Enrol now for our new online tutoring program. Learn from the best tutors. Get amazing results. Learn more.

May 11, 2021, 11:36:36 pm

### AuthorTopic: VCE Chemistry Question Thread  (Read 1474869 times) Tweet Share

0 Members and 4 Guests are viewing this topic.

#### p0kem0n21

• Trailblazer
• Posts: 39
• Respect: +24
##### Re: VCE Chemistry Question Thread
« Reply #9060 on: February 21, 2021, 08:47:24 pm »
+4
Hey

Would love some help with this question. Answer is stated as B. A step by step would be great!

Recall that pressure and volume are inversely proportional. Thus, if volume increases by a certain factor k, then pressure must decrease by that same factor k. In this question, the volume increases from 2.0 L to 3.2 L. Using simple division (3.2/2.0), we can work out that the volume increases by a factor of 1.6 (i.e. we multiply 2.0 L by our factor of 1.6 to get 3.2 L).

Now we must divide our pressure of 1.1 atm by the factor 1.6. This gives us 0.6875 atm, which we convert to kPa by multiplying by 101.3. This gives us 69.6 kPa (to 3 s.f.).

Note that the extra information about moles of oxygen and temperature are probably there to throw you off and trick you into thinking that PV=nRT is needed.

#### miyukiaura

• Forum Regular
• Posts: 85
• per aspera ad astra
• Respect: +3
##### Re: VCE Chemistry Question Thread
« Reply #9061 on: February 22, 2021, 03:56:07 pm »
0
Do we need to know about natural gas? It's not on the SD but lots of resources mention it.

#### ArtyDreams

• MOTM: Jan 20
• Victorian Moderator
• Forum Obsessive
• Posts: 481
• Fly against the wind. Not with it.
• Respect: +564
##### Re: VCE Chemistry Question Thread
« Reply #9062 on: February 22, 2021, 04:09:35 pm »
+1
Do we need to know about natural gas? It's not on the SD but lots of resources mention it.

Yes - if I remember correctly you need to know briefly about Coal Seam Gas and LPG which are both natural gases.

#### berryland

• Fresh Poster
• Posts: 2
• Respect: 0
##### Re: VCE Chemistry Question Thread
« Reply #9063 on: February 26, 2021, 07:51:14 am »
0
Hey Im cant find the answer for this question, can someone tell me how to do it?

What volume of air (21.0% oxygen, O2, by volume), measured at SLC, would be required to fully combust 68.5 g of C3H8?

thank youuu

#### ArtyDreams

• MOTM: Jan 20
• Victorian Moderator
• Forum Obsessive
• Posts: 481
• Fly against the wind. Not with it.
• Respect: +564
##### Re: VCE Chemistry Question Thread
« Reply #9064 on: February 26, 2021, 11:20:46 am »
+7
Hey Im cant find the answer for this question, can someone tell me how to do it?

What volume of air (21.0% oxygen, O2, by volume), measured at SLC, would be required to fully combust 68.5 g of C3H8?

thank youuu

Hi!

So to figure out this question your first step would be to create a balanced equation of the complete combustion of C3H8.

The equation would look like this [I've emitted states to save time, but please don't forget states in questions].
C3H8 + 5O2 -> 3CO2 + 4H2O

Now, we know that we have 68.5 grams of Propane. Therefore, to find out how much oxygen is needed to react with this amount of propane to make it combust, we need to find out how many mols of Propane we have.

Using n=m/fm
68.5/44 = 1.56mol

Next, we know we need 5 times as much O2 to react with this amount of propane, using the chemical equation. Hence,
n(O2) = 1.56 * 5 = 7.8 mol.

Then, as this question is asking for Volume, we need to find out how much Litres of O2 we need. As the experiment is being conducted under SLC, we use n = V/Vm.

V = 7.8 *24.8 = 193.44 L

Now, finally, we need to find out how much air gives us this much of oxygen. 21% of air is oxygen, so we can form an equation like this and rearrange.

