HSC Chemistry Question Thread

[Mate2425]

Thank you kiwiberry, very helpful much appreciated! 

Hi!! So the equation for fermentation is
The mass change is due to the CO2 released, so 3.33g of CO2 was released. Now that we have the mass, we can find moles using n=m/M:
and then the volume using n=v/MV (assuming this is at 0^oC so MV=22.71, if it's at 25^oC then MV=24.79)
So the volume of CO2 released is 1.72L. Hope that helps!

[Gkumar]

hey could you help me with these questions?

[Lear]

a) N (g) + O2 (g) ---> NO2 (g)
b) n (N) = m/M
=  25 / 14.07
= 1.78... moles
since stoick ratio is 1:1:1
n (NO2) = m /m
m = nM
= 25 x 1.78...
= 44.42 g
c) n (NO2) = V / Vm
1.78... = V / 24.79
V = 44.05 L

I believe nitrous oxide is actually N20 but might be incorrect.

[clovvy]

I believe nitrous oxide is actually N20 but might be incorrect.

You are right it is N2O, the whole calculations got messed up because it is a wrong compound and NO2 is nitrogen dioxide..
Therefore the calculations should be:
a) 2(N2)(g) + O2(g) --> 2N2O (nitrogen element don't exist by itself, just like other halogens)
b) ratio 2:1->2
nitrogen: 25/14.007=1.78482.... mols...
therefore the mass of N2O to be produced are: 1.78482 x (16 + 14.007 x 2) = 78.557 grams
c) At 25°C and 100 kPa, the molar volume of all gases is 24.79 L
 therefore V= 1.78482 x 24.79 = 44.22 L (2 d.p.)...

Anyway that is how the question is done

[clovvy]

Hi, I need some help figuring out my independent variable (not sure if more than one is allowed) for my chemistry experiment.

Heading: Identifying and comparing the properties of particular household emulsions.
Aim: To compare the differences in properties for different types of emulsions (oil-in-water emulsions and water-in-oil emulsions) in everyday household products.

So I'm testing five properties on five different emulsions (yes, that's 25 trials without repetition yet) at home (so no fancy chemistry lab equipment).
Emulsions: Mayonnaise, Salad dressing, Ice-cream, Butter, Hand cream (Nivea)

Property 1 is just the physical feel of the emulsion on the skin... so I'm basically feeling the coolness or greasiness of the emulsion (dependent variable) which helps me differentiate between o/w and w/o emulsions. But what would be my independent variable here?

Skipping down to property 3 regarding the stability of the emulsions. I put equal amounts of each emulsion onto separate plates and left it in the sun for a few hours. Oil-in-water emulsions are less stable since some of their higher water proportion evaporates off first. Anyway, what would be my independent variable here?

Also, I'm not sure if I'm meant to do an independent variable for each property or one independent variable applicable to all properties (if it works for these 2 properties, it should work for the rest).

[PS: Sorry this post doesnt have that much structure.. I guess it represents my confusion about this task..]

Thanks in advance. 

Sorry to say this but the way you described the experiment is all over the place and it is very confusing to read, generally for sciences you need to write down your procedures very clearly (what are your steps for the experiment, what is your hypothesis, records of results).... Clearly yours is lacking this... What is property 1,2 and 3? Please lay it out properly next time... (Title, aim- that part is ok until the next bit- hypothesis, experiment procedures, results, conclusion, discussions i.e. risk assessments etc)

Anyway straight to the point, for property 1, I would say the substance you are using (oil and water) and I don't know what your aim is because you are very very vague with your explanations.... Property 3 I would say the time is the independent variable because you are looking at change in something overtime (again I don't understand what you are doing)... What are you refering to with property 2? If you don't know what independent variable is, in science (not just chem) it is basically a variable that is controlled or changed to test the effects of the dependent variable

I would suggest that you try to change your lay-out for the  experiment and make it neater and clearer so you know what you need to do, what to do etc so all the information is properly set instead of having them all over the place which makes it confusing... If unsure feel free to ask anyone in this forum (the forum exist for that reason anyway so don't stress too much)

[arii]

Sorry to say this but the way you described the experiment is all over the place and it is very confusing to read, generally for sciences you need to write down your procedures very clearly (what are your steps for the experiment, what is your hypothesis, records of results).... Clearly yours is lacking this... What is property 1,2 and 3? Please lay it out properly next time... (Title, aim- that part is ok until the next bit- hypothesis, experiment procedures, results, conclusion, discussions i.e. risk assessments etc)

Anyway straight to the point, for property 1, I would say the substance you are using (oil and water) and I don't know what your aim is because you are very very vague with your explanations.... Property 3 I would say the time is the independent variable because you are looking at change in something overtime (again I don't understand what you are doing)... What are you refering to with property 2? If you don't know what independent variable is, in science (not just chem) it is basically a variable that is controlled or changed to test the effects of the dependent variable

I would suggest that you try to change your lay-out for the  experiment and make it neater and clearer so you know what you need to do, what to do etc so all the information is properly set instead of having them all over the place which makes it confusing... If unsure feel free to ask anyone in this forum (the forum exist for that reason anyway so don't stress too much)

