Some chem questions.
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1) Graphite and Diamond are allotropes. Complete combustion of equal masses of graphite and diamond produces equal masses of carbon dioxide as the only product. Why is this so
2) Is graphite not a carbon compound? Why?
3) A compound P is formed when 2 vols of ammonia reacts with one vol of co2. Whats is the most likely formula of P. NH2-CO2-NH4 OR NH4-CO2-NH4 and why?
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Originally posted by davidche:
1) Graphite and Diamond are allotropes. Complete combustion of equal masses of graphite and diamond produces equal masses of carbon dioxide as the only product. Why is this so
2) Is graphite not a carbon compound? Why?
3) A compound P is formed when 2 vols of ammonia reacts with one vol of co2. Whats is the most likely formula of P. NH2-CO2-NH4 OR NH4-CO2-NH4 and why?
>>> 1) Graphite and Diamond are allotropes. Complete combustion of equal masses of graphite and diamond produces equal masses of carbon dioxide as the only product. Why is this so <<<Because it proves that both graphite and diamond are pure C, no other elements present.
>>> 2) Is graphite not a carbon compound? Why? <<<
The above (Q1) has already shown why it is an ALLOTROPE of C, not a compound of C.
>>> 3) A compound P is formed when 2 vols of ammonia reacts with one vol of co2. Whats is the most likely formula of P. NH2-CO2-NH4 OR NH4-CO2-NH4 and why? <<<
In terms of structural formula, all of the 4 possible options in that MCQ question are flawed. You cannot draw the Kekule structures as suggested by the condensed formulae given. That's at 'A' levels, where everything matters.
At 'O' levels, it's far more simple - only NH2-CO2-NH4 fits in terms of the ratio of N to H to C to O atoms, based on the reactants (2 mols of NH3 and 1 mol of CO2).
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cool thanks.
got more qns XD. Like always, all qns have been simplified and shortened to reflect my doubts.
4) 1 mol of copper sulphate reacts completely with 1 mole of sodium carbonate to gib 1 mol of sodium sulphate and 1 mol of copper carbonate.
8 mol of copper sulphate is mixed with 4 mol of sodium carbonate
What is left in the reaction vessel after rxn is complete?
is it green ppt in blue solution or colourless solution?
i chose blue but it is wrong. why so?i thought copper ions means blue colour?
5) which fertiliser provides most nitrogen per mole?
NH4NO3 or (NH4)3 -PO4
6)I particularly dont know how to do these kinda qns.
it was found that 0.001 mole of chloride of metal X requires 0.0005 of silver nitrate for complete reaction. What is the formula of the chloride?
7) Which particle is present in the least conc. in sulphuric acid.
H2O(l) molecule or SO4(aq)
8) how does lead(ii) carbonate+ nitric acid and sodium iodide give lead(ii) iodide. Since lead carbonate is insoluble, wld nitric acid react with it?
9) when aq ammonium chloride was heated with aqY, a gas is evolved which turns indicator paper blue. What ion is present in Y.
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Originally posted by davidche:
cool thanks.
got more qns XD. Like always, all qns have been simplified and shortened to reflect my doubts.
4) 1 mol of copper sulphate reacts completely with 1 mole of sodium carbonate to gib 1 mol of sodium sulphate and 1 mol of copper carbonate.
8 mol of copper sulphate is mixed with 4 mol of sodium carbonate
What is left in the reaction vessel after rxn is complete?
is it green ppt in blue solution or colourless solution?
i chose blue but it is wrong. why so?i thought copper ions means blue colour?
5) which fertiliser provides most nitrogen per mole?
NH4NO3 or (NH4)3 -PO4
6)I particularly dont know how to do these kinda qns.
it was found that 0.001 mole of chloride of metal X requires 0.0005 of silver nitrate for complete reaction. What is the formula of the chloride?
7) Which particle is present in the least conc. in sulphuric acid.
H2O(l) molecule or SO4(aq)
8) how does lead(ii) carbonate+ nitric acid and sodium iodide give lead(ii) iodide. Since lead carbonate is insoluble, wld nitric acid react with it?