V(air) * 0.21 = 193.44

V = 921.14 L of air.

I hope this helps! Let me know if you have more questions! I've attached my rough working below in case that helps better.

#### Jamie2003

• Fresh Poster
• Posts: 4
• Respect: 0
##### Re: VCE Chemistry Question Thread
« Reply #9065 on: February 26, 2021, 06:34:49 pm »
0
Need serious Chem help...
How do you do percentage efficiency questions in Unit 3 Chemistry? Especially when they give you the %, maybe the mass and it tells you its in SLC.

A certain fuel contains 91% Octane, C8H18, by mass with the remainder being Ethanol C2H5OH.
When 2.50kg of this fuel is completely burnt at 25 degrees celcius and 100kPa, the amount of energy produced, in Megajoules, would be...

#### ArtyDreams

• MOTM: Jan 20
• Victorian Moderator
• Forum Obsessive
• Posts: 481
• Fly against the wind. Not with it.
• Respect: +564
##### Re: VCE Chemistry Question Thread
« Reply #9066 on: February 26, 2021, 07:28:04 pm »
+5
Need serious Chem help...
How do you do percentage efficiency questions in Unit 3 Chemistry? Especially when they give you the %, maybe the mass and it tells you its in SLC.

A certain fuel contains 91% Octane, C8H18, by mass with the remainder being Ethanol C2H5OH.
When 2.50kg of this fuel is completely burnt at 25 degrees celcius and 100kPa, the amount of energy produced, in Megajoules, would be...

Hi!

I've attached my working below (it takes me way too long to write chem on the forums) but I thought I'll supply a few hints for you first! See if you can figure out how to solve this problem with my hints first before looking at my solution

1. This is actually not a percentage efficiency question! Rather, you need to break up your steps to work out the individual energies that each fuel produces when combusted.
2. See if you can work out the mass of each individual fuel we have using the percentage given and the total mass.
3. The experiment is in SLC so that means you can use your data book to find out the Heat of Combustion of the fuel. I'm working in grams below, so I'm going to use the kj/gram value. Alternately, you can convert the masses into mols and use the kj/mol value.
4. Once you find out the energy released by the individual fuels, add them together to get the total.

I hope this helps a bit! My solution is below if you get stuck. Let me know if you have further questions!!

#### amanaazim

• Forum Regular
• Posts: 80
• Respect: 0
##### Re: VCE Chemistry Question Thread
« Reply #9067 on: March 04, 2021, 09:29:49 pm »
0
if a question asks to write down the thermochemical equation example ethanol do we assume that it is a complete combustion reaction and the products are carbon dioxide and water?

#### amanaazim

• Forum Regular
• Posts: 80
• Respect: 0
##### Re: VCE Chemistry Question Thread
« Reply #9068 on: March 04, 2021, 09:47:22 pm »
0
hey guys i need help with this question

What is the net volume of carbon dioxide released when 5.00 MJ of
energy is obtained from the combustion of ethanol?

#### ArtyDreams

• MOTM: Jan 20
• Victorian Moderator
• Forum Obsessive
• Posts: 481
• Fly against the wind. Not with it.
• Respect: +564
##### Re: VCE Chemistry Question Thread
« Reply #9069 on: March 04, 2021, 10:07:19 pm »
+5
if a question asks to write down the thermochemical equation example ethanol do we assume that it is a complete combustion reaction and the products are carbon dioxide and water?
Ideally a question would specify if its a complete combustion reaction or not (i.e. write down the thermochemical equation for the complete combustion of ethanol) however, for VCE chemistry purposes it is okay to assume that it is a complete combustion with the products being carbon dioxide and water.) The data book values for the heat of combustion of the all the substances assume those products.

hey guys i need help with this question

What is the net volume of carbon dioxide released when 5.00 MJ of
energy is obtained from the combustion of ethanol?
I'll give you a few hints to help solve this! Let me know if they still don't help and I'll put up a more detailed solution.

1. Start by writing a thermochemical equation for the combustion of ethanol.
2. Convert the 5MJ to KJ = you'll get a big number but it will be easier to work with as the Heat of Combustion values from the data book are in Kj/mol. (you could do this the other way around too)
3. Work out the amount of mols of ethanol required to be combusted to release this amount of energy. You can do this via the formula DeltaH = q/n
4. Use stoichiometric ratios to find out the mol of CO2 released when that amount of mols of ethanol is combusted.
5. The heat of combustion values from the data book assume the experiment was conducted under standard conditions. Therefore you can use the formula n=v/vm to find out the volume of CO2 released!

Hope this helps

#### wingdings2791

• Trailblazer
• Posts: 43
• I'm going back to the start
• Respect: +29
##### Re: VCE Chemistry Question Thread
« Reply #9070 on: March 05, 2021, 09:17:28 pm »
0
Hi everyone! My practical report SAC is coming up soon and I am absolutely lost on how to approach the discussion. The aim of the experiment is to order six unknown half-cells (A-F) by setting up a galvanic cell for every combination of half-cells and recording the voltages produced.

I think I understand the theory behind this practical. However, I don't at all know how I should present my results (currently it's in a two-way table, no graph), and don't know how to write 'observations' (of the data- the rubric says that this should 'compare and contrast the collected data'). I'm also not sure how to separate observations from 'links to chemical concepts', as it seems very difficult to explain the data without references to theory.

I've attached my results for reference. Thanks so much in advance for any help!
2019- Chinese SL [42]
2020- Biology [43] Music Performance [49]
2021- Chemistry, Methods, English Language

#### Bri MT

• VIC MVP - 2018
• ATAR Notes Legend
• Posts: 4578
• invest in wellbeing so it can invest in you
• Respect: +3534
##### Re: VCE Chemistry Question Thread
« Reply #9071 on: March 06, 2021, 08:21:53 am »
+3
Hi everyone! My practical report SAC is coming up soon and I am absolutely lost on how to approach the discussion. The aim of the experiment is to order six unknown half-cells (A-F) by setting up a galvanic cell for every combination of half-cells and recording the voltages produced.

I think I understand the theory behind this practical. However, I don't at all know how I should present my results (currently it's in a two-way table, no graph), and don't know how to write 'observations' (of the data- the rubric says that this should 'compare and contrast the collected data'). I'm also not sure how to separate observations from 'links to chemical concepts', as it seems very difficult to explain the data without references to theory.

I've attached my results for reference. Thanks so much in advance for any help!

Did you run any replicates? (I can't see replication from the table)

If you need to have a graph you could change the two-way table into a clustered bar chart.

Remember that explaining the results belongs in the discussion; the results section is just for saying what the results are.

#### miyukiaura

• Forum Regular
• Posts: 85
• per aspera ad astra
• Respect: +3
##### Re: VCE Chemistry Question Thread
« Reply #9072 on: March 13, 2021, 02:52:46 pm »
0
Which is more energy efficient - fuel cells or galvanic cells?

#### lm21074

• MOTM: JAN 19
• Victorian Moderator
• Forum Obsessive
• Posts: 436
• Respect: +473
##### Re: VCE Chemistry Question Thread
« Reply #9073 on: March 13, 2021, 08:48:19 pm »
+2
Which is more energy efficient - fuel cells or galvanic cells?
Galvanic cells are more energy efficient (60-90% efficient compared to fuel cells, being 40-60% efficient).

Note that fuel cells can be up to 85% efficient if heat produced by the cell is also used to generate electricity.

#### ali6352

• Fresh Poster
• Posts: 1
• Respect: 0
##### Re: VCE Chemistry Question Thread
« Reply #9074 on: March 15, 2021, 08:01:18 pm »
+1
Hi
I have a SAC coming up about Le chateliers principle and equilibrium where I need to create a mind map linking all the concepts. Can someone help?