HSC Chemistry Question Thread

[bsdfjnlkasn]

Thank you SO much Jake!! I understood everything

I have one more question (attached). Do you have any suggestions for tackling pH questions or are they really not that hard

[jakesilove]

Thank you SO much Jake!! I understood everything

I have one more question (attached). Do you have any suggestions for tackling pH questions or are they really not that hard

Most of the time, pH questions are just simple application of the log formula. However, sometimes they get trickier, like the question you attached. In those cases, just slowly work your way through, finding out any information you can. Then, see if you can string it together to form an answer.

First, we need a chemical equation



Clearly, the reaction is 1:1. Now, the moles of each substance is




Clearly, HCl is the limiting reagent. All of the acid will react, leaving



moles of the base left. We need to find the concentration of the Hydroxide ions, which will be

[bluecookie]

What are the indicators suitable for these titrations:
b) 0.2M potassium hydroxide and 0.15M acetic acid.
c) 0.25M butanoic acid and 0.20 sodium carbonate
d) 0.2M hydrochloric acid and 0.2M sodium carbonate

[Annie657]

Hi, just wondering what saturated and unsaturated means in regards to alkenes and alkanes, because I keep using them as words but dont actually know what they mean! 
Thankyou

[Bubbly_bluey]

Hi, just wondering what saturated and unsaturated means in regards to alkenes and alkanes, because I keep using them as words but dont actually know what they mean! 
Thankyou

Hey! I have a pretty "dumb" way (if you like to call it) of remembering it. So alkanes have more hydrogens bonding to the carbons because of single bonds. So they are kind of like "saturated with hydrogens" . Chemically I believe it means that since that all carbons are filled with single bonds, they dont have the potential to react with other compound (ie. all the bonds are full)
Alkenes have less hydrogens because of double bonding. They can react with other compounds by breaking up that bonding.
Not sure if this is the best way to explain it, but I hope it helps 

[jakesilove]

What are the indicators suitable for these titrations:
b) 0.2M potassium hydroxide and 0.15M acetic acid.
c) 0.25M butanoic acid and 0.20 sodium carbonate
d) 0.2M hydrochloric acid and 0.2M sodium carbonate

Hey! You generally ignore the concentration of the solution, and just look at the acid/base used.

If a strong acid and a strong base is used, then the appropriate indicator is Bromothymol Blue, which has an end point around a pH of 7

If a strong acid and a weak base is used, then the appropriate indicator is Methyl Orange, which has an end point around a pH of 4-6

If a weak acid and a strong base is used, then the appropriate indicator is Phenophthalein, which has an end point around a pH of 8-10

[shreya_ajoshi]

Hi,
how would you do this?
If 1kg of 10% solution of glucose in water in apparatus weighing 350g was completely fermented, and any gas formed allowed to escape, by what amount would the mass of the apparatus and its contents decrease? What percentage decrease does this correspond to?

[shreya_ajoshi]

Hi how would you answer these?
The answers don't have sample answers.
thank you

[jakesilove]

Hi,
how would you do this?
If 1kg of 10% solution of glucose in water in apparatus weighing 350g was completely fermented, and any gas formed allowed to escape, by what amount would the mass of the apparatus and its contents decrease? What percentage decrease does this correspond to?

Hey! First, let's look at the chemical reaction for fermentation.



Assuming that the reaction goes to completion (which, in reality, it won't. Remember that the yeast dies at 15% concentration of Ethanol), all Glucose will be converted into Ethanol and Carbon dioxide.

We know we have 100g of Glucose. In moles, this is



So, this is how many moles of Glucose reacts. Thus, as the reaction is 1:2:2, 0.55*2=1.10 moles of Carbon dioxide is produced. This gas is LOST! So, we can find the mass of the Carbon dioxide, and subtract that from the total mass to get our answer.

[Fahim486]

Hi everyone, I'm having trouble understanding these questions so could someone pls explain them.
Thanks!

[Fahim486]

Sorry and I'm also having trouble answering this question.
Cheers!

