HSC Chemistry Question Thread

[seventeenboi]

hellooo
could someone help me with this question?? thanks the answer is B

[kiwiberry]

hellooo
could someone help me with this question?? thanks the answer is B

Hello!
If a 10 ppm solution has an absorbance of 0.4, then a solution with an absorbance of 0.5 will have a concentration of \(\frac{10}{0.4} \times 0.5=12.5 \text{ ppm} = \frac{0.0125 \text{ g/L}}{63.55 \text{ g/mol}} = 1.966...\times 10^{-4} \text{ mol/L} \) copper ions. Now considering the reaction between copper ions and sodium carbonate:

Here, copper carbonate is the precipitate. We're able to find the number of moles of Cu²⁺ since we have its concentration and volume, and so we're able to find the mass of CuCO₃

[bsdfjnlkasn]

Can someone please help me out with the following question. I've tried looking at limiting reagents but I don't think it's relevant here.

What mass of 1,2 - dibromoethane is produced when 2 L of ethylene gas is reacted with 11.18g of bromine at SLC?

Any help would be greatly appreciated

[kiwiberry]

Can someone please help me out with the following question. I've tried looking at limiting reagents but I don't think it's relevant here.

What mass of 1,2 - dibromoethane is produced when 2 L of ethylene gas is reacted with 11.18g of bromine at SLC?

Any help would be greatly appreciated

Hey!

Whenever the amounts of both reactants are given, the limiting reagent must be found so the correct mole ratio is used. Remember that SLC = 24.79 L
Therefore bromine is the limiting reagent, and we can use the mole ratio to find the mass of C₂H₄Br₂

[bsdfjnlkasn]

Hey!

Whenever the amounts of both reactants are given, the limiting reagent must be found so the correct mole ratio is used. Remember that SLC = 24.79 L
Therefore bromine is the limiting reagent, and we can use the mole ratio to find the mass of C₂H₄Br₂

Thank you - legend!



I just have a few more questions, if that's ok (for anyone to answer)

I sort of understand why the answer here is C, it's the most obvious, but if someone could possibly illustrate the point with a chemical equation that would probably help clarify things further. I'm just not sure which one to write 
The pH of a solution of sodium acetate was measured to be 9.05. Which of the following    statements best explains the measured pH?

   (A) sodium ions donate protons to water
   (B) acetate ions donate protons to water
   (C) sodium ions accept protons from water
   (D) acetate ions accept protons from water


How are we supposed to know this one? It's probably got something to do with the BP's but i'm not sure what

Ethanol (boiling point 78.1°C) and butanoic acid (boiling point 163.5°C)will react under reflux    to produce ethyl butanoate (boiling point 120.0°C). When the reflux mixture is distilled, four    pure liquids are collected in separate beakers labelled 1, 2, 3 and 4 in order of their    collection.
   Which beaker contains the ester?
   (A) 1
   (B) 2
   (C) 3
   (D) 4

Are we supposed to know the entire flowchart for both anion and cation tests?

What would the method for this question be?
The concentration of argon in the atmosphere is 0.93% (v/v). What is the concentration    expressed in ppm?

   (A) 9.3 ppm
   (B) 93 ppm
   (C) 930 ppm
   (D) 9300 ppm

Would it be valid to discuss Pb²⁺ as a product of industrial or agricultural processes and then justify we need to monitor it's levels? The sample answer discusses phosphate but for lead, what agricultural process does it come from?

For the attached question, does the following reasoning sound ok? So if X was able to precipitate with Cu²⁺ then it means that it is more reactive, so higher on the standard potentials? Does it mean that it's a better reducing agent because it's forcing the Cu to oxidise? I'm just not really sure what i'm looking for in this table and how I can use it to answer the question ...


Thanks so much again

[kiwiberry]

I sort of understand why the answer here is C, it's the most obvious, but if someone could possibly illustrate the point with a chemical equation that would probably help clarify things further. I'm just not sure which one to write 
The pH of a solution of sodium acetate was measured to be 9.05. Which of the following    statements best explains the measured pH?

   (A) sodium ions donate protons to water
   (B) acetate ions donate protons to water
   (C) sodium ions accept protons from water
   (D) acetate ions accept protons from water

May be wrong but I think the answer is D? The sodium ion is not able to accept or donate protons because it's a cation. Because acetate is the conjugate base of a weak acid, it tends to accept protons from water, leaving behind hydroxide ions which make the solution basic

How are we supposed to know this one? It's probably got something to do with the BP's but i'm not sure what

Ethanol (boiling point 78.1°C) and butanoic acid (boiling point 163.5°C)will react under reflux    to produce ethyl butanoate (boiling point 120.0°C). When the reflux mixture is distilled, four    pure liquids are collected in separate beakers labelled 1, 2, 3 and 4 in order of their    collection.
   Which beaker contains the ester?
   (A) 1
   (B) 2
   (C) 3
   (D) 4

Distillation separates liquids with different bp's- the mixture is heated and the liquid with the lowest bp evaporates first and is collected first. So the liquids will be collected from lowest to highest bp. Remember that water (100^oC) is also produced! Therefore, the order will be ethanol, water, ethanol butanoate, butanoic acid- the ester is 3

Are we supposed to know the entire flowchart for both anion and cation tests?

