February 25, 2020, 01:20:24 pm

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#### Hawraa

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« Reply #3795 on: November 04, 2019, 02:02:02 pm »
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Hi Hawraa,

We can rule out option A because nitrous acid will partially dissociate to produce H+ ions and NO2- ions. The presence of H+ ions will reduce pH (noting that pH = -log10[H+], so greater [H+] means lower pH). Option B is ruled out for a similar reason.

Now, to discern between C and D we look at pKa. Since nitrous acid has a higher pKa than chlorous acid, this means that nitrous acid is a weaker acid - so its conjugate base (NO2-) is a stronger base. This means a solution containing NO2- ions has a greater pH than a solution containing the same concentration of ClO2 ions, so C must be the answer. Note also that Na+ ions don't really react with water - it's the anion in this instance that governs pH.

So to sum up, C has a higher pH than A simply because there are no H+ ions in its structure. C has the highest pH since its anion is the strongest base present.

That's great thanks a lot. Just another question please. You know how you said that nitrous acid is weak and so its conjugate base is strong, I read something similar on the internet but didn't really understand it. So is it like whenever we have a weak acid, its CB is strong? And vice versa? And is the same thing applies for bases, so a weak base would have a strong CA?
Thanks

#### louisaaa01

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« Reply #3796 on: November 04, 2019, 02:21:26 pm »
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That's great thanks a lot. Just another question please. You know how you said that nitrous acid is weak and so its conjugate base is strong, I read something similar on the internet but didn't really understand it. So is it like whenever we have a weak acid, its CB is strong? And vice versa? And is the same thing applies for bases, so a weak base would have a strong CA?
Thanks

Yes, that's exactly right!

We can come to this conclusion based on how the various acids and bases dissociate. For instance, HCl is a strong acid, completely dissociating into H+ and Cl-. Cl- is the conjugate base of HCl - since HCl dissociates so readily, Cl- is highly unlikely to accept that H+ back to form HCl, so it's a very weak base (by the Bronsted-Lowry theory). A similar scenario occurs for strong bases - since a strong base will readily accept a proton, once that proton has been accepted and the conjugate acid is formed, it won't readily 'donate' it back.

A weak acid, on the other hand, will form an equilibrium in water. Take, for example, the fairly weak ethanoic acid, CH3COOH, which dissociates in water as per the following equilibrium:

CH3COOH (aq) + H2O (l) <--> CH3COO- (aq) + H3O+ (aq)

The conjugate base of CH3COOH, being CH3COO-, is a moderately strong base since it has the tendency to accept a proton back from the hydronium ion and form CH3COOH again (reverse reaction). A similar thing occurs for a weak base and its conjugate acid.

Hope this clarifies the concept a little more!
« Last Edit: November 04, 2019, 02:23:17 pm by louisaaa01 »
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#### Aaron Lillis

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« Reply #3797 on: November 04, 2019, 07:42:32 pm »
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Hey all!!

Could someone please provide a complete solution to this question? I'm finding it extremely confusing - particularly the equation given and how it helps, how the molarity of sodium ethanoate relates to anything mathematically and which dissociation equation you're meant to use for Ka. This is my worst type of question in chemistry, any help would be appreciated

#### louisaaa01

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« Reply #3798 on: November 04, 2019, 08:09:31 pm »
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Hey all!!

Could someone please provide a complete solution to this question? I'm finding it extremely confusing - particularly the equation given and how it helps, how the molarity of sodium ethanoate relates to anything mathematically and which dissociation equation you're meant to use for Ka. This is my worst type of question in chemistry, any help would be appreciated

I have attached my attempt at this question - hope this helps! If you're still confused with anything, don't hesitate to ask.

EDIT: x may also be solved using the quadratic formula - however, since x is small, it is generally safe to assume that, when subtracted from the molarity of the solution, the difference is negligible.
« Last Edit: November 04, 2019, 08:11:51 pm by louisaaa01 »
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#### Aaron Lillis

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« Reply #3799 on: November 04, 2019, 08:55:24 pm »
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I have attached my attempt at this question - hope this helps! If you're still confused with anything, don't hesitate to ask.

EDIT: x may also be solved using the quadratic formula - however, since x is small, it is generally safe to assume that, when subtracted from the molarity of the solution, the difference is negligible.

Thank you so much, oh my lord I didn't realise it was the base dissociation equation given. Thank you x100

#### RACHEL1111

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« Reply #3800 on: November 05, 2019, 11:22:26 pm »
+1
Hi!
Could anyone help me with this confusion : when the volume of the equilibrium reaction increases, would there be a sudden drop of the curve representing concentration of chemical species in the concentration vs. time graph? or the concentration just  slowly increases or decrease?
I saw these two different versions on different textbooks and it really confuses me. Any help is appreciated!!

#### Hawraa

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« Reply #3801 on: November 05, 2019, 11:54:34 pm »
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Hi!
Could anyone help me with this confusion : when the volume of the equilibrium reaction increases, would there be a sudden drop of the curve representing concentration of chemical species in the concentration vs. time graph? or the concentration just  slowly increases or decrease?
I saw these two different versions on different textbooks and it really confuses me. Any help is appreciated!!

