HSC Chemistry Question Thread

[MisterNeo]

Hi! Please can you explain how to do this question?
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you

Was the answer 3mL? To find the moles of KOH, you need to multiply moles of H₂SO₄ by 2 stoichiometrically, which would be the same as multiplying by 2 to get moles of H⁺. Doing it either way would get you the same answer because both make you multiply by 2.

[winstondarmawan]

Hello!
What's the term for when a salt in solution is in such high concentration that it won't dissolve anymore?
I can't seem to remember..

[MisterNeo]

Hello!
What's the term for when a salt in solution is in such high concentration that it won't dissolve anymore?
I can't seem to remember..

Saturated

[beau77bro]

What does it mean "if no precipitate forms is ismade of alkaline"?
Towards the bottom^

Mod Edit: Merged post.

[angelahchan]

Hi! Please can you explain how to do this question?
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you

I thought I"d give it a go
First we work out the moles of H2SO4 .
 Moles = concentration * volume = 0.0005*0.015=  0.0000075
Since this is a 1:2 ratio of H2SO4 to KOH
Multiply 0.0000075 by 2 = 0.000015 mols of KOH
Volume = moles/concentration = 0.000015/0.005= 0.003
which is 3 ml
I'm not sure if I did it correctly though....tell me if I'm wrong because I totally could be , and I'm also not sure how they calculated H+
I just worked out the moles of H2SO4 rather than the moles of H+, and then multiplied by 2 to find moles of KOH, since there's a 1:2 ratio.
And then from the moles of KOH I worked out the volume via volume = moles/concentration.
Maybe (idk tho) try to work with the moles of the substances rather than H+ and OH- in this case...? I have no clue, maybe you can post the answers for someone else to have a look?

whoops i was looking at 2nd last page rather than last, didn't notice someone already posted a reply

[winstondarmawan]

Shipwrecks
What are some sources of sulfate in deep ocean wrecks? Not sure if we are required to know this because I can't seem to find it in my notes.
CORE TOPICS
1. Are concentrated and dilute acids defined by >1M and <1M respectively?
2. Also, is there an equation for photochemical smog that we are advised to know?
3. When water is added to a weak acid, does pH increase or decrease?
2012 HSC:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/22811213_1360059024119617_5363852_n.jpg?oh=30d80c4c98a69709c377d13fc9d5ba6b&oe=59F1FF4E
Not sure how filter funnels are used in the preparation of standard solutions.
TIA.

[MisterNeo]

(Image removed from quote.)
What does it mean "if no precipitate forms is ismade of alkaline"?
Towards the bottom^

Sulfide reacts with hydrogen ions to form aqueous hydrogen sulfide, which doesn’t bond with those metal ions to form precipitates. Hence, the solution is made alkaline to get rid of the hydrogen ions so that sulfide can have a chance to bond with the metal ions rather than the hydrogen ion. Through this, we can tell if those cations are present due to sulfide precipitates being formed.

[bridie_2345]

Hi there for industrial chemistry, i was wondering if someone could please explain the how soap, water and oil together form an emulsion with the soap acting as an emulsifier?

also how in depth should i be about the enivornmental impacts of soaps and detergents?

thanks so much

[aryak]

Heyy, I am having a little bit of trouble in solving this question. I dont know how to do the question itself. Is it ok if you could explain to me the steps in solving this question, cause I dont understand how the answer is C.

[MisterNeo]

Heyy, I am having a little bit of trouble in solving this question. I dont know how to do the question itself. Is it ok if you could explain to me the steps in solving this question, cause I dont understand how the answer is C.

Hi there for industrial chemistry, i was wondering if someone could please explain the how soap, water and oil together form an emulsion with the soap acting as an emulsifier?
also how in depth should i be about the enivornmental impacts of soaps and detergents?
thanks so much

An emulsion is when a two usually immiscible liquids are dispersed in solution, such as soap making water and oil mix. Soap is made of a hydrophillic (water-loving) negatively charged head and a hydrophobic (water-fearing) hydrocarbon tail. The hydrocarbon tail will want to get out of the water due because it is non-polar, and will adsorb into oil particles. Upon scrubbing, the oil particles will be agitated and break up into smaller droplets, then more soap will adsorb into the oil droplets to form micelles. These micelles have the hydrophillic negative head on the outside, so they repel each other and disperse themselves along with the oil throughout the solution.

[beau77bro]

Sulfide reacts with hydrogen ions to form aqueous hydrogen sulfide, which doesn’t bond with those metal ions to form precipitates. Hence, the solution is made alkaline to get rid of the hydrogen ions so that sulfide can have a chance to bond with the metal ions rather than the hydrogen ion. Through this, we can tell if those cations are present due to sulfide precipitates being formed.

ohh ok so it said "it is made alkaline" thanks neo - the explanation really helps this late late study

[aryak]

Hii,
I can't seem to do AAS calculation questions. Is it ok if you could explain to me this question?
A sample of fish was analysed to determine its mercury concentration. A 2.00g of sample was digested and made upto 100 ml of solution. This was then analysed using an atomic absorption spectrometer, and the standard Hg concentration was 0.3 ppm for the dilute sample? Calculate the concentration of mercury in the orignal sample in ppm?

Also, do you by any chance have a list of AAS questions of this type by any chance?

Thank you

[ilovemycat]

hii

i should probs know this but why exactly does celluloses carbon chain structure make it a good alternative for petrochemicals? is it simply the fact that it already contains carbon chains, so its easy to add stuffs to it..?..?

and also in your slides jake,  (a true blessing and gift to this planet - god/other bless your soul!!) you include the history, production, properties and life story of phb but do we need to know alllllllll of that? because its a helluva lot of info

thanks!

mish

[Baylsskool]

Is the whole point behind why we titrate, to get to 7pH and produce water or to reach equivalence point??

[kauac]

Is the whole point behind why we titrate, to get to 7pH and produce water or to reach equivalence point??

Titrations are usually used to find the concentration of an unknown solution. So in most cases, the aim is to reach the equivalence point, as this allows the ratio of moles of acid and base to match the stoichiometric ratio of the equation. The equivalence point is not always at a pH of 7, if the titration involves a strong base and weak acid, or vice versa.

Hope this is alright?

hii

i should probs know this but why exactly does celluloses carbon chain structure make it a good alternative for petrochemicals? is it simply the fact that it already contains carbon chains, so its easy to add stuffs to it..?..?

and also in your slides jake,  (a true blessing and gift to this planet - god/other bless your soul!!) you include the history, production, properties and life story of phb but do we need to know alllllllll of that? because its a helluva lot of info

thanks!

mish

As for your first question, what you have said is correct, as far as I know... Cellulose has a long carbon chain, and can be broken down into glucose by acid hydrolysis, which then ferments with yeast to produce ethanol, which is dehydrated to produce ethene. I think that cellulose having the ability to be turned into ethene is the key point here, as like you said, ethene can then be used to create a range of monomers, thus polymers.

Mod edit: Merged, use the “Modify” button to add replies