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March 29, 2024, 03:57:06 am

Author Topic: VCE Chemistry Question Thread  (Read 2313271 times)  Share 

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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #435 on: March 04, 2014, 10:41:26 pm »
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Chapter 5, Q22 a) of the heinamann textbook.
It seems like that's what they've done.
Anyone else confirm that I can cancel the Hydrogens like this?

Cheers

Ok. Its just you said that Fe solid was produced. You never mentioned oxide ions.

So

Fe2O3(s) + 3H2O (l) + 3e ----> 2Fe(l) + 3O2-(aq) + 6H+(aq)

Now I've seen the textbook answer, but I'm stumped. Why would they cancel out hydrogens? I think I need an explanation for that too.

Snorlax

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Re: VCE Chemistry Question Thread
« Reply #436 on: March 04, 2014, 10:47:57 pm »
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Ok. Its just you said that Fe solid was produced. You never mentioned oxide ions.

So

Fe2O3(s) + 3H2O (l) + 3e ----> 2Fe(l) + 3O2-(aq) + 6H+(aq)

Now I've seen the textbook answer, but I'm stumped. Why would they cancel out hydrogens? I think I need an explanation for that too.

I'm not sure what you're trying to do..LOL.
Why did you...wut... your charges don't even balance..?

Pretty sure my equation was right. I just need to know how (if there's a rule or whatnot) hydrogens are allowed to cancel

Also, I did state Iron3 oxide converts into metallic iron

« Last Edit: March 04, 2014, 10:50:07 pm by Snorlax »
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Capristo

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Re: VCE Chemistry Question Thread
« Reply #437 on: March 04, 2014, 10:48:05 pm »
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Hey, guys..Just wondering if anyone could help me with this. UVvis-Spectroscopy

A 2.5g sample of fertilizer was weighed out and the iron II content was dissolved by stirring it with 25ml of 2M sulphuric acid. The solution was filtered into a 250ml volumetric flask. 0.01M KMno4 was added to the solution untill the first appearance of pink to ensure all of the Fe2+ were oxidised to Fe3+ ions. Finally, deionised water was added to make the solution up to a volume of 250ml.

50ml of the fertilizer solution was then transferred to a flask and 25ml of KSCN and HCL was added. Deionisated water was then added to bring the solution up to 250ml.

The solution was analysed using a spectrophotometer.

After sketching a calibration graph using the absorbance for the known concentration Fe3+ solutions, the concentration of the sample was determined to be 1.8ppm.

Detirmine the percentage by mass of fe2+ in the fertilizer.

Any help would be really appreciated, thanks!
« Last Edit: March 04, 2014, 10:50:21 pm by Capristo »
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #438 on: March 04, 2014, 10:58:12 pm »
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I'm not sure what you're trying to do..LOL.
Why did you...wut... your charges don't even balance..?

Pretty sure my equation was right. I just need to know how (if there's a rule or whatnot) hydrogens are allowed to cancel

Also, I did state Iron3 oxide converts into metallic iron
Remember that redox does not always need water to happen. Especially in a thermite reaction one, in an extreme heat condition, you can not get water from anywhere.
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #439 on: March 04, 2014, 11:05:41 pm »
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Remember that redox does not always need water to happen. Especially in a thermite reaction one, in an extreme heat condition, you can not get water from anywhere.
Oh so because it was a thermite process, water wasn't produced to begin with?
Simply the Iron3 oxide converted into it's Iron and oxgygen?
That makes more sense
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #440 on: March 04, 2014, 11:31:43 pm »
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Oh so because it was a thermite process, water wasn't produced to begin with?
Simply the Iron3 oxide converted into it's Iron and oxgygen?
That makes more sense

I thought thermite reaction was between aluminium and iron oxide. All that happens is the aluminium is oxidised to a trivalent cation and the iron cations are reduced.

Hey, guys..Just wondering if anyone could help me with this. UVvis-Spectroscopy

A 2.5g sample of fertilizer was weighed out and the iron II content was dissolved by stirring it with 25ml of 2M sulphuric acid. The solution was filtered into a 250ml volumetric flask. 0.01M KMno4 was added to the solution untill the first appearance of pink to ensure all of the Fe2+ were oxidised to Fe3+ ions. Finally, deionised water was added to make the solution up to a volume of 250ml.

