Some last min chem questions `o`
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2) An ionic compound forms a white ppt with acidified barium chloride solution, no change observed with aqeuous ammonia. Is the ionic compound ammonium sulphate or calcium sulphate?
3) One mole of hydrocarbon X reacts with 2 mol of hydrogen gas. What is the formula of X? a) C5H8 b)C8H18
I wrote b as answer but they go mark me wrong.
4)(this one is daunting me for quite a long while)
Mass of test tube = 21.28g
Mass of test tube + lead oxide = 27.26
Mass of test tube + lead = 26.46
What is the emprical formula of this oxide of lead?
5) Is the formula of a ionic cmpd same as a covalent cmpd?
6) Do Isotopes have similar chemical properties?
7) What are the differences of chemical and physical properties?
8) Whats the meaning of F, subscript 60 (F= fullerene)
What a long chain of questions. Thanks a TON!!!!!!!!!!!!!!!!!!!!
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2) Both. Or ammonium sulphate IF calcium sulphate is concentrated.
3) One mole of hydrocarbon X reacts with 2 mol of hydrogen gas. Hydrocarbon has two double bonds.
Seems like none of the above leh, anymore choices? Most likely a polyunsaturated hydrocarbon.
4) idiot question go straight to the point PLEASE.
Mass of lead = 5.18g
No of moles of Pb = 5.18/207 = 0.025 mol
Mass of oxide = 5.98 - 5.18 = 0.8g
No of moles of oxide (a.k.a. oxygen) = 0.8/16 (or 32?) = 0.05 mol
Ratio of Pb : O = 1:2
Empirical formula = PbO2
Not sure ah. Wait for UltimaOnline or tintin to check.
EDIT: I think for oxygen it should be 32. PbO2 doesn't seem to make sense. PbO makes much more sense lor...
5) The way you write the formula? Yes.
6) Yes.
7) Chemical properties can only be observed with a chemical reaction, a.k.a. the substance undergoes a chemical change. Physical properties can be observed without the substance undergoing a chemical change. