HSC Chemistry Question Thread

[kojijo]

Hello!

Why is it that for precipitation tests the minimum ion concentration should be 0.1 molar? Is it just standard solution?

Thank you!

[Mate2425]

Hey guys, i have a practical chemistry tommorow test on identifying the cations and anions in a solution. We are not using the flame test, but rather the solubility - precipitate test. I was wondering if someone could please help me by offering a few pointers in these focus questions; justify the appropriateness of a particular investigation plan + Evaluate the accuracy and reliability that this could be improved investigation ... relation to the Prac test and AAS.

Thank you. 

[KT Nyunt]

I guess this is a bit late now but you might as well still get a reply

justify the appropriateness of a particular investigation plan

To justify your method you want to think about
- Why use these reagents in this order? The answer would always be: because [insert element here] is the most reactive and would give a positive result for all tests, confusing results, thus this element is tested for first.... (and so on - going to the next element reactive, finishing with the least reactive element)
- Why use certain equipment? For precipitation reactions, the main thing would be: using test tubes infant of a black background - allowing colour changes to be clearly identified.

Evaluate the accuracy and reliability that this could be improved investigation ... relation to the Prac test and AAS.

To evaluate the accuracy of your investigation: (for this you would have to think about how sensitive/precise are you measuring equipment at ever step)
- consider having precipitates in front of a black background to ensure the correct colour is viewed
- consider using test tubes allow the sample to be larger and therefore the result can more effectively be determined
- consider if the first tests match up with the expected results from the confirmatory tests it is likely to be accurate
- consider if equal amounts of reagent were used in each test to determine whether a precipitate formed

To evaluate the reliability of your investigation:
- consider if experiment was repeated
- consider if results were consistent among tests

Accuracy and reliability of AAS:
- accuracy: measures amount of specific metals present in ppm / ppb (hence, it is very sensitive way to measure metals in a substance quantitatively)
- reliability: it is a fast a simple procedure, thus it can be repeated multiple times, an average can be taken - thus it is a procedure that can produce reliable results

Hope this was helpful! (sorry it was a bit late)

[cocopops201]

Hi,

How can I answer this question:
'Justify the procedure you used to prepare an ester in a school laboratory. Include relevant chemical equations in your answer.'

Thank you

[MisterNeo]

Hi,

How can I answer this question:
'Justify the procedure you used to prepare an ester in a school laboratory. Include relevant chemical equations in your answer.'

Thank you

Hi
Esterification is a slow equilibrium reaction between a carboxylic acid and an alkanol. (Include an equation here to show the reversible properties of the reaction) The conditions to produce esters requires high temperatures and a sulfuric acid catalyst to force the equilibrium to the right. However, the alkanols used in the esterification process are volatile and flammable, which means that we cannot use a Bunsen burner to directly heat the chemicals. Hence a hot plate or a water bath should be used to reduce risk. A reflux stand is also used in the experiment because of volatile nature of the chemicals which ensures the chemicals are not lost and allows the equilibrium to take place under higher temperatures to speed up the process.

Hope this helps

[jaredmayo01]

hey im doing an open ended investigation on sulphate in 2 different lawn fertilisers, im just wondering what the difference between quantitative results and calculations are as they are both 3 marks, and what would my observations be in this experiment, thanks!!

[mxrylyn]

Heyy

I'm not sure how to identify the concentration of hydrogen ions and hydroxide ions in pure water at 25 degrees Celsius

And part 2 of the question is to "Define the ionic product"

[cocopops201]

Finding forensic notes are soo hard, does anyone know where to find them or have any?? would appreciate so much 

[mxrylyn]

Finding forensic notes are soo hard, does anyone know where to find them or have any?? would appreciate so much 

Heyy, I don't have any notes for Forensic  Chem, but I did a quick google search for some.

http://www.acehsconline.net/topic-forensic-chemistry/

https://www.scribd.com/doc/7727247/Forensic-Chemistry

https://easychem.com.au/forensic-chemistry/ (click on the dot points and topic areas you want to learn) -  a very good source of HSC Chem information

https://sites.google.com/site/chem4hsc/forensic-chemistr

[cocopops201]

Heyy, I don't have any notes for Forensic  Chem, but I did a quick google search for some.