>>> 4) 1 mol of copper sulphate reacts completely with 1 mole of sodium carbonate to gib 1 mol of sodium sulphate and 1 mol of copper carbonate.
8 mol of copper sulphate is mixed with 4 mol of sodium carbonate
What is left in the reaction vessel after rxn is complete?
is it green ppt in blue solution or colourless solution?
i chose blue but it is wrong. why so?i thought copper ions means blue colour? <<<
Determine limiting reactant (by comparing experiment mole ratio of reactants, with equation mole ratio of reactants). See what's completely used up, what's present at end of reaction.
>>> which fertiliser provides most nitrogen per mole?
NH4NO3 or (NH4)3 -PO4 <<<
Your school gave you a flawed answer to this MCQ question. The correct answer is the latter.
Even better version of this question, would say per gram (or per tonne). Then first calculate no. of moles per gram/tonne, then multiply by no. of N atoms per molecule (or formula unit, since it's an ionic compound).
>>> 6)I particularly dont know how to do these kinda qns.
it was found that 0.001 mole of chloride of metal X requires 0.002 of silver nitrate for complete reaction. What is the formula of the chloride? <<<
Your question's figures are flawed. I've changed 0.0005 to 0.002.
0.002 mol of Ag+ ions reacts with 0.002 mol of Cl- ions to give AgCl(s).
Since this means that 0.001 mol of chloride of metal contains 0.002 mol of Cl- ions, hence 1 mol of chloride of metal contains 2 mol of Cl- ions.
Empirical formula is MCl2 (eg. MgCl2)
>>> Which particle is present in the least conc. in sulphuric acid.
H2O(l) molecule or SO4(aq) <<<
Again, original MCQ question and available options are flawed.
There are the following species in present, listed from highest to lowest molarity (assuming it's dilute sulfuric acid).
H2O(l)
H+(aq)
HSO4-(aq)
SO4 2-(aq)
OH-(aq)
>>> 8) how does lead(ii) carbonate+ nitric acid and sodium iodide give lead(ii) iodide. Since lead carbonate is insoluble, wld nitric acid react with it? <<<
nitric acid reacts with any carbonate, to give soluble nitrate salt. Here, you get lead(II) nitrate solution.
Mixing aqueous sodium iodide with aqueous lead(II) nitrate, you obtain solid lead(II) iodide.
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>>> 9) when aq ammonium chloride was heated with aqY, a gas is evolved which turns indicator paper blue. What ion is present in Y. <<<
OH-(aq) + NH4+(aq) --> NH3(g) + H2O(l)
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Determine limiting reactant (by comparing experiment mole ratio of reactants, with equation mole ratio of reactants). See what's completely used up, what's present at end of reaction.
sodium sulphate, copper carbonate, copper sulphate is left.
copper carbonate is green so green ppt.
copper sulphate gives blue colour so green ppt in blue solution? But the answer sheet says its green ppt in colouless solution.
Your school gave you a flawed answer to this MCQ question. The correct answer is the latter.
woot how did you know the school's ans is NH4NO3? XD
There are the following species in present, listed from highest to lowest molarity (assuming it's dilute sulfuric acid).
H2O(l)
H+(aq)
HSO4-(aq)
SO4 2-(aq)
OH-(aq)
Hmm i tot the H20 molecules are all ionised to H+ and OH- Thats why i chose h20 as least conc.
nitric acid reacts with any carbonate,
For a moment i actually thought acid cant react with insoluble carbonates. omg.
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btw, when a soluble oxide dissoves in water, it becomes a hydroxide like sodium oxde-> sodium hydroxide. But non soluble oxides can also have hydroxide form through a salt's reaction with sodlium hydroxide(QA)
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Originally posted by davidche:
sodium sulphate, copper carbonate, copper sulphate is left.
copper carbonate is green so green ppt.
copper sulphate gives blue colour so green ppt in blue solution? But the answer sheet says its green ppt in colouless solution.
woot how did you know the school's ans is NH4NO3? XD
Hmm i tot the H20 molecules are all ionised to H+ and OH- Thats why i chose h20 as least conc.