50ml of the fertilizer solution was then transferred to a flask and 25ml of KSCN and HCL was added. Deionisated water was then added to bring the solution up to 250ml.

The solution was analysed using a spectrophotometer.

After sketching a calibration graph using the absorbance for the known concentration Fe3+ solutions, the concentration of the sample was determined to be 1.8ppm.

Detirmine the percentage by mass of fe2+ in the fertilizer.

Any help would be really appreciated, thanks!

1.8 ppm => 1.8 mg/l
In 250 ml this means we had 0.45 mg of Fe3+
These ions were initially in 50 ml of fertiliser solution that was taken from 250 ml in total of Fe2+ solution, so in the first 250 ml flask we had five times 0.45 mg = 2.25 mg of Fe2+. This is in 2.5 g so % by mass is 2.25 mg / 2.5 g = 0.09%

I'm typing on my phone and I'm tired so if there's a mistake do tell me.
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Re: VCE Chemistry Question Thread
« Reply #441 on: March 05, 2014, 08:37:04 pm »
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If there is 0.075M of Cr2O7 2- , what is the concentration in a 150mL sample?

I multiplied 0.075 but it's molar mass and then multiplied this value by 150/1000 to find that there are 2.43g per 150mL. Is this correct?? Is g per mL a unit for concentration?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #442 on: March 05, 2014, 09:44:24 pm »
+1
If there is 0.075M of Cr2O7 2- , what is the concentration in a 150mL sample?

I multiplied 0.075 but it's molar mass and then multiplied this value by 150/1000 to find that there are 2.43g per 150mL. Is this correct?? Is g per mL a unit for concentration?

The concentration is just 0.075 M. M is the unit for molar concentration. The volume is actually irrelevant.
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Re: VCE Chemistry Question Thread
« Reply #443 on: March 05, 2014, 10:06:11 pm »
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I keep getting the complete wrong answer for every part of this practice question ... Could someone help D:

An impure sample of limestone, mainly calcium carbonate, was analysed by using a back titration. Approximately 1 g of the finely powdered limestone was weighed accurately into a conical flask. An excess of hydrochloric acid, exactly 50.00 mL, was added to the limestone. The mixture was stirred for 15 min with a magnetic stirrer to allow the reaction to be completed.
The mixture was then titrated with a standard solution of sodium hydroxide and the following results were obtained:
- Mass of watch glass = 8.7954 g
- Mass of anhydrous Calcium carbonate and watch glass = 9.8460 g
- Concentration of standard sodium hydroxide solution = 0.0489 M
- Titration value of sodium hydroxide obtained = 22.32 mL
- Concentration of hydrochloric acid = 0.395 M

a   Write balance equations for the two reactions that occur.
b   Determine the moles of hydrochloric acid in excess after the reaction with the limestone.
c   Calculate the total moles of hydrochloric acid added to the limestone.
d   How many moles of hydrochloric acid reacted with the limestone?
e   Calculate the number of moles of calcium carbonate in the limestone.
f   What is the percentage of calcium carbonate in the limestone?


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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #444 on: March 05, 2014, 11:00:17 pm »
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I keep getting the complete wrong answer for every part of this practice question ... Could someone help D:

An impure sample of limestone, mainly calcium carbonate, was analysed by using a back titration. Approximately 1 g of the finely powdered limestone was weighed accurately into a conical flask. An excess of hydrochloric acid, exactly 50.00 mL, was added to the limestone. The mixture was stirred for 15 min with a magnetic stirrer to allow the reaction to be completed.
The mixture was then titrated with a standard solution of sodium hydroxide and the following results were obtained:
- Mass of watch glass = 8.7954 g
- Mass of anhydrous Calcium carbonate and watch glass = 9.8460 g
- Concentration of standard sodium hydroxide solution = 0.0489 M
- Titration value of sodium hydroxide obtained = 22.32 mL
- Concentration of hydrochloric acid = 0.395 M

a   Write balance equations for the two reactions that occur.
b   Determine the moles of hydrochloric acid in excess after the reaction with the limestone.
c   Calculate the total moles of hydrochloric acid added to the limestone.
d   How many moles of hydrochloric acid reacted with the limestone?
e   Calculate the number of moles of calcium carbonate in the limestone.
f   What is the percentage of calcium carbonate in the limestone?