THANK YOU

[clovvy]

An antacid tablet is known to contain calcium carbonate. To determine the mass of calcium carbonate in the tablet, the following procedure was used.
- The tablet was crushed and then placed in a beaker
- A pipette was used to add 25.0mL of 0.600mol/L HCl to the crushed tablet
- Once the reaction between HCL and calcium carbonate had stopped, phenolphthalein indicator was added to the reaction mixture
- A teflon-coated burette was then used to add 0.100 mol/L sodium hydroxide to the beaker to neutralise the excess HCl
- The phenolphthalein changed from colourless to pink after 14.2 mL of the sodium hydroxide solution has been added

Calculate the mass of calcium carbonate in the original antacid tablet.

[Kate Science]

An antacid tablet is known to contain calcium carbonate. To determine the mass of calcium carbonate in the tablet, the following procedure was used.
- The tablet was crushed and then placed in a beaker
- A pipette was used to add 25.0mL of 0.600mol/L HCl to the crushed tablet
- Once the reaction between HCL and calcium carbonate had stopped, phenolphthalein indicator was added to the reaction mixture
- A teflon-coated burette was then used to add 0.100 mol/L sodium hydroxide to the beaker to neutralise the excess HCl
- The phenolphthalein changed from colourless to pink after 14.2 mL of the sodium hydroxide solution has been added

Calculate the mass of calcium carbonate in the original antacid tablet.

I hope this is a useful model answer to use in future. Just remember that it's always best to identify the exact step in a calculation where you get stumped because then you get help on that bit in particular. These questions are the hardest types of calculation you will encounter in the HSC because it's a combination of gravimetric analysis and titration so there are lots of fiddly steps where it can understandably be easy to make errors. Let me know if there's a section in there that doesn't make sense still or if you need more clarification! 

CaCO3 + 2HCl  CaCl2 + CO2 + H2O
mol HCl initial = 0.025 L * 0.600 mol L-1
      = 0.015 mol
HCl + NaOH  H2O + NaCl
mol HCl titrated = 0.0142 L * 0.100 mol L-1
      = 0.00142 L
mol HCl reacted with CaCO3 = initial – titrated
      = 0.015 – 0.00142
      = 0.01358 mol
 therefore, mol CaCO3 in tablet = 0.5 * mol HCl reacted (molar ratio)
      = 0.5 * 0.01358
      = 0.00679 mol
mass CaCO3 = mol * molar mass
      = 0.00679 mol * (40.09 + 12.01 + 3 * 16.00) mol g-1
      = 0.00679 * 88.10
      = 0.598199 g
      = 0.598 g (3 sigfig)

[lexi24216]

Hi I'm not sure how to work out the heat produced when 1.0L of ethanol undergoes complete combustion, given the molar heat of combustion as 1370KJ/mol and its density 0.80g/mL
Thanks

[justwannawish]

Hello, everyone!

For the first dotpoint of Chemical monitoring the one about a chemist in a named industry, do we need the name of a real-life chemist or is it just a hypothetical role? I had my notes around what an analytical chemist at a consulting company and a production chemist at an ammonia producing factory would do but can't find any real life people to use! Any ideas? Thanks

[KT Nyunt]

Hello, everyone!

For the first dotpoint of Chemical monitoring the one about a chemist in a named industry, do we need the name of a real-life chemist or is it just a hypothetical role? I had my notes around what an analytical chemist at a consulting company and a production chemist at an ammonia producing factory would do but can't find any real life people to use! Any ideas? Thanks

Unfortunately I don't know a named chemist for those roles but yes, you'd need a names chemist. A classic example almost everyone uses is Burhan Gemiknalki and there's info about him on pretty much any HSC site. But I have heard that you can make up a chemist because the main idea is that you name a profession/branch within the world of chemistry and you describe the role accurately - don't take my word for it though, I suggest waiting for a moderator to confirm or just use a real chemist just to be safe