I think it should be ok just to know which ions are used to precipitate what, although it may be good to know things like why carbonate is tested first and why sulfate is tested before chloride etc. because I saw a hsc question on that somewhere. You should be fine with that if you know your solubility rules though! Sorry that was a vague answer, but personally I memorised the order the ions were tested in to be on the safe side.

What would the method for this question be?
The concentration of argon in the atmosphere is 0.93% (v/v). What is the concentration    expressed in ppm?

   (A) 9.3 ppm
   (B) 93 ppm
   (C) 930 ppm
   (D) 9300 ppm

0.93% (v/v) refers to 0.93mL/100mL. Multiplying both sides by 10000, we get 9300mL/1000000mL which equals 9300 ppm

Would it be valid to discuss Pb²⁺ as a product of industrial or agricultural processes and then justify we need to monitor it's levels? The sample answer discusses phosphate but for lead, what agricultural process does it come from?

Yes you're able to discuss lead! Not sure about agricultural processes, but for industrial processes lead can be a product of the manufacturing of lead-acid batteries, as well as from old paints.

For the attached question, does the following reasoning sound ok? So if X was able to precipitate with Cu²⁺ then it means that it is more reactive, so higher on the standard potentials? Does it mean that it's a better reducing agent because it's forcing the Cu to oxidise? I'm just not really sure what i'm looking for in this table and how I can use it to answer the question

That's right! So X will be higher on the standard potentials than all the other ions.

Hope this helps

[bsdfjnlkasn]

Thank you so much kiwiberry! I really appreciate your help

For the question on how the pH of a solution of sodium acetate would be affected, the answer was actually D I must have slipped on the keyboard and hit C, but your explanation is really clear. Thanks again for everything , I'll probably be sending a lot more questions through now considering I only recently started revising ... whoops

[bsdfjnlkasn]

Hey there,

I was just wondering why "reduction of sulfur to hydrogen sulphide" is an environmental concern of the Frasch process? Is it because it then oxidises to form sulphurous acid (which then leads to acid rain)? What would the chemical equation for this reduction be?

Also does thermal pollution lead to eutrophication?

Thanks you

[bsdfjnlkasn]

I guess you can talk about hydrogen sulfide, however it gets oxidised into sulfur dioxide and water.

It's these products that then go on to react to form acidic rain. But sulfur dioxide can also be formed if the molten sulfur isn't cooled fast enough. So if you talk about sulfur dioxide, I wouldn't talk about hydrogen sulfide since they both end up as acid rain.
Hydrogen sulfide does smell like crap, and is a very poisonous gas to humans...so you can talk about that as an environmental issue.

With your second question, thermal pollution doesn't directly cause eutrophication (high nutrient levels) Heating water causes decreased gas solubility, which reduces dissolved oxygen levels and kills aquatic life and promotes algae blooms.
If you have time to go into more depth (ie. long response), you can argue that warm water dissolves minerals easier, hence increasing the nutrient levels in waterways. You can also say that the dead fish decompose into nutrients.
Basically, thermal pollution CAN cause eutrophication, but not directly. So I wouldn't make that link if you don't have enough writing space.
Hope this helps

Thanks dude

Was just wondering why increasing temperature (for an exothermic reaction) decreases yield? Surely increasing the number of collusions will allow more to react and so more product to form?

I guess you can talk about hydrogen sulfide, however it gets oxidised into sulfur dioxide and water.

It's these products that then go on to react to form acidic rain. But sulfur dioxide can also be formed if the molten sulfur isn't cooled fast enough. So if you talk about sulfur dioxide, I wouldn't talk about hydrogen sulfide since they both end up as acid rain.
Hydrogen sulfide does smell like crap, and is a very poisonous gas to humans...so you can talk about that as an environmental issue.

With your second question, thermal pollution doesn't directly cause eutrophication (high nutrient levels) Heating water causes decreased gas solubility, which reduces dissolved oxygen levels and kills aquatic life and promotes algae blooms.
If you have time to go into more depth (ie. long response), you can argue that warm water dissolves minerals easier, hence increasing the nutrient levels in waterways. You can also say that the dead fish decompose into nutrients.
Basically, thermal pollution CAN cause eutrophication, but not directly. So I wouldn't make that link if you don't have enough writing space.
Hope this helps

Hey thanks for that

Was just wondering what you would write then for the environmental issues of the Frasch process? Would you mostly be concerned with earth subsidence and the possibility for acid rain to occur? 

Also which equation should I write to show that an increase in heat will decrease gas solubility?

Thanks again

[kiwiberry]

Thanks dude

Was just wondering why increasing temperature (for an exothermic reaction) decreases yield? Surely increasing the number of collusions will allow more to react and so more product to form?
Hey thanks for that

Was just wondering what you would write then for the environmental issues of the Frasch process? Would you mostly be concerned with earth subsidence and the possibility for acid rain to occur? 

