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March 29, 2024, 10:58:33 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040810 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #285 on: April 30, 2016, 04:07:21 pm »
+1
Hey!
My second 3/4 chem sac is regarding aspirin and oil of wintergreen (methyl salicylate).
Is they key structural difference between the two the fact that aspirin lacks a distinct hydroxyl functional group or the positioning of the methyl functional group on aspirin?

Hey! I think you're looking for the VCE Chemistry question board! Sorry, won't be able to help you out here :)

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HighTide

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Re: Chemistry Question Thread
« Reply #286 on: May 02, 2016, 07:19:25 am »
+1
Hey!
My second 3/4 chem sac is regarding aspirin and oil of wintergreen (methyl salicylate).
Is they key structural difference between the two the fact that aspirin lacks a distinct hydroxyl functional group or the positioning of the methyl functional group on aspirin?
Apologies for invading the HSC thread... the structural difference would be more towards the functional groups present. Methyl salicylate has a hydroxyl group whilst aspirin has a carboxyl group. But also, if you draw it out, you'll see another difference in the components to form the ester.
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Re: Chemistry Question Thread
« Reply #287 on: May 02, 2016, 07:45:32 pm »
0
Hey
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
     = 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
          = 5.248 x 10-4 mol

From equation, mole ratio of HCl ≡ NH3   is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
        = 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted)   = 5.248 x 10-3 mol
Concentration of NH3   = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3

Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1

Therefore, weight-volume percentage of NH3 in window cleaner 
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%

Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou  :)
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RuiAce

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Re: Chemistry Question Thread
« Reply #288 on: May 02, 2016, 09:10:49 pm »
0
Hey
i have a titration assessed practical coming up soon
the assessment is the we have to find the percentage of household ammonia in a cleaning product by tritrating it against HCl... with the solution diluted to 100mL and an aliquot of 10mL
I've done the calculation part to one...but we are also given density...and i don't quite know where the density fits into it?
the HCl has a concentration of 0.1082
Average volume of HCl titrate used
     = 4.85 cm3
No. of moles of HCl reacted = 0.1082 x (4.85 x10-3)
          = 5.248 x 10-4 mol

From equation, mole ratio of HCl ≡ NH3   is 1:1,
no. of moles of NH3 reacted = 5.248 x 10-4 mol
No. of moles of NH3 in volumetric flask (diluted) = (100 ÷ 10) × 5.248 x 10-4 mol
        = 5.248 x 10-3 mol
No. of mole of NH3 in 10mL (undiluted)   = 5.248 x 10-3 mol
Concentration of NH3   = (5.248 x 10-3) ÷ (10 x 10-3) = 5.248 x 10-1 moldm-3

Molar mass of NH3 = 14.01 + 3 (1.0079)
= 17.0337 g mol-1

Therefore, weight-volume percentage of NH3 in window cleaner 
= ((17.0337 x 5.248 x 10-1)/ 103) × 100%
= 0.894%

Percentage of ammonia in the window cleaner is: 0.894%
found the percentage of ammonia...but didnt use the density given...could you please tell me where i went wrong?
i'd really appreciate the help
Thankyou  :)

Density = Mass / Volume

So if they give you the density and you have the volume, you can use it to find the mass

Take note that n = m/M, not n = V/M. Also take note that only WATER has a density of 1. For water, m = V

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Re: Chemistry Question Thread
« Reply #289 on: May 02, 2016, 09:21:02 pm »
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Density = Mass / Volume

So if they give you the density and you have the volume, you can use it to find the mass

Take note that n = m/M, not n = V/M. Also take note that only WATER has a density of 1. For water, m = V

so using that method could you bypass all the parts where in my solution i have worked it out through moles...
would u just do density x av titre (volume)= mass?
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amandali

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Re: Chemistry Question Thread
« Reply #290 on: May 03, 2016, 12:13:08 am »
0
1. How to explain "why can multiple indicators be used in titration of strong acid?"


2. You are given a titration graph of weak acid + strong base with curve ending at the basic region
is the reason for that due to weak acid producing strong conjugate base which reacts with H2O to produce OH- that increases pH

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Re: Chemistry Question Thread
« Reply #291 on: May 03, 2016, 07:00:00 pm »
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Hey guys,

Do both ionic and covalent compounds contain intermolecular and intramolecular bonds? Thanks!!
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Elizawei

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Re: Chemistry Question Thread
« Reply #292 on: May 03, 2016, 10:49:35 pm »
+1
Hey guys,

Do both ionic and covalent compounds contain intermolecular and intramolecular bonds? Thanks!!

Yup, they both do  ;)

EDIT: SORRY didn't realise this was the HSC chem thread, oops  :P
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katherine123

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Re: Chemistry Question Thread
« Reply #293 on: May 03, 2016, 11:46:30 pm »
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Explain the effect of liquefying the ammonia on the yield of reaction  2 marks

N2(g) + 3H2(g) <--> 2NH3(g)
 [NH3(g)] decreases as NH3(g) --> NH3(l)   so does it shift to right because the concentration of NH3(g) has decreased  or because the reaction is under high pressure so it will shift to reduce the number of gas molecules to reduce pressure

amandali

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Re: Chemistry Question Thread
« Reply #294 on: May 03, 2016, 11:58:36 pm »
0
Can you check my response? thanks

a) since pressure is inversely proportional to volume, the decrease in volume will increase the pressure. According to Le chatalier's principle, the equilibrium will shift right so that there will be fewer total gaseous molecules in the system since for every 2  molecules that react only 1 is formed which reduces pressure. This shift will cause an increase in concentration of Z(g) and also heat released as forward reaction is exothermic.

b) catalyst does not affect the position of equilibrium because it lowers the activation energy which increases the rate of both forward and reverse reactions equally. Therefore, equilibrium state is reached sooner but the yield of product is not increased.

