply #247 on: April 02, 2016, 03:13:56 pm »
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Hi,

When detailing the development of Acid-Base theories (Lavoisier, Davy, Arrhenius and Brøsted-Lowry) there was one HSC question which asked how each developed from the other

I am aware of what each theory, but could you help in clarifying exactly how each theory improved the former

Thanks
Nathan

During the HSC I wrote a full 8 marker on acid-base theories. I was able to find it somehow.

Please refer to below:

Past theories of acids and bases tend to purely rely on what was believed to be the chemical composition of all acids and their properties. Lavoisier's theory stated that acids were substances which contained hydrogen, as he found that when compounds involving oxygen dissolved in water, they produced acidic properties, such as CO2, NO2 and P4O10. This also meant that some acids were recognised to be acids, e.g. H2SO4. However, this is now known to be significantly flawed, as some substances containing oxygen are in fact basic (e.g. Na2CO3, Na2O) and why some other acids did not contain oxygen (e.g. HCl). Davy then proposed that acids were substances that contained hydrogen through the discovery of HCl, through means of dissolving gaseous hydrogen chloride into water and producing a highly acidic solution. Davy also found that metals could displace the hydrogen in acids: Metal + Acid -> Salt + Hydrogen gas. Yet, this was also flawed because other substances such as CH4 are not acidic, yet also contain hydrogen.
Arrhenius then developed a theory that was, at the time, much more revolutionary and a more accurate definition of acids, and bases. Arrhenius showed that acids were substances that when dissolved in water, ionised to form H+ ions, and similarly for bases albeit OH- ions. Arrhenius was also able to initiate the definitions of relative strengths of acids through his theory from an understanding of degrees of ionisation, and also demonstrated how acids and bases can undergo a neutralisation process: Acid + Base -> Salt + Water. This is reasonably accurate, and is sometimes taught as a simplified model of how acids and bases work, however it did not cater for other bases such a Na2CO3 and NH3.
The most accurate definition of acids and bases was proposed by Johannes Bronsted and Thomas Lowry. The Bronsted-Lowry theory defines acids and bases in terms of their chemical behaviour, that is, acids were substances that could donate a proton/protons, whereas bases could accept them. This theory focused much more on the properties of acids and bases rather than their chemical composition and an understanding of what conjugate acids and bases were. By consideration of the ability to donate/accept H+, it was then finally understood why substances were acidic, basic or neutral. (It also led into the identification of amphiprotic species with reference conjugate acids and bases.)

RhyannahHamer

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Re: Chemistry Question Thread
« Reply #248 on: April 03, 2016, 05:46:09 pm »
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Hey there,
I'm doing notes under the syllabus right now, and I don't think I've got any sufficient notes on the following dot point:
'describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions'
If you could help me out, it'd be much appreciated.
Thanks :)

jakesilove

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Re: Chemistry Question Thread
« Reply #249 on: April 03, 2016, 10:07:59 pm »
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Hey there,
I'm doing notes under the syllabus right now, and I don't think I've got any sufficient notes on the following dot point:
'describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions'
If you could help me out, it'd be much appreciated.
Thanks :)

Hey!!

Basically, for that dot point I would have a few things up your sleeve.

Definitions of strong and weak acids

Always start by defining things. A strong acid is, by definition, an acid that ionises completely in solution. A weak acid is just any acid that does NOT ionise completely in solution (and usually only ionises to a tiny extent, eg. 1-10%).

Formulas

But what do the above statements even mean? The easiest way to describe strong and weak acids in terms of equilibrium is to use equations. So, let's do that!

Let's start by looking at a strong acid. I always like to use HCl, because its the one used most often in the HSC.

We write the reaction of HCl and Water to show ionisation.



Now, we know that HCl is a strong acid, and so ionises completely. In terms of equilibrium, this means that the reaction lies entirely to the right!

Now, let's look at weak acids or weak bases. You can use whatever weak acid or base you want.

First, we write out the equation.



Now, we know that Ammonia is a weak base. Therefore, in terms of equilibrium, we can say that the reaction lies largely to the left!

That's really all you need to know in terms of acids/bases and equilibrium. Hope it helps!

Jake
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Maz

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Re: Chemistry Question Thread
« Reply #250 on: April 05, 2016, 01:10:31 am »
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hey
i've tried to do this titration question a couple of times- i just don't know how to do it :(
can you please help me?
the question:
9.32g of 'coudy ammonia' was dissolved in water and made up to 250mL in a volumetric flask. 20mL portions of this solution were tritrated with 0.98 mol L^-1 HCl. An average of 25.8 mL HCl was required for the neutralisation using methyl orange indicator. Calculate the percentage by mass of ammonia in the 'cloudy ammonia'.
i'd really appreciate it if you could help me out...
thankyou  :)
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browny2409

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Re: Chemistry Question Thread
« Reply #251 on: April 06, 2016, 09:38:43 am »
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Thanks for your pervious answer,

Just another question, when summarising the chemistry of the fermentation process (Production of Materials), do we need to mention acid hydrolysis and enzyme digestion, because i have heard of these terms but don't have them in my notes - all i refer to is the invertase enzyme and the zymase enzyme brought about by the yeast

Thanks
Nathan

jakesilove

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Re: Chemistry Question Thread
« Reply #252 on: April 06, 2016, 09:52:51 am »
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hey
i've tried to do this titration question a couple of times- i just don't know how to do it :(
can you please help me?
the question:
9.32g of 'coudy ammonia' was dissolved in water and made up to 250mL in a volumetric flask. 20mL portions of this solution were tritrated with 0.98 mol L^-1 HCl. An average of 25.8 mL HCl was required for the neutralisation using methyl orange indicator. Calculate the percentage by mass of ammonia in the 'cloudy ammonia'.
i'd really appreciate it if you could help me out...
thankyou  :)

Hey!

I'm so sorry for the late response, I totally missed your question! This is a tough one, that requires multiple applications of the titration formulas. My working is below, I hope it helps!




Jake
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jakesilove

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Re: Chemistry Question Thread
« Reply #253 on: April 06, 2016, 09:59:14 am »
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Thanks for your pervious answer,

Just another question, when summarising the chemistry of the fermentation process (Production of Materials), do we need to mention acid hydrolysis and enzyme digestion, because i have heard of these terms but don't have them in my notes - all i refer to is the invertase enzyme and the zymase enzyme brought about by the yeast

Thanks
Nathan

Hey Nathan,

Whenever I tutor students, and whenever I answered questions on this topic in my HSC, I always recommend one method for exactly this problem: Mention the terms, but that's it.

An important step in the process is going from Cellulose to Glucose. At the moment, this process is expensive and time consuming, and requires either acid hydrolysis and enzyme digestion. However, you definitely don't need to know more information about either of those processes. I would definitely mention that they are expensive and time consuming, and name-drop the terms, but that's it.

Not only would you get full marks by mentioning it, but as it is really an 'addition' to the course the marker will look really favorably on you for going above and beyond, despite the fact that maybe you don't understand the terms you are using (as I definitely don't either!).
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Re: Chemistry Question Thread
« Reply #254 on: April 06, 2016, 10:21:39 am »
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Hey guys!

Got my Chemistry half yearly today and I've got a stupid question D: This is the question:

Which of the following aqueous solutions has a pH greater than 7?
a) Sodium ethanoate
b) Sodium chloride
c) Ammonium nitrate
d) Ammonium chloride

What's a good way to tell if you're unsure of the compound? Thanks!

Neutron