For a moment i actually thought acid cant react with insoluble carbonates. omg.
>>> sodium sulphate, copper carbonate, copper sulphate is left.
copper carbonate is green so green ppt. copper sulphate gives blue colour so green ppt in blue solution? But the answer sheet says its green ppt in colouless solution. <<<
Trust yourself.What's important is not this particular single question, but that you understand the underlying concepts and can apply to all similar questions. As long as you know the key is to determine limiting reactant, you're ok.
>>> Hmm i tot the H20 molecules are all ionised to H+ and OH- Thats why i chose h20 as least conc. <<<
If that was the case, then pure water would conduct electricity.
>>> For a moment i actually thought acid cant react with insoluble carbonates. omg. <<<
Welcome The Future (Let the Moment Pass).
>>> btw, when a soluble oxide dissoves in water, it becomes a hydroxide like sodium oxde-> sodium hydroxide. But non soluble oxides can also have hydroxide form through a salt's reaction with sodlium hydroxide(QA) <<<
Yes, correct. Solid state hydroxides exist, like calcium hydroxide solid, magnesium hydroxide solid, zinc hydroxide solid etc.
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I only know that H2O ionises when there are ionic cmpds dossolved inside. So for the dilute sulphuric acid case, not all H2O ionises.
So copper sulphate is blue in colour.
just did another 50 mcq qns so...
1) After acidification, a colourless solution of X reacts with silver nitrate(aq) to give a white ppt. Is X copper(ii) chloride or lead(ii) chloride? why?
2) What is the OS of N in NH3? i am thinking +3 (i got 2 qns wrong cos of this, how to know if N is positive or negative OS in NH3 or other covalent cmpds tat has H inside?)
3) I wonder am i supposed to know that potassium manganate(VII) + iron(ii) sulphate is a redox reaction. If so, what are the products? is it iron(iii) sulphate?
4) Se is in the same grp as sulphur. What is the correct formula of potassium solenide? K2Se or K2SeO4?
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Originally posted by davidche:
I only know that H2O ionises when there are ionic cmpds dossolved inside. So for the dilute sulphuric acid case, not all H2O ionises.
So copper sulphate is blue in colour.
just did another 50 mcq qns so...
Not quite. H2O auto-ionizes to form H+ ions and OH- ions, regardless of whether there are ionic compounds dissolved or not.The (endothermic) auto-ionization constant for water at room temperature, is very low. The molarity of protons [H+] and hydroxide ions [OH-] in pure water is only 1.0 x 10^-7. Which is why pH of pure water at room temperature is 7.
pH = p[H+] = -log(1.0 x 10^-7) = 7
When ionic compounds are dissolved into water, the water molecules form ion-dipole electrostatic interactions with the cations and anions, creating hydration shells of water molecules arranged in geometric patterns around the cations and anions. Beautiful stuff.
But water molecules do not ionize more when they are ionic compounds present. That's a false myth you picked up (from your school teacher?).
>>> So copper sulphate is blue in colour. <<<Yes, that's due to non-degenerate d-d* orbital splitting into distinct energy levels due to electron repulsion between electron orbitals of the complexed transition metal cations and the electron orbitals of the electron donor ligand molecules, the different magnitudes depending on their VSEPR (Valence Shell Electron Pair Repulsion theory) geometry, which results in different frequency wavelengths of light being absorbed and reflected as calculated by E = hv (where h represents Planck's constant), which results in beautiful colours as perceived by the human eye (retina absorbs wavelengths, sends impulses along the optic neurons to the sight processing center of the brain, and you go "Wah Swee lah, blue blue colour one!!!").
You'll
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I'll? lol nice one. Like i mentioned before, i like your sense of humour.
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But water molecules do not ionize more when they are ionic compounds present. That's a false myth you picked up (from your school teacher?).
here me out first. Pure water does not ionise. When there are stuffs dissolves in it, the water molecule ionises. Thats why stuffs in our tap (which has salts i think like sodium) can conduct electricity. Sounds all logical what. And yeah my teacher taught me this.