a) First is the calcium carbonate with the hydrochloric acid, the other is the hydrochloric acid with the sodium hydroxide
b)n(HCl)excess = n(NaOH) = cV = 0.0489*0.02232
   n(HCl)reacted = (0.395*0.05) - (0.0489*0.02232)
c) & d) from above.
e) n(CaCO2) = 1/2 * n(HCl)reacted
f) m(CaCO2) = n(CaCO2) * Mr
   then that mass over mass of initial sample, multiply by 100.
« Last Edit: March 06, 2014, 07:57:05 pm by Snorlax »
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jgoudie

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Re: VCE Chemistry Question Thread
« Reply #445 on: March 06, 2014, 07:45:02 am »
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Looks like the excess and reacted are around the wrong way:

"b)n(HCl)xs = n(NaOH) = cV = 0.0489*0.02232
   n(HCl)reacted = (0.395*0.05) - (0.0489*0.02232)
c) & d) from above."

a) First is the calcium carbonate with the hydrochloric acid, the other is the hydrochloric acid with the sodium hydroxide
b)n(HCl)reacted = n(NaOH) = cV = 0.0489*0.02232
   n(HCl)excess = (0.395*0.05) - (0.0489*0.02232)
c) & d) from above.
e) n(CaCO2) = 1/2 * n(HCl)reacted
f) m(CaCO2) = n(CaCO2) * Mr
   then that mass over mass of initial sample, multiply by 100.
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Re: VCE Chemistry Question Thread
« Reply #446 on: March 06, 2014, 07:25:00 pm »
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Does anyone know any health risks associated with Atomic absorption spectroscopy, IR spectroscopy and UV-Vis spectroscopy?
Will be very helpful :D
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #447 on: March 06, 2014, 09:17:52 pm »
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A question from the Heinamann book:
In order to standardise a solution of hydrochloric acid, a
student titrated the solution against 20.00 mL aliquots of
a standard solution of sodium carbonate. Methyl orange
indicator was used to identify the end point of the reaction:
2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
The sodium carbonate solution had been prepared by
dissolving 1.236 g of anhydrous Na2CO3 in water and making
the solution up to 250.0 mL in a volumetric fl ask. The titres
recorded were 21.56 mL, 20.98 mL, 20.96 mL
and 21.03 mL.
a What value for the titre of hydrochloric acid solution
should the student use in the calculation of the acid
concentration? Explain your answer.
b What is the molarity of the sodium carbonate solution?
c Calculate the concentration of the HC l, in mol L−1.


Having trouble with c). I'm getting 1.111M while the answer suggests 0.08892 M


Thanks!
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Gutthi

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Re: VCE Chemistry Question Thread
« Reply #448 on: March 06, 2014, 09:36:55 pm »
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While titrating Iodine against Wine (to determine the wine's Sulfur Dioxide content) why is the Iodine titrated QUICKLY?

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Re: VCE Chemistry Question Thread
« Reply #449 on: March 06, 2014, 11:13:07 pm »
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Looks like there might be a couple of things you are looking over here:

1) The Sodium Carbonate was added to a 250ml volumetric flask.
2) The ratio of HCl to Na2CO3 is not 1:1


A question from the Heinamann book:
In order to standardise a solution of hydrochloric acid, a
student titrated the solution against 20.00 mL aliquots of
a standard solution of sodium carbonate. Methyl orange
indicator was used to identify the end point of the reaction:
2HCl(aq) + Na2CO3(aq) → 2NaCl(aq) + H2O(l) + CO2(g)
The sodium carbonate solution had been prepared by
dissolving 1.236 g of anhydrous Na2CO3 in water and making
the solution up to 250.0 mL in a volumetric fl ask. The titres
recorded were 21.56 mL, 20.98 mL, 20.96 mL
and 21.03 mL.
a What value for the titre of hydrochloric acid solution
should the student use in the calculation of the acid
concentration? Explain your answer.
b What is the molarity of the sodium carbonate solution?
c Calculate the concentration of the HC l, in mol L−1.


Having trouble with c). I'm getting 1.111M while the answer suggests 0.08892 M


Thanks!
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