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Chapter 6: Acids and Bases

Titrations

Postby shehzad6090 » Fri Feb 10, 2012 12:13 am

On pages 66 and 67,
It says: "As excess NaOH is added, the ionisation of the base "" is suppressed by and the pH is determined only by the excess [OH-]. Therefore, the titration curve beyond the equivalence point follows that for the titration of a strong acid."
And vice versa on the next page...
I dont understand what this sentence is implying at all. I understand that the curve will reverse and the pH will fall again, but why? Could you explain in layman's terms because I cannot understand the explanation in the book.
Thankyou!
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Postby goldstanda3269 » Thu Feb 16, 2012 1:34 pm

First notice that the curves are titrations using base. This means that we expect the curve will increasingly rise (i.e. the pH continues to climb) as we add more base (OH-). Whether the titration is with a strong acid or with a weak acid, when we get beyond the equivalence point, the curves are essentially the same: www.files.chem.vt.edu/chem-ed/titration ... g-weak.gif
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Postby shehzad6090 » Thu Feb 23, 2012 7:15 pm

thankyou!
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Re: Chapter 6: Acids and Bases

Postby JordanS » Tue Mar 10, 2015 1:27 pm

For question 3 of Q & A 1. My original inclination was to choose answer A because it would form a very unstable carbanion. I then, however, for some reason decided to see what would happen after that and assumed that the Hydrogen below would immediately be attracted to this formal negative charge to form a double bond and cyclohexene molecule. Hence I thought that molecule B would similarly form a secondary carbanion that would stabilize by drawing a Hydrogen from the left and forming ethene? That may be an incorrect deduction.

So my question is, for answering questions like this do we avoid going to the next step and purely just look at the stability of the intermediate conjugate base and hence molecule A would have the most unstable conjugate base.

Thanks.
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Re: Chapter 6: Acids and Bases

Postby JordanS » Tue Mar 10, 2015 1:56 pm

When do you use the formula M1V1=MfVf (where M = Molarity) and when do you simply just use n=CV? For example with reference to Q & A 2, Q. 11.

Thanks.
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Thu Mar 12, 2015 11:50 am

Though it is good to be aware of those formulas, neither is required for the GAMSAT. The questions based on these formulas should be solved with reasoning and dimensional analysis which in combination will reduce the chance of error. These 2 tools permit the solving of many more problems that just concentration problems.

Also, notice that M1V1=MfVf and n=CV are identical. The first equation is the same as saying MV is constant and since M is concentration in specific units, that is the same as saying c (constant) = n = CV.
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Re: Chapter 6: Acids and Bases

Postby JordanS » Sun Mar 15, 2015 2:00 pm

Yes, I see that now, thanks.
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Thu Aug 06, 2015 5:05 pm

"How to know strong, weak acid? Memorise?

As long as I know, strong acid is completely ionized in solution such as HCl
But like bellowing question, how can I resolve? memorise common strong weak acid and base??"

CHM-75, in the "Memorize" box is "Example of strong/weak acids/bases" and you will find the list of acids at the bottom of CHM-76.


"Bellowing is Chapter question.
2) Which of the following organic compounds is the strongest acid? "

Frankly, if you have no science background, then don't worry about this question. By Chapter 8 organic chemistry (Carboxylic acids), the answer will become clear to you.


"As per H-H equation, pH=pKa + log([salt]/[acid]). I know this.
pH= 5 + log([salt]/[10^3])
Then, how do you calculate log[salt]??"

You don't have to calculate log[salt]. You only need to correctly interpret what this means: "the concentration of acetic acid will be 10^3 that of its conjugate base" which means that...

[salt]/[acid] = 1/10^3 = 10^-3

The concentration of the conjugate base and salt is always the same. For example:

Cl- is the conjugate base and NaCl is the salt. Or in this question #5, acetate is the conjugate base and sodium acetate is the salt (CHM 6.3, 6.7, 6.8).
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Q: 6

Postby ElleRT » Fri Aug 21, 2015 10:08 pm

Hi,

I was wondering for Q.6 on your Q&A where The pKa of acetic acid is 5. What is the pH at which the concentration of acetic acid will be 10-6 that of its conjugate base, acetate?

Why is it +6 and not -6 like it was -3 for the previous question?

Thanks
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Sat Aug 22, 2015 7:03 am

The overall technique for both questions is identical but you must carefully following the exponents.

Notice that Question 5 starts with a positive exponent which becomes negative because it goes in the denominator. Question 6 starts with a negative exponent which becomes positive because it also goes in the denominator.
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Thu Sep 17, 2015 7:49 pm

ilbe wrote:Queries about App (GS flashcards) question.