Hi clovvy, thanks so much for the fast reply. I wrote up a thorough explanation of what I was doing so hopefully that helps. If it still doesn't, I might just paste in my complete method on the docs. https://docs.google.com/document/d/1EPvqE_DqAa8UYyYg60mEwfdktmx-J8V8HWU-BmDWDBQ/edit?usp=sharing

[clovvy]

Hi clovvy, thanks so much for the fast reply. I wrote up a thorough explanation of what I was doing so hopefully that helps. If it still doesn't, I might just paste in my complete method on the docs. https://docs.google.com/document/d/1EPvqE_DqAa8UYyYg60mEwfdktmx-J8V8HWU-BmDWDBQ/edit?usp=sharing

Much better than before, however still have a lot to improve (it's ok, I have failed a prac task before so personally I know the pain)... You can't just jump into results without showing the procedures of the prac because it kinda look like you don't really know what you are doing.... You are not quite there yet but I can see some improvements which is good... I know that you are afraid of plagiarism so it make sense if you do not want to post your actual work there (I know)... To give you a rough idea on what your report should look like, read this https://www.adelaide.edu.au/writingcentre/docs/new_practical_report_guide.pdf
or https://sjesci.wikispaces.com/file/view/prac_report_proforma.png/187658559/682x1048/prac_report_proforma.png

[arii]

Much better than before, however still have a lot to improve (it's ok, I have failed a prac task before so personally I know the pain)... You can't just jump into results without showing the procedures of the prac because it kinda look like you don't really know what you are doing.... You are not quite there yet but I can see some improvements which is good... I know that you are afraid of plagiarism so it make sense if you do not want to post your actual work there (I know)... To give you a rough idea on what your report should look like, read this https://www.adelaide.edu.au/writingcentre/docs/new_practical_report_guide.pdf
or https://sjesci.wikispaces.com/file/view/prac_report_proforma.png/187658559/682x1048/prac_report_proforma.png

I'm planning out the experiment (which is the bit before actually doing the experiment and writing up the report). It needs me to figure out my variables, so what do you think my independent variable would be? I'm not sure if it is time or the ratio (of oil and water in the emulsions tested).

(By the way, this is in the option topic. Industrial Chem)

[clovvy]

I'm planning out the experiment (which is the bit before actually doing the experiment and writing up the report). It needs me to figure out my variables, so what do you think my independent variable would be? I'm not sure if it is time or the ratio (of oil and water in the emulsions tested).

(By the way, this is in the option topic. Industrial Chem)

I am not doing Industrial Chem yet (your school did it a different way which is fine)... After reading your stuff over and over again I think the independent variable would be the time and you will need different values or graphs to record your result. Ratios and amounts of items you put in I would call that a 'controlled variable'....

(If anyone could pop in and help out here it would be greatly appreciated thanks)

[meditatingwalrus]

hey! im doing a question from a past paper, and i keep getting the same, incorrect, answer.. but i dont know what im doing wrong? ive attatched a photo of my working out, and if you have some spare time, i would appreciate it so much if anyone could help me out.

14. Calculate the mass of carbon dioxide formed when 74.4 L of oxygen gas at 100kPa and 25 oC completely combust a sample of ethanol.
(A)   44 g
(B)   50 g
(C)   88 g
(D)   132 g

thanks

[arii]

You applied the ratio thing in the wrong part.

Finding the mass of CO_2(g):
n(O₂) = 74.4/24.79 = 3.00121... (So you were correct until here)
n(CO₂) = 2/3 * n(O₂) = 2/3 * 3.00121... = 2.000806... moles
m(CO₂) = (2.000806)(12.01+2*16.00) = 88.0555014...grams.

Therefore, your closest answer would be (C).

[ARCHITECT]

Hey Could someone help with the following,

If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?

CO2+H2O--->H2CO3

thanks

Mod edit: merged double post - if you need to fix up your post or want to add something, just click the modify button on the post

[clovvy]

Hey Could someone help with the following,

If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?

CO2+H2)--->H2CO3

thanks

You are told that you have 2030 grams of CO2 dissolved in 5L of water.. all you need to do is convert 2030g/5L to 406g/L

[RuiAce]

You are told that you have 2030 grams of CO2 dissolved in 5L of water.. all you need to do is convert 2030g/5L to 406g/L

I don't think it's that simple. When CO2 dissolves it doesn't just do what NaCl does and dissociate, but rather it reacts (in accordance to the equation given).

Having said that,

Hey Could someone help with the following,

If this mass of carbon dioxide (2.03 Kg) dissolves in 5 L of water, what is the concentration, in grams per litre, of the solution
produced?

CO2+H2O--->H2CO3

thanks


Mod edit: merged double post - if you need to fix up your post or want to add something, just click the modify button on the post

What I'm doing here is assuming that ALL of the CO2 given dissolved.




Of course, at the same time I don't know if this passes a sensibility check.

[meditatingwalrus]

hey! im really struggling with this question - i keep getting a humongous answer? can someone help me out if possible? it means a lot,

1.   Calculate the pH of the solution when 25.0ml of 0.750mol L-1 hydrochloric acid solution is added to 10ml of 0.500mol L-1 barium hydroxide solution.

(A)    0.456
(B)   0.602
(C)   1.862
(D)   2.058



thanks so much