Also which equation should I write to show that an increase in heat will decrease gas solubility?

Thanks again

For an equilibrium reaction with an exothermic forward reaction, increasing temperature would increase the rate of both the forward and reverse reactions due to the increased number of collisions, but not the amount of yield. By Le Chatelier's Principle, equilibrium will shift to favour the reaction which decreases the temperature, ie. the reverse endothermic reaction, and as a result yield will decrease.

Yeah I would talk about the subsidence and the fact that hydrogen sulfide and sulfur dioxide are both serious air pollutants, as well as the fact that sulfur dioxide can cause acid rain.

Not sure about the gas solubility but someone else will probably be able to answer that

[bsdfjnlkasn]

Hey there,

Was just wondering how the number of moles for OH^- were calculated? The concentrations and volumes were all substituted correctly but you can see the question below. I'm just not sure why they multiplied it by two? Are they sort of considering the internals mole ratio in Ba(OH)₂? Like how Ba:OH is 1:2? I've seen this a few times and if this is what is happening, I'd love an explanation because I don't really understand what's going on aha and why they do it.

Additionally, could someone explain why the attached method (my working) is wrong? I still got a pH of 13, but on the lower side...



Thanks again!

More questions:

1. How are we to determine which compounds are the most volatile? Can all those that we've encountered be ordered or is there a structural way of deducing this?

2. For the MC attached (Q7), why are A and C wrong? The answer is B. Sorry if this is annoying, but could someone please illustrate with oxidation numbers for the elements involved? I'm still really rusty on it all. Thank you!!

3. How do we figure out Q14?

4. How can we tell from an equilibrium graph whether it is exo/endothermic?

5. How would I do part 3 of the equilibrium constant question? I'm still not really familiar with the ICE method and would appreciate an explanation


THANK YOU

[bsdfjnlkasn]

Hope this helps Let me know if something is unclear!

Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)

Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?

Thanks again for all your help!

[jakesilove]

Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)

Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?

Thanks again for all your help!

An oxidising agent is something that causes another substance to oxidise. Thus, it itself reduces. Does that make sense/answer your first question?

For the equilibrium constant, we just use the chemical equation that MisterNeo posted above. We know that 0.2 moles of CO reacted, therefore we multiple that by 2 to find how much Hydrogen reacted (0.4 moles) and know that 0.2 moles of the product was produced. Since the initial concentration of the product was zero, there is now a total of 0.2 moles of the product. Divide this by the volume to get a concentration, and you're done!

[MisterNeo]

Hey thanks for that! I'm still a bit confused about why D in question 7 is incorrect. Maybe it's because I don't completely understand the meaning of "oxidising agent that undergoes reduction". Isn't oxygen just a spectator because it's oxidation number doesn't change through the whole reaction? I thought an oxidising agent was a species that made something else reduce ... I was never too confident on all the terminology :/ Hopefully you can make sense of my confusion and offer an explanation (don't worry if you can't lol)

Also with calculating the equilibrium constant, I've attached the solution but don't understand how they deduced the equilibrium values for the products - any ideas?

Thanks again for all your help!

No problem
So the definition of a reducing agent is a species that causes the reduction of another, and underoes oxidation itself.
Oxygen is always an oxidising agent because it only has oxidation states of 0 and -2 (reduction is gain).
Hence, the answer can't be D.
If oxygen was a reducing agent, this means that it could undergo oxidation to lose electrons, but it can't (oxygen can't be +1 or +2)
For example, rusting of iron is the oxidation of iron caused by oxygen being an oxidising agent.
It is quite confusing at first, but you'll get the hang of it after a few practice Redox questions.

For the equilibrium question, the K constant is the conditions once it has reached equilibrium.
-You need to write out the reaction equation (see my other answer).
-You first calculate the concentration of each reactant/product initially. (Divide moles by the 5L volume of the chamber- there was no product to begin with since it is initial.)

Same with hydrogen because they both start with 1mol.
-So after the equilibrium has reached equilibrium, 0.2mol of product was formed. You have to find out how many moles of each reactant was used according to the stoichiometry of the equation. Then determine the remaining reactants concentration.
The moles of CO used was 0.2mol (1:1 ratio with methanol).
The moles of H₂ used was 0.4mol (2:1 ratio with methanol)
Now subtract these used values from the initial value.
So at equilibrium, the concentration is:
-CO: (1.0-0.2)/5=0.16mol/L
-H₂: (1.0-0.4)/5=0.12mol/L
-CH₃OH: 0.2/5=0.04mol/L
Now put these values into the K-constant formula (see my other answer), products go on the numerator/reactants on the denominator.



It's better if you express decimal constants in scientific notation.

[jakesilove]

No problem
So the definition of a reducing agent is a species that causes the reduction of another, and underoes oxidation itself.
Oxygen is always an oxidising agent because it only has oxidation states of 0 and -2 (reduction is gain).
Hence, the answer can't be D.

It's better if you express decimal constants in scientific notation.

Why do I even try: MisterNeo, as always you are a legend, and your answer is impeccable