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October 18, 2019, 08:56:22 am

Author Topic: Chemistry Question Thread  (Read 509850 times)  Share 

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SynX

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Re: Chemistry Question Thread
« Reply #3765 on: August 06, 2019, 03:45:08 pm »
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Use the Henderson-hasselbalch equation .  It is given like this :

pH = pKA + log base 10 of (conjugate base concentration/ conjugate acid concentration).
Since we have the pH, manipulate the equation to find the concentration of acetic acid and ion.
Correct me if I am wrong.
Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...
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horse9996

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Re: Chemistry Question Thread
« Reply #3766 on: August 12, 2019, 06:18:53 pm »
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Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...

Ka is literally just any old equilibrium constant, the a just means that its for an acid. pKa is the same as pH, except with Ka rather than [H+]. This is to make values easier to work with and more comparable. Also, pKa + pKb = 14 (pKb is the same thing but the dissociation of a weak base rather than acid - denoted by b rather than a). To remember the Henderson-Hasselbalch equation, remember 'aha' - since the log is [A-]/[HA] = [base]/[acid]
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louisaaa01

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Re: Chemistry Question Thread
« Reply #3767 on: September 04, 2019, 02:56:58 pm »
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Can someone please help out with these questions?

For 17 - I've just assumed that the acetic acid completely dissociates. I know that isn't the case, but they haven't given any Ka values - are we expected to just know Ka for acetic acid? (But even if it only partially dissociates, shouldn't the mass required be greater than what I've calculated?)

Thank you.

horse9996

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Re: Chemistry Question Thread
« Reply #3768 on: September 08, 2019, 03:27:16 pm »
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Can someone please help out with these questions?

For 17 - I've just assumed that the acetic acid completely dissociates. I know that isn't the case, but they haven't given any Ka values - are we expected to just know Ka for acetic acid? (But even if it only partially dissociates, shouldn't the mass required be greater than what I've calculated?)

Thank you.

For 17 you need the Ka value - I would assume it should be given to you? Check the data sheet. Form a RICE table. The equilibrium concentration of H+ ions can be determined from the pH and then let the initial concentration of acetic acid be x. By using the equilibrium constant expression and the Ka value, you should be able to calculate x and therefore the mass of acetic acid needed.

For 18 it is as follows:
- Not A - cellulose is a polymer which requires glucose as its monomer so this doesn't make sense as coming from ethylene
- Not C - glucose can't be formed from ethylene in 1 step as far as I know
- Not D - ethanol can't undergo polymerisation and doesn't form any polymers
- B - styrene can be formed from ethylene (one of the hydrogens is replaced with a benzene) and this undergoes addition polymerisation to form a polymer (polystyrene)
Hence the answer must be B.

I hope that makes sense
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Katie-E

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Re: Chemistry Question Thread
« Reply #3769 on: September 13, 2019, 08:51:41 pm »
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Hey all,

Does anyone know of any tutors in the Bankstown/ 2200 postcode area who are available for some year 12 HSC Chemistry tutoring?

Thanks sooooooooooo much in advance  ;)

classof2019

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Re: Chemistry Question Thread
« Reply #3770 on: September 28, 2019, 12:04:51 pm »
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Having a bit of trouble with this question:

In an experiment, 4 - hydroxybutanoic acid [HO(CH2)3COOH] forms a polymer containing 1000 monomer units. Which of the folowing is closest to he approximate molar mass (in g/mol) of this polymer?

I ended up with 8.6 x 104 - I figured (this could be wrong) that condensation polymerisation occurs, so the repeating unit is really O(CH2)3CO (H2O expelled). However, answers say 1.0 x 105 - basically multiplying the molar mass of the monomer by 1000

Which approach is correct?

DamnDhruv

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Re: Chemistry Question Thread
« Reply #3771 on: October 11, 2019, 07:55:31 pm »
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hello I have the atar notes topic notes ook for chemistry wherein i found out that they wrote potassium chloride is not soluble. Now is this  a genuine mistake or am just missing something?
here is the image for reference:

Erutepa

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Re: Chemistry Question Thread
« Reply #3772 on: October 11, 2019, 08:14:16 pm »
+2
hello I have the atar notes topic notes ook for chemistry wherein i found out that they wrote potassium chloride is not soluble. Now is this  a genuine mistake or am just missing something?
here is the image for reference:
Potassium chloride is definitely soluble. Unfortunately, you will come across the odd mistake/typo in pretty much all resources from textbooks to study notes like these, so just be wary.
« Last Edit: October 11, 2019, 08:33:38 pm by Erutepa »
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DamnDhruv

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Re: Chemistry Question Thread
« Reply #3773 on: October 11, 2019, 11:52:25 pm »
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Wow, that’s great! I just have a hard time memorizing “Henderson-hasselbalch equation” all I know about this is the pH=-log(aH+), by doing some research I found out that pKa=-log(Ka), which is really similar to the pH equation, as well as pOH equation. But the definition of Ka is “where "Ka" is the “equilibrium constant for the ionization of the acid”. Seems the pH and pOH value of a substance adds up to a constant around 14. Just learning these at school. But the concept of pKa is confusing!
Plus, I just figured out the function of the log button by playing the calculator a few weeks ago...

its on the fomula sheet i think