HSC Chemistry Question Thread

[MisterNeo]

I found three isomers so far
1-butanol
2-butanol
2-methyl-propan-2ol
i'm not sure what the fourth isomer is

2-methylpropan-1-ol

[Mathew587]

Hey guys,
Anyone know how to this q- http://imgur.com/a/T2SuY
the answer is in bold.
ty

[MisterNeo]

Hey guys,
Anyone know how to this q- http://imgur.com/a/T2SuY
the answer is in bold.
ty

Hey!
Firstly, write out the equation.

You need to find the moles of both HCl and NaOH (conc. times volume), and identify the limiting reagent.
n(HCl)= 0.0025mol
n(NaOH)= 0.00318mol
Since HCl is the limiting reagent (1:1 mole ratio), you subtract it from NaOH to find excess NaOH.
n(NaOH)= 0.00068mol
Now you divide these remaining moles by the total volume (0.055L) for the concentration.
Then use the pH formula with this conc. and subtract it from 14.
pH= 12.1

[Mathew587]

Thanks for that
I didn't know we had to convert the moles into conc. before using the ph formula lolol. Saved me

[Aaron12038488]

is a milky precipitate same as a cloudy ppt?

[Natasha.97]

is a milky precipitate same as a cloudy ppt?

Hi! I think it is

[Jenny_trn]

Hi, what would be the answer to the following question and why?

Which of the following substances is best analysed by atomic absorption spectroscopy (AAS)?
(A) Calcium
(B) Iodine
(C) Nitrogen
(D) Silicon

Thanks!

[MisterNeo]

Hi, what would be the answer to the following question and why?

Which of the following substances is best analysed by atomic absorption spectroscopy (AAS)?
(A) Calcium
(B) Iodine
(C) Nitrogen
(D) Silicon

Thanks!

Hey!
AAS is used to detect concentrations of metal ions.
Iodine, nitrogen, and silicon are not metals.
Calcium is a metal, so the answer is A.

[Jenny_trn]

Hi ! I'm not sure how to do this;

Solutions containing copper ions were analysed by AAS. A standard solution of 10ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentration was found to have an absorbance of 0.500.

100mL of this second solution was reacted with excess sodium carbonate solution. The precipitate was then dried and weighed.
What mass of precipitate was formed?

(A) 1.25 x 10^-3] g
(B) 2.45 x 10^-3 g
(C) 1.54 g
(D) 2.43 g

Thanks!

[kiwiberry]

Hi ! I'm not sure how to do this;

Solutions containing copper ions were analysed by AAS. A standard solution of 10ppm copper had an AAS absorbance of 0.400. A second solution of unknown concentration was found to have an absorbance of 0.500.

100mL of this second solution was reacted with excess sodium carbonate solution. The precipitate was then dried and weighed.
What mass of precipitate was formed?

(A) 1.25 x 10^-3] g
(B) 2.45 x 10^-3 g
(C) 1.54 g
(D) 2.43 g

Thanks!

Hey!
If a solution with concentration 10 ppm has absorbance 0.4, a solution with absorbance 0.5 will have concentration \(\frac{10}{4} \times 5 = 12.5 ppm = 0.0125g/L\). Dividing by the molar mass of copper, this concentration equals 1.96.. x 10^-4 mol/L. Now, writing out the reaction between copper ions and sodium carbonate:
Copper carbonate is the precipitate here. We can use c=n/v to find the moles of copper, use the mole ratio to find moles of CuCO₃ and then find its mass
Strange, I might have made a calculation error somewhere but I guess the closest option is B?

[Jenny_trn]

Hi! currently stuck on part (b) of this question

ty

[Natasha.97]

Hi! currently stuck on part (b) of this question

ty

Hi!

1) Addition of excess HNO₃ ensures that all CO₃^2- ions have been taken out of the solution
(CO₃^2- + 2H⁺ → CO₂ + H₂O)

2) Barium is usually used to precipitate sulphate ions to become BaSO₄, which is white. Barium chloride is soluble, so none of the precipitate formed would be barium chloride.
(Ba²⁺ + SO₄^2-  → BaSO₄)

3) Silver chloride is used to precipitate chloride ions to become AgCl, which is also white. Silver sulphate (Ag₂SO₄) is insoluble, so if silver was added first, the white precipitate may contain samples of both substances (silver chloride and silver sulphate), rendering the sample invalid.
(Ag⁺ + Cl^-  → AgCl)

Hope this helps

Edit: Net ionic equation for first step (had an equation involving sodium nitrate for some reason!)

[beau77bro]

why is the ground based spectrometer for monitoring O3 concentrations limted by pointing directly overhead? the books all say since it is constantly changing but what does that mean?

What is an Ozone measuring instrument? why did it replace TOMS

[bsdfjnlkasn]

Hey there!

I was wondering if someone could please help me find the concentration of chloride ions in the original undiluted seawater for the following method? The average titre was found to be 12mL

1.   Dilute estuary water by pipetting a 20 mL sample into a 100 mL volumetric flask and making it up to the mark with distilled water.
2.   Pipette a 10 mL aliquot of diluted estuary water into a conical flask and add about 50 mL distilled water and 1 mL of chromate indicator. The solution should be pale yellow, at this stage.
3.   Titrate the sample with 0.1 mol L−1 silver nitrate solution. The silver chloride that forms will be a white precipitate. The endpoint of the titration is identified as the first appearance of a red-brown colour of silver chromate. It is a good idea to perform a rough titre, so you know what the end point should look like. Your teacher has put colour photos on your lab tray to help.
4.   Repeat the titration with further aliquots of diluted estuary water until concordant results (titres agreeing within 0.1 mL) are obtained.

THANK YOU

[bsdfjnlkasn]

why is the ground based spectrometer for monitoring O3 concentrations limted by pointing directly overhead? the books all say since it is constantly changing but what does that mean?

What is an Ozone measuring instrument? why did it replace TOMS

Hey there,

I believe for that outcome we're only required to know the method of measurement and a brief description of how it works. Not so much the limitations of the different technologies/progression of their use. I don't have these in my notes and after comparison haven't needed to know the specifics for TOMS and Ground-based Dobson U.V. Spectroscopy. Would love to hear other people's opinions though!