Hey,
When the volume of the vessel increase then the pressure decrease, therefore, the concentration of the species decreases represnted by a sharp decrease on the graph. I've seen this also in some past papers sample answers. Check 2009 Q23 and also 2005 Q27(this one is the other case with reduced volume).
Hope this clarifies the confusion.

#### RACHEL1111

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« Reply #3802 on: November 06, 2019, 06:14:55 am »
+1
Hey,
When the volume of the vessel increase then the pressure decrease, therefore, the concentration of the species decreases represnted by a sharp decrease on the graph. I've seen this also in some past papers sample answers. Check 2009 Q23 and also 2005 Q27(this one is the other case with reduced volume).
Hope this clarifies the confusion.

Yes! It does! Thank you very much!

#### Hawraa

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« Reply #3803 on: November 06, 2019, 01:35:43 pm »
+1
Hi everyone,
Just regarding this graph question from ATAR Notes chemistry topic test. So it says that the trend line(line of best fit) should not be linear but should go near most points. For me it looks like the dotted line has been connected by hand. Is it wrong to draw it with a ruler ( like I did, if you can see the line in the middle) because I'm confused. My teacher said it should be linear but the answer here says it should not. So do we connect the dots or just draw a line approximately through them.
Can someone please explain this to me? Thanks.

#### Hawraa

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« Reply #3804 on: November 06, 2019, 09:31:15 pm »
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Hi everyone,

Ethanol is widely used fuel that can be made from both renewable and non renewable resources. Using relevant chemical equations, outline two methods that can be used to produce Ethanol.

The methods I would use are fermentation (equation of glucose turning into Ethanol and CO2) and hydration of ethene. Is the second method wrong? Because the answer I have talks about fermentation and the substitution of halogenated alkanes with water? Are they both correct?
Any help would be appreciated guys, thanks.

#### keltingmeith

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« Reply #3805 on: November 18, 2019, 09:27:28 pm »
+3
Hi everyone,
Just regarding this graph question from ATAR Notes chemistry topic test. So it says that the trend line(line of best fit) should not be linear but should go near most points. For me it looks like the dotted line has been connected by hand. Is it wrong to draw it with a ruler ( like I did, if you can see the line in the middle) because I'm confused. My teacher said it should be linear but the answer here says it should not. So do we connect the dots or just draw a line approximately through them.
Can someone please explain this to me? Thanks.

Would help if we could see the question

Hi everyone,

Ethanol is widely used fuel that can be made from both renewable and non renewable resources. Using relevant chemical equations, outline two methods that can be used to produce Ethanol.

The methods I would use are fermentation (equation of glucose turning into Ethanol and CO2) and hydration of ethene. Is the second method wrong? Because the answer I have talks about fermentation and the substitution of halogenated alkanes with water? Are they both correct?
Any help would be appreciated guys, thanks.

No need to worry, hydration of ethene is still a correct answer
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#### mani.s_

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« Reply #3806 on: November 22, 2019, 07:48:30 pm »
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Hi, how would I find the pH of 0.020 mol/L citric acid without using Ka values??? Thanks

#### louisaaa01

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« Reply #3807 on: November 22, 2019, 07:58:21 pm »
+3
Hi, how would I find the pH of 0.020 mol/L citric acid without using Ka values??? Thanks

Hi,

Citric acid is a weak triprotic acid, so to accurately find pH you would have to use Ka. However, if Ka isn't provided, the question could be solved (though somewhat erroneously) relying on the assumption that citric acid is a strong acid.

Since citric acid is triprotic, assuming it is strong and thus completely dissociates, a 0.020 mol/L solution of citric acid will have [H+] = 3 x 0.020 = 0.060 mol/L. You can then use the formula, pH = -log[H+] to determine pH.

Hope this helps!
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#### milie10

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« Reply #3808 on: January 30, 2020, 06:22:37 pm »
+1
Hi!
I tried doing some of the past HSC questions on acids and bases, and I'm having a bit of trouble doing these two.

For this, I tried using c1v1=c2v2 but ended up with 10L Is this the right method to use here?

I'm still confused about limiting reagent type questions- in this question, when I use n=cxv, does this find n(2HCl), or does it only find n(HCl)? Also, is there a quicker way to do this question?

Thanks so much!

#### r1ckworthy

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« Reply #3809 on: January 30, 2020, 06:59:02 pm »
+6
Hey!
Hi!
I tried doing some of the past HSC questions on acids and bases, and I'm having a bit of trouble doing these two.

(Image removed from quote.)
For this, I tried using c1v1=c2v2 but ended up with 10L Is this the right method to use here?
You are right! 10L is the answer, so you choose the answer that is closest to this value, which is C!

(Image removed from quote.)
I'm still confused about limiting reagent type questions- in this question, when I use n=cxv, does this find n(2HCl), or does it only find n(HCl)? Also, is there a quicker way to do this question?

Thanks so much!
Here is my working out of the question (click on the image to make it larger):

When you use $n=c \times v$, it only finds n(HCl). I find limiting reagent questions confusing, so I like to write statements as I did in my working. In those statements, I calculated how many moles a substance will react with/ use up (according to stoichiometric ratios). I compare the two statements and pretty quickly find out which substance is the limiting reagent.
I am not sure of a quicker way to do this question (hopefully someone can chip in!).

Let me know if you need any more help!
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