HSC Chemistry Question Thread

[Fahim486]

Hey so I was just wondering is the precipitation test only for cations or can it also be or anions? Because recently I did a prac in class to test for anions and so after adding 5 drops of HNO3 followed by 5 drops of Ag+ to my phosphate anion, I got a yellow precipitate but i'm not sure whether that identified my silver cation or phosphate anion.

Cheers!

[MisterNeo]

Hey so I was just wondering is the precipitation test only for cations or can it also be or anions? Because recently I did a prac in class to test for anions and so after adding 5 drops of HNO3 followed by 5 drops of Ag+ to my phosphate anion, I got a yellow precipitate but i'm not sure whether that identified my silver cation or phosphate anion.

Cheers!

Hi!
The precipitation test can be used for both cations and anions.
Cations can be identified using:
-Flame test
-Atomic Absorption Spectroscopy
-Precipitation
Anions can be identified using:
-Precipitation

Precipitating Anions
For example, you have two unknown solutions of sulfate ions and chloride ions but you forgot which is which.
How do you know which anion is present?
Add a barium nitrate solution to the both solutions and the precipitating solution is sulfate. Barium sulfate is INSOLUBLE and will precipitate as a white solid.

Precipitating Cations
Another example, you have two unknown solutions of sodium ions and calcium ions, how do you identify them?
Add a sulfate solution to both and the precipitating solution is calcium. All Group 1 are soluble thus sodium will remain in solution. Calcium sulfate is insoluble.

Your school prac
The solution started as a phosphate solution only I assume.
Adding nitric acid would introduce nitrate ions to the solution, no biggie, all nitrates are soluble.
Adding silver ions would precipitate the phosphate ions because it forms a yellow precipitate. You identified the anion present as phosphate. The silver ion is a known solution that you added, so it was used to identify phosphate not the other way around.

I hope this clears thing up

[Fahim486]

Hi!
The precipitation test can be used for both cations and anions.
Cations can be identified using:
-Flame test
-Atomic Absorption Spectroscopy
-Precipitation
Anions can be identified using:
-Precipitation

Precipitating Anions
For example, you have two unknown solutions of sulfate ions and chloride ions but you forgot which is which.
How do you know which anion is present?
Add a barium nitrate solution to the both solutions and the precipitating solution is sulfate. Barium sulfate is INSOLUBLE and will precipitate as a white solid.

Precipitating Cations
Another example, you have two unknown solutions of sodium ions and calcium ions, how do you identify them?
Add a sulfate solution to both and the precipitating solution is calcium. All Group 1 are soluble thus sodium will remain in solution. Calcium sulfate is insoluble.

Your school prac
The solution started as a phosphate solution only I assume.
Adding nitric acid would introduce nitrate ions to the solution, no biggie, all nitrates are soluble.
Adding silver ions would precipitate the phosphate ions because it forms a yellow precipitate. You identified the anion present as phosphate. The silver ion is a known solution that you added, so it was used to identify phosphate not the other way around.

I hope this clears thing up

That was a great explanation. Thanks so much!!!!

[marioedd]

Need help with part d.
Thanks in advance

[MisterNeo]

(Image removed from quote.)
Need help with part d.
Thanks in advance

Hi!
If the concentration of the sodium chloride was too low, the chlorine gas will not form at the anode but the hydroxide ions will become discharged to give water and oxygen gas. This is because hydroxide is favoured over chlorine as chlorine ions drops in concentration due to reduced sodium chloride.



Also, for Part C, you need 2 chlorine ions as you only wrote one.



Hope this helps!! Sorry for the late reply, I spent like 30mins learning how to type equations

[arunasva]

Ok I am so stuck. How are there 4 sample when there were 3 divisions made ? Overall just stuck man. Please help, Ill be grateful 

[RuiAce]

Ok I am so stuck. How are there 4 sample when there were 3 divisions made ? Overall just stuck man. Please help, Ill be grateful 

The first three are standardized, i.e. those concentrations were known from the start. The other four samples are random samples you've obtained and know nothing about their concentration. There's a difference between known concentrations and unknown concentrations; you don't have to have the same number of known and unknown concentrations.

To draw the calibration curve, you first plot the standard concentration against absorbance (may be other way around; I've forgotten the right order), and draw the line of best fit through only those three points and (0,0). Then, you use your graph to infer the values of A, B, C and D based off the given absorbances.

[legorgo18]

Hello, how do i answer this:

Why is the water flow in microscopic membrane filters parallel to the surface rather than perpendicular?

a) The holes in the polymer membrane are produced at an angle to the surface.
b) In fact, the water flow is perpendicular to increase pressure.
c) It increases the rate of flow.
d) The water pressure would be too high, clogging the pores up.

Tyvm

[Kekemato_BAP]

Could someone explain how to do calibration curves for AAS? This question is new content for me and is confusing. 

[RuiAce]

Could someone explain how to do calibration curves for AAS? This question is new content for me and is confusing. 
(Image removed from quote.)

This same question was posted only a few posts ago. Please check out that post first and then come back for further help

[marioedd]

Hi!
If the concentration of the sodium chloride was too low, the chlorine gas will not form at the anode but the hydroxide ions will become discharged to give water and oxygen gas. This is because hydroxide is favoured over chlorine as chlorine ions drops in concentration due to reduced sodium chloride.



Also, for Part C, you need 2 chlorine ions as you only wrote one.



Hope this helps!! Sorry for the late reply, I spent like 30mins learning how to type equations

Thanks so much

[beau77bro]

im really struggling with monitoring and management because im not sure how much of it to know or not. it seems ridiculous memorise heavy. i have atarnotes, would i be correct in saying i would need to memorise a majority of that info for the course? i need some direction to continue as i am lacking much direction.

[beau77bro]

im really struggling with monitoring and management because im not sure how much of it to know or not. it seems ridiculous memorise heavy. i have atarnotes, would i be correct in saying i would need to memorise a majority of that info for the course? i need some direction to continue as i am lacking much direction. like im reading all of this info and its going in one ear and out the other because i dont know what is actually important.

[MisterNeo]

im really struggling with monitoring and management because im not sure how much of it to know or not. it seems ridiculous memorise heavy. i have atarnotes, would i be correct in saying i would need to memorise a majority of that info for the course? i need some direction to continue as i am lacking much direction.

Hey! Glad you asked!
Monitoring and management can be quite content heavy and requires some degree of memorising but also being able to apply the knowledge.
Things you should memorise/be familiar with
-Solubility rules are a must for identifying cations and anions.
-Flame test colours of the syllabus elements (Ca, Ba, Pb, Fe, Cu, Li, etc)
-Equilibrium conditions of the Haber process. How to maximise yield of ammonia. How to get the reactants. Why does the process have to be carefully monitored. History and Germany uses. Haber process is a big chunk of memorising really
-CFC's, we haven't done this yet, but you need to memorise the layers of the atmospheres and the conditions of each layer. Also equations of ozone depletion by free radical.
-Also, water quality. Turbidity, hardness, etc.

So yeah, the topic does have a lot of memorising involved. Howeve, Band 4-5 students know how to memorise the course whereas Band 6 know how to apply the information effectively. You catch my drift? 
Hope this helps

[seventeenboi]

Hi!
What is the purpose of using standard solutions in atomic absorption spectroscopy???
Thanks