[jakesilove]

Hi how would you answer these?
The answers don't have sample answers.
thank you

Hey! I'll just do one as an example, and you can use my response to work out the answers to the others. Let's look at petrol.

We have kJ/mol, and we want kJ/g. This is pretty easy, if you think of the question in terms of units. Remember that



Therefore,



I usually think about this by asking whether I expect the number to get smaller or larger. Clearly, the energy released per gram will be less than the energy released per mole. Thus, we divide.

Now, the find the kJ/L, we use the answer we just found. Remember,



So, if one gram of Petrol releases 48.2kJ of energy, and there are 690g of petrol in a litre, then



Easy! Use the same working out for the rest.

In discussing the potential use of a fuel source as an alternative to petrol, first and foremost is renewability. Due to the limited nature of petrochemicals, being able to produce substances such as Ethanol is very important.

Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.

[jakesilove]

Hi everyone, I'm having trouble understanding these questions so could someone pls explain them.
Thanks!

Hey! The first question is sort of just a by-definition one; radioactivity is when elements shed electrons/helium atoms due to an instability in the nucleus. This can occur because of three reasons; One, the ratio of neutrons to protons is too high. Two, the ratio of neutrons to protons is too low. Three, there are just too many neutrons and protons. Thus, the answer is definitely D.

Question 9 is the same; the salt bridge is literally used just to complete the circuit. Remember from like year 8 science that, if you want a circuit to work, it needs to be a complete loop. Thus, the electrons might flow one way (through the external circuit), but the 'left over' electrons/ions need some way to continue to flow. Thus, we use a salt bridge soaked in a conductive substance.

For your last question, looks like you need to do some research on radioactive isotopes. The questions you need to ask yourself are;

- Is a half life of 12 days long or short? If it is long, then you don't want it in the human body (as it will continue to damage tissue for ages!). If it is short, it is likely suitable.
- What is beta decay? Is it harmful to the human body?
- What is low energy gamma radiation? Is it harmful to the human body?

As this is only a two marker, a very brief over view of the above points is likely enough. Generally, however, isotopes used for medicine have a much shorter half-life (ie. 6 hours), and will only emit very low energies of one type of radiation. So, this substance is unlikely to be suitable for medical purposes.

[shreya_ajoshi]

Hey! First, let's look at the chemical reaction for fermentation.



Assuming that the reaction goes to completion (which, in reality, it won't. Remember that the yeast dies at 15% concentration of Ethanol), all Glucose will be converted into Ethanol and Carbon dioxide.

We know we have 100g of Glucose. In moles, this is



So, this is how many moles of Glucose reacts. Thus, as the reaction is 1:2:2, 0.55*2=1.10 moles of Carbon dioxide is produced. This gas is LOST! So, we can find the mass of the Carbon dioxide, and subtract that from the total mass to get our answer.

Thanks!!!

[shreya_ajoshi]

[/quote] Then, look to the table. Does the fuel source provide as much energy per gram as Petrol? How about energy per litre? Will cars need to fill up with more fuel if they use a non-petrol substance? Is the substance even a liquid? How can it be stored? Will it corrode the engine? Those are the sorts of discussion points that will get you full marks.
[/quote]

Thanks so much
So could I say that Ethanol  and Kerosene do not provide as much energy per gram but hydrogen does. Only kerosene  releases as much and more energy  per Litre. If cars are to use these fuels, they would need to use them such as Ethanol as a petrol extender. Hydrogen is not even liquid hence will be very hard to contain and use as a fuel. When the ethanol is subjected to fractional distillation after fermentation, it will still have water in it and this will corrode the engines. Hence, Kerosene and ethanol have the most potential as alternative fuels based on their physical and chemical properties. However, Kerosene is obtained from crude oil which is a non-renewable resource, hence its supply is slowly decreasing. Hence the only alternative left is ethanol which is derived from a renewable resource (biomass) but it does have disadvantages as the engines will need modification.
I know it's a messy response but I'm just making sure my ideas are correct and then I would structure it.