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>>> 1) After acidification, a colourless solution of X reacts with silver nitrate(aq) to give a white ppt. Is X copper(ii) chloride or lead(ii) chloride? why? <<<
Colourless, so lead(II) chloride.
>>> 2) What is the OS of N in NH3? i am thinking +3 (i got 2 qns wrong cos of this, how to know if N is positive or negative OS in NH3 or other covalent cmpds tat has H inside?) <<<
N is more electronegative than H, so N has priority of O.S. For N to have stable octet, give it O.S. of -3. Hence in a neutral molecule NH3, H will have O.S. of +1.
>>> 3) I wonder am i supposed to know that potassium manganate(VII) + iron(ii) sulphate is a redox reaction. If so, what are the products? is it iron(iii) sulphate? <<<
Yes.
Under acidified conditions, purple MnO4- is reduced to colourless Mn2+. If neutral or alkaline conditions, purple MnO4- is reduced to brown ppt of MnO2(s).
>>> 4) Se is in the same grp as sulphur. What is the correct formula of potassium solenide? K2Se or K2SeO4? <<<
Potassium selenide is K+ and Se2- combined, so K2Se.
K2SeO4 is potassium selenate.
Sulfide, sulfate.
Selenide, selenate.
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Originally posted by davidche:
here me out first. Pure water does not ionise. When there are stuffs dissolves in it, the water molecule ionises. Thats why stuffs in our tap (which has salts i think like sodium) can conduct electricity. Sounds all logical what. And yeah my teacher taught me this.
Not quite. The water molecule remains largely as covalently bonded water molecule. Tap water can conduct electricity because of the mobile ions (eg. Na+, Cl-, etc) present in tap water.The term "tap water" includes all species present :
1) Covalently (ie. non-ionized) water molecules,
2) Dissolved aqueous ions (that can conduct electricity)
3) etc.
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Colourless, so lead(II) chloride.
o shucks careless
N is more electronegative than H, so N has priority of O.S. For N to have stable octet, give it O.S. of -3. Hence in a neutral molecule NH3, H will have O.S. of +1.
So since N lacks 2 electrons while H only lacks one or has to give away one, N takes priority to be negative?
Potassium selenide is K+ and Se2- combined, so K2Se.
K2SeO4 is potassium selenate.
Sulfide, sulfate.
Selenide, selenate.
This qn wld be out of syllabus what, since it is testing selenate and selenide. Who would know this? Who at my lvl.
Not quite. The water molecule in tap water or any ionic cmpd solution remains largely as covalently bonded water molecule.
Right? point noted.
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>>> So since N lacks 2 electrons while H only lacks one or has to give away one, N takes priority to be negative? <<<
NH3 is a covalent compound. The N and H atoms do not gain or lose electrons. They share electrons to achieve a stable duplet (for H) octet (for N).
Oxidation state is not the same as ionic (or formal) charge. Oxidation state has to do with "electronegativity", which is only properly taught at 'A' levels (coming very soon for you Sec 4 graduates!)
At 'O' levels, simply remember that the oxidation state of H can only be +1, 0, or -1.
+1 ~ when H is covalently bonded to any other element, eg. HCl, NH3, etc.
0 ~ when H is bonded to itself, ie. H2.
-1 ~ when H is ionically bonded to metals (which are "electropositive"), eg. NaH is Na+ and H-.
>>> The water molecule in tap water or any ionic cmpd solution remains largely as covalently bonded water molecule. Right? <<<
Right.
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i can see why this is the homework forum... everyone come here and ask for answer for their homework... gone are the days kids figuring their homework out...
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Cool that you can see it
1) whats the thing about aluminium developing an oxide layer easily? why doesnt it happen on other metals? I know al and iron(can rust).
2) What has less tendancy to form a positive ion? Magnesium or iron? If this has sth to do with properties of transition metals, then it shld be out of syllabus.
3) In the polymerisation of ethene, there is no change in -desnity or mass or molecular formula?
