HSC Chemistry Question Thread

[f_tan]

Hi! Could someone please explain how to answer these questions? Thanks!

1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L

[Natasha.97]

Hi! Could someone please explain how to answer these questions? Thanks!

1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L

Hi!
1) Write out a BALANCED chemical equation (important)
2) Calculate the moles of HCl using c = n/v (make sure that volume is in litres!)
3) Calculate moles of magnesium carbonate, and determine the LIMITING reagent (i.e. is there sufficient A to react with B)
4) Calculate the moles of CO₂ produced using limiting reagent
4) Refer to data sheet for molar volumes of gas (I think for this particular Q, it is 22.71L for 1 mole of gas)
5) Multiply the value calculated in step 4 with 22.71, and you should get the answer

Hope this helps!

Note: Not 100% sure if this is correct, can anyone else confirm?

[MisterNeo]

Hi! Could someone please explain how to answer these questions? Thanks!

1. 5.00 g of magnesium carbonate has reacted with 25.0 mL of 0.500 M HCl. The volume of CO2 produced at 0 degrees C and 100 kPa is closest to:
A) 0.142 L
B) 0.155 L
C) 0.284 L
D) 1.35 L

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Hey!
The first thing you should always do is write out the chemical equation.

This requires you to find the limiting reagent (the one that runs out first).
So you find the moles of magnesium carbonate used by dividing mass by molar mass.
5 / 84.31 = 0.0593mol
And you find the moles of the acid by multiplying the concentration by the volume.
0.5 x 0.025 = 0.0125mol
^This is the limiting reagent since it has less moles than the other reactant.
So there will be 0.00625mol of CO₂ because of the 1:2 mole ratio.
To find volume of gas, multiply the moles of gas by 22.71L at 0'C
0.00625 x 22.71 = 0.142L
Thus the answer is A.

For the galvanic cell, you'd refer to the reduction table for the nickel potential (-0.25 negative means it prefers oxidation over reduction).
Chlorine's potential is (+1.4 positive means it prefers reduction over oxidation).
Anode: Oxidation
Cathode: Reduction
Thus, the anode is nickel, and the cathode is chlorine gas at the inert platinum electrode.
It will flow from nickel to chlorine because it always flows from anode to cathode across the external wire.

The chlorine ions in solution do not produce a gas because it does not lose electrons, but nickel does.
The E* value is just nickel plus chlorine (remember to multiply nickel by -1 since it undergoes oxidation.
E*=0.25+1.4=1.65V (spontaneous reaction coz it's positive)
The colour change observed would be that the anode cell (nickel) becomes more green as more ions of nickel are produced.
Hope this helps

[f_tan]

Can anyone explain how you get the answer to these two questions?
Thank you!

[Graceful]

Hey Jake, thanks for all your help,
What is the monomer for PHB?

[kiwiberry]

Can anyone explain how you get the answer to these two questions?
Thank you!

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Hello!
Remember that for condensation polymerisation, both monomers must have a functional group on either end. B is the only option which has monomers which fit this criteria - the first monomer has an OH group and the second one has a COOH group, so this is the answer. Ethene and chloroethene don't have a functional group and instead undergo addition polymerisation.
13) seems odd, I'm pretty sure they're all conjugate bases of weak acids, unless I'm missing something

[kiwiberry]

Hey Jake, thanks for all your help,
What is the monomer for PHB?

Hey! The monomer for PHB is 3-hydroxybutanoic acid

[winstondarmawan]

Hello! Just a few questions on conductivity of ions.
Do both anions and cations allow for the conduction of electricity?
Does the extent of conductivity depend entirely on the charge of the ions? Or do other factors such as molar mass play a role.
EDIT: Also any tips for practical exams (especially titration) are appreciated!
TIA

[seventeenboi]

HELLO COULD SOMEONE PLS EXPLAIN THIS

[MisterNeo]

HELLO COULD SOMEONE PLS EXPLAIN THIS

The answer would likely be A.


It doesn't fully ionise that second hydrogen, so it would release less than the hydrochloric acid even though the concentrations are 1:2.

Hello! Just a few questions on conductivity of ions.
Do both anions and cations allow for the conduction of electricity?
Does the extent of conductivity depend entirely on the charge of the ions? Or do other factors such as molar mass play a role.
EDIT: Also any tips for practical exams (especially titration) are appreciated!
TIA

It's mainly the concentration of ions in solution that impact the electrical conductivity. The current is carried by both cations and anions (all charged particles) because you can't have a solution of only cations and no anions, vice versa.
For titration pracs, I'd recommend revising the roles of each equipment and the rinsing procedure.
This is a decent guide to titration methods.
Knowing how to calculate the concentration is vital because that's the whole point of the experiment.

[J.B]

"A white crystalline substance is found on a shelf in a chemical storeroom, however the label is unreadable. It is suspected to be either lead(II) carbonate, barium carbonate or barium chloride.

Outline a procedure that could be used to positively identify this substance."

In this question does the fact that it's a crystalline substance make a difference to the tests you would use?

[MisterNeo]

"A white crystalline substance is found on a shelf in a chemical storeroom, however the label is unreadable. It is suspected to be either lead(II) carbonate, barium carbonate or barium chloride.

Outline a procedure that could be used to positively identify this substance."

In this question does the fact that it's a crystalline substance make a difference to the tests you would use?

Hey!
If it's a crystalline substance, it means that it is a solid salt not yet in contact with water.

This would be my method:
Barium chloride (the only soluble salt possibility) can be ruled out if the solid does not dissolve when placed in water. (Utilise the info given)
So you're left with either lead/barium carbonate...
Add HCl to decompose the carbonate to leave behind barium chloride or lead chloride.
Lead chloride is insoluble, barium chloride is not.
Write this into a flowchart with those (Yes?/No?) arrows and fancy boxes.

ps. I would avoid mentioning flame test because lead ion vapour is toxic.

[elizag]

Hey, I am currently preparing for my trial Chemistry exam, and was just wondering which questions are considered to be the common extended responses
Any guidance would be appreciated! Thank you

[MisterNeo]

Hey, I am currently preparing for my trial Chemistry exam, and was just wondering which questions are considered to be the common extended responses
Any guidance would be appreciated! Thank you

Based on the past papers I have done, the most common long responses are those on the syllabus with a high-order verb.
"Assess the potential of ethanol as an alternative fuel..."
Look for the dotpoints in the syllabus that begin with "analyse", "assess", "discuss".
I would write a good response for each of these to prepare for any long response they might throw at you.

[phoebegresham]

Hey! When writing a molecular formula, the general formula for alkanes is C(n)H(n+2) and for alkenes C(n)H(2n), isn't it? When something is cyclo, does it change it to alkanes: C(n)H(2n) and alkenes: C(n)H(2n-2)? Very confused  thank you!!