Rishi97

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Re: Chemistry Question Thread
« Reply #295 on: May 04, 2016, 10:37:54 am »
+2
Can you check my response? thanks

a) since pressure is inversely proportional to volume, the decrease in volume will increase the pressure. According to Le chatalier's principle, the equilibrium will shift right so that there will be fewer total gaseous molecules in the system since for every 2  molecules that react only 1 is formed which reduces pressure. This shift will cause an increase in concentration of Z(g) and also heat released as forward reaction is exothermic.

b) catalyst does not affect the position of equilibrium because it lowers the activation energy which increases the rate of both forward and reverse reactions equally. Therefore, equilibrium state is reached sooner but the yield of product is not increased.

Good responses :)
Just a small fix up is needed for response 2. The questions asks about the effect of a catalyst on the equilbrium mixture. However, you started off talking about the position of equilbrium straight away. This is correct but maybe just structure it a bit differently. So start of by explaining the role of a catalyst and then apply it to the situation/question given. For eg. a catalyst is a substance that increases the rate of reaction by lowering the activation energy. The catalyst increases the rate of both the forward and reverse reactions. Thus, the equilbrium state is reached sooner but the amount of yield is not affected.
Hope this makes sense :) great effort just need a bit of fixing up...but if your teacher is not a harsh marker, you may have still gotten the marks :)
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meretz.etai

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Re: Chemistry Question Thread
« Reply #296 on: May 05, 2016, 11:26:50 am »
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Hey Jake,

Im having some problems understanding the idea equilibrium, can help explain?


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Re: Chemistry Question Thread
« Reply #297 on: May 08, 2016, 01:24:20 pm »
+1
Hey Jake,

Im having some problems understanding the idea equilibrium, can help explain?

Hey meretz!

Clearly I'm not Jake, but I would be more than happy to help you out here! In year 11, all the equations that we have learnt about are all complete reactions, which means that the reaction only goes one way because the reaction would proceed until all the reactants are used up. A good example is combustion reaction (2C8H18(l) + 25O2(g) ----> 18H2O(l) + 16CO2(g). So if you think about it, when you burn a fuel, do you ever see the products H2O and CO2 reacting to form the original fuel again? Definitely not. In contrast an equilibrium reaction is basically any reactions that do not go to complete and therefore the reaction can be reversed. For instance, the Haber Process N2(g) + H2(g) --> / <--- NH3(g) is a reversible process. In this case, when nitrogen reacts with hydrogen to form ammonia, some ammonia decomposes back into nitrogen and hydrogen as well. The amount of reactants/products forms are governed by Le Chatelier's principle which talk about the impact of temperature, pressure and concentration upon the equilibrium reaction. Be mindful that this does not affect complete reactions because it is a ONE-WAY reaction and therefore the products cannot decompose to form reactants again.

When we talk about chemical equilibrium, we are referring to a state of a close chemical system in which the concentrations of both reactants and products do not change with time and the rate of the forward reaction is equal to the rate of the reverse reaction. For instance, consider a sealed bottle of soda water. There is dynamic equilibrium happening because carbon dioxide is dissolving in the liquid and carbon dioxide being form as gas in the air above the liquid simultaneously. The equilibrium is maintained unless concentration changes, pressure changes or temperature changes which will cause a disturbance.

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Re: Chemistry Question Thread
« Reply #298 on: May 11, 2016, 07:39:14 pm »
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Hi, I am really struggling to answer the question below. Please help!?


A particular brand of vinegar claimed to 4.3% acetic acid. 10mL of the vinegar was diluted to make a 100mL solution. The claim was tested using a titration where 16mL of 0.1M sodium hydroxide was needed to neutralise 25mL of the 10% vinegar solution. Calculate the molarity (moles/L) and the percentage (w/v, ie - g/100mL = % ) of acetic acid in the original vinegar. Comment on the 4.3% claim.

jakesilove

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Re: Chemistry Question Thread
« Reply #299 on: May 12, 2016, 02:17:22 pm »
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Hi, I am really struggling to answer the question below. Please help!?


A particular brand of vinegar claimed to 4.3% acetic acid. 10mL of the vinegar was diluted to make a 100mL solution. The claim was tested using a titration where 16mL of 0.1M sodium hydroxide was needed to neutralise 25mL of the 10% vinegar solution. Calculate the molarity (moles/L) and the percentage (w/v, ie - g/100mL = % ) of acetic acid in the original vinegar. Comment on the 4.3% claim.

Hey! I've attached my answers below. These sorts of questions can be tough, as they require a whole bunch of steps and a lot of maths. Once you've done a few of them, however, you should be able to work through others!



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