4)When excess calcium carbonate is added to dilute HCL, the reaction gradually slows down and stops. Why is this so?
calcium carbonate is covered by CO2
Conc. of HCL gradually reduces to zero.(my choice)
5)For electrolysis, the concentration of aqueous solution doesnt affect very reaction metals like calcium-potassium right?
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Originally posted by davidche:
Cool that you can see it
1) whats the thing about aluminium developing an oxide layer easily? why doesnt it happen on other metals? I know al and iron(can rust).
2) What has less tendancy to form a positive ion? Magnesium or iron? If this has sth to do with properties of transition metals, then it shld be out of syllabus.
3) In the polymerisation of ethene, there is no change in -desnity or mass or molecular formula?
4)When excess calcium carbonate is added to dilute HCL, the reaction gradually slows down and stops. Why is this so?
calcium carbonate is covered by CO2
Conc. of HCL gradually reduces to zero.(my choice)
5)For electrolysis, the concentration of aqueous solution doesnt affect very reaction metals like calcium-potassium right?
>>> 1) whats the thing about aluminium developing an oxide layer easily? why doesnt it happen on other metals? I know al and iron(can rust). <<<
Yes, all reactive metals do. But aluminium is 'special', because it develops the oxide layer very rapidly, like sodium. But unlike sodium, the aluminium oxide layer is insoluble (hence 'protecting' the raw aluminium underneath from further corrosion or oxidation).
>>> 2) What has less tendancy to form a positive ion? Magnesium or iron? If this has sth to do with properties of transition metals, then it shld be out of syllabus. <<<
When you learn that magnesium is more "reactive" than iron, it actually means magnesium has greater tendency to be oxidized to cation (compared with iron).
>>> 3) In the polymerisation of ethene, there is no change in -desnity or mass or molecular formula? <<<
I already posted on it here. Do a Find (control-F) for "ADDITION polymer".
http://www.sgforums.com/forums/2297/topics/336442?page=2
>>> 4)When excess calcium carbonate is added to dilute HCL, the reaction gradually slows down and stops. Why is this so?
calcium carbonate is covered by CO2
Conc. of HCL gradually reduces to zero.(my choice) <<<
Correct. Concentration of reactant HCl(aq) decreases as it is used up by the reaction, hence speed of reaction decreases.
>>> 5)For electrolysis, the concentration of aqueous solution doesnt affect very reaction metals like calcium-potassium right? <<<
Correct. These metals are too reactive, hence their cations are too stable, to ever be reduced in aqueous solution.
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>>> 1) After acidification, a colourless solution of X reacts with silver nitrate(aq) to give a white ppt. Is X copper(ii) chloride or lead(ii) chloride? why? <<<
Colourless, so lead(II) chloride.
How do you get lead(II) chloride SOLUTION?
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ar left 50 more qns to do to complete 300 qns total.
regarding salt bridge,
As electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate.
I still dont quite understand esp the bolded part XD
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Originally posted by Garrick_3658:
How do you get lead(II) chloride SOLUTION?
oh sry , it is sodium. But doesnt mater. The thing is that i didnt see that copper chloride is not colourless.
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Originally posted by Garrick_3658:
How do you get lead(II) chloride SOLUTION?
Good point. Which means both options by the question are wrong. The question is flawed.>>> regarding salt bridge, as electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. <<<
Remember that all solutions, all compounds, everything in this Universe, MUST be electrically neutral. That's (in fact) why you know how to write formula.
Al3+ cation
CO3 2- anion
You know the formula must be Al2(CO3)3. Why? To ensure electrical neutrality. If there were no salt bridge, electrons would stop flowing in the external wire, in order to maintain electrical neutrality.
In fact, that's why lightning occurs (electrical discharge due to built up of ionic charges) during a rain storm. But that's physics and physical geography and whatever.
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i kinda see. I am attempting the last 50 qns now.
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K, going out for dinner/supper liao. By the time I'm back, y'all koon liao. So make sure u guys have enough sleep, wake up refreshed for tmrw's exam paper, and makan enough breakfast so ur brain got glucose and can perform during the exam. Have fun!
