Failed chem test. But still dont know how to do some qns. :(
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1) When 20 cm3 of a gaseous hydrocarbon were sparked with 150cm3 of oxygen and the residual gases cooled to rtp, a contraction of 60cm3 occured. A further contraction of 80cm3 took place when the residual gases were subjected to aq NaOH. All the vols were measured at 20 degrees.
This is my workings.
Initial volume: CxHy = 20 X+y/4 O2 = 150
FInal volume: excess volume of O2 = Z Volume of Co2 = 110-Z
After Naoh: Co2 removed, 110-Z = 80
Z=30
Thus, X+ y/4 =150-30(excess)
X = 80 (volume of Co2)
X:Y
=80:160
=1:2
Thus CxHy = CH2
2) Write a balenced equation between KNO2 and KMnO4 given tat half eqn for oxidation of NO2- to NO3- is H20 + NO2- -> NO3- + 2H+ + 2e-
Basically i dont know whats the skeleton eqn. Or i dont really know how to determine the products of KNO2 and KMnO4
3)25cm3 of solution containing sodium hydroxide and sodium carbonate required 22 cm3 of hcl acid with phenolphthalein as indicator and a further 6.40cm3 of it with methy orange as indicator.
Just wanna know is the vol of HCL required to react with NAOH = 22 -6.4 ?
Also when asked to calculate the conc of sodium carbonate, do i use 12.80cm3 as volume?
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what grade question is this?
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Q1. Vol of H2O produced = initial contraction = 60 cm3
Vol of CO2 produced = 80 cm3 after NaOH
Therefore vol of CO2 : H2O = 4:3
2CxHy + O2 ---> 8CO2 + 6H2O
Therefore hydrocarbon is C4H6
2. Likely reduction product of MnO4- is Mn2+
MnO4- + 8H+ + 5e ---> Mn2+ + 4H2O = 2MnO4- + 16H+ + 10e ---> 2Mn2+ + 8H2O
H20 + NO2- -> NO3- + 2H+ + 2e- = 5H20 + 5NO2- -> 5NO3- + 10H+ + 10e-
Combining both halves,
2MnO4- + 6H+ + 5NO2- ----> 2Mn2+ + 3H2O+ 5NO3-
Adding K + to neutralise -ve charges
2KMnO4 + KNO2 + H+ ----> 2Mn(NO3)2 + KNO3 +3H2O +K+
Realising acidic medium is need add HCl
Then 2KMnO4 + KNO2 + HCl ----> 2Mn(NO3)2 + KNO3 +3H2O +KCl
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A level
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Combining both halves,
2MnO4- + 6H+ + 5NO2- ----> 2Mn2+ + 3H2O+ 5NO3-
Adding K + to neutralise -ve charges
2KMnO4 + 5KNO2 + 6H+ ----> 2Mn2+ + 5KNO3 +3H2O
Realising acidic medium is need add HCl
Then 2KMnO4 + 5KNO2 + 6HCl ----> 2MnCl2 + 5KNO3 +3H2O + 2KCl
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lol its only 1 mark qn nia. I just realise form notes tat when they say balence it doesnt have to be complete molecules. can be in ions. XD
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Chemfreak022, the contraction of 60cm^3 is not attributed to the production of H2O.
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davidche, the empirical formula of your hydrocarbon is CH2, which you already got.
However you cannot just chin chye reduce the ratio to empirical formula... according to your formula X:Y = 80:160. So must divide by 20 since the amount of hydrocarbon is 20cm3.
i.e. "C80H160" (it's stupid to put it like that) = 20 C4H8
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the correct method to work will be:
CxHy + (x + y/4) O2 ---> x CO2 + y/2 H2O.
Amount of CO2 absorbed by aq. NaOH = 80cm^3.
Ratio of CxHy : CO2 = 20 : 80 = 1:4
=> x = 4.
There is a contraction of 60cm^3 => final volume is 110cm^3, of which 80cm^3 is CO2. The other 30cm^3 is excess oxygen (since no water left)
=> 120cm^3 of oxygen used.
Ratio of CxHy : O2 = 20 : 120 = 1:6
i.e. x + y/4 = 6
since x = 4
y/4 = 6 - 4 = 2
y = 8
Hence your hydrocarbon is C4H8.
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H2O + NO2- ---> NO3- + 2H+ + 2e ------------------------ (1)
MnO4- + 8H+ + 5e ---> Mn2+ + 4 H2O ---------------- (2)
Balance the electrons, i.e. 5 (1) + 2 (2):
5 H2O + 5 NO2- + 2MnO4- + 16H+ + 10e ----> 5NO3- + 10H+ + 10e + 2Mn2+ + 8 H2O
Cancel out the extra water and electron and H+ from both side
5NO2- + 2MnO4- + 6H+ ---> 5NO3- + 2Mn2+ + 3 H2O
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skeleton equation can get from the chemical formula booklet they give you.
because of how the syllabus is, if the equation you need is not inside, they will be required to provide it to you (e.g. the NO2- one)
which means, during exam if u find high find low cannot find equation in the booklet, either look for it in the question or re-do the question coz you do wrongly already.
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For the record, Chemfreak022 and SBS261P are both correct, depending on how you choose to interpret this ambiguous question (contraction of product gases or contraction of reactant gases?).
In the actual 'A' or 'O' level exam, should such critical abiguity be found in a question, the intelligent candidate will write out both alternative workings and answers, but with qualification and explanation. "Dear Examiner/Marker Sir/Mdm, I regret to inform that the question has failed to unambiguously state whether the contraction of gases referred to the product gases or the reactant gases. As such, I am left with no choice but to provide alternative workings and answers below. On the left below, I assume contraction of product gases. On the right below, I assume contraction of reactant gases. Thank you very much Sir/Mdm, for your kind understanding in this critical matter."
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however using chemfreak022's method, you'll have an unbalanced equation when you check back. anyway it's the question's fault...
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Thanks alot guys and babes!