The question is
What will be the approximate pH (acidic/neutral/basic) of a solution containing NaC2H3O2 (sodium acetate)?

The answer is
weak base.

1. I understand it is not acidic as it doesn't have proton, so my reasoning is
that the solution maybe 'neutral or basic'. But the answer is weak base. How do you make decision?


I don't have the app in front of me but I'm told that the explanation to the answer is given: The pH will be basic (pH > 7) because the acetate ions act as a weak base. Note that sodium acetate, which is usually written CH3COONa, has no protons that could dissociate thus it cannot be acidic (C-H bonds are very strong).

In other words: Acetate is a proton acceptor (= definition of a base) and a classic example of a weak base (see CHM 6.7; but also CHM 6.1, 6.6; ORG 8.1).
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Fri Oct 09, 2015 12:21 am

"Query 1. What is the differences between 'aq' and 'l'?
Aquous is existing as ions and liquid is pure liquid??"

- Essentially, yes.
- CHM 5.2 provides the formal definition of aqueous.


"Query 2. In this rxn, NaCl is salt like table salt what we eat. Right?"
- Correct.

"If I dissolve NaCl with H2O, then it turn into HCl and NaOH? Maybe not because we eat salt every day and not so dangerous as strong base and strong acid.
To do so, maybe 'heat' or 'energy' is required?? Does it possible to do reverse rxn?"

No.

The arrow only points to the right (i.e. it is a one-way reaction; it is not a double sided arrow).

HCl + NaOH ---> NaCl + H2O

CHM 1.5.1: The above is a neutralization reaction, which is a special type of double-displacement reaction.

Note: Please always also provide the section number from the book because the page numbers are different in different editions. Thanks for understanding.
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Sun Oct 11, 2015 9:39 am

ilbe wrote:The section number is 6.7

In the last part I can see dounle arrowed rxn as
(1) Strong acid + stron base
HCl + NaOH <-> NaCl + H2O

This maybe typo?


Yes, a one-sided arrow is more precise for a strong acid/strong base.



ilbe wrote:Query with regards to acronym ' HOAc/ AcOH' of Acetic acid.


1.Ac means Acetic? right?
2.What is OH? hydroxyl group? If so, where are carbonyl group?
3.Are there any acronyms to be seen?


thanks a lot


1. Ac = Acetyl
2. OH = hydroxyl
Carbonyl is part of acetyl Ac https://en.wikipedia.org/wiki/Acetyl

3. Memorising the meaning of the acronym Ac is not a GAMSAT question (i.e. they would normally remind you of its meaning in the passage or question stem). All acronyms will be seen as you complete ACER and GS problems (i.e. ph phenyl, et ethyl, me methyl, etc.).
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Sun Oct 11, 2015 2:07 pm

ilbe wrote:Queries with regards to memorising acids (CHM 6.1)

1. Is there any reason that the column has been divided strong/weak/strong/weak rather than strong/weak at once?

2. Any good way to memorise this table please?

Thanks a lot.


1. No.

2. Repetition and attention.

However, it is not necessary to memorise the table since the Ka or pKa values are normally given (unless it is one of the powerful acids that even the newspaper boy knows like HCl and sulfuric acid).
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Re: Chapter 6: Acids and Bases

Postby goldstanda3269 » Sun Oct 11, 2015 2:21 pm

ilbe wrote:Queries with regards to meaning of [H+], [OH-].

I can see [H+][OH-] = 10^-14.
In water (neutral solution), pH=-log10 [H+] thus '[H+] = 10^-7'

Here, [H+] means molar concentration which is this 'number of moles of H+ ion in one liter'.

1. So H+ ions are 10^-7 M (molar concentration). right?
2. value of M can be -ve?
I M = 6.02 x 10^23 molecules so try to calculate into number of molecules
10^-7 x 6.02 x 10^23 = 6.02 x 10^16. Thus 6.02 x 10^16 H+ molecules exist. right?
3. Can I make decision as following?
Value of M or [ ] can not be down under 10^-23 as Avogadros' number is 6.02x 10^23?

Thanks a lot


1. Yes, in pure water at standard temperature and pressure.

2. The value of M cannot be negative.

"10^-7 x 6.02 x 10^23 = 6.02 x 10^16. Thus 6.02 x 10^16 H+ molecules exist"

- It is possible.

3. The value of M or [ ] can be 0 and any number above 0 including any number between 0 and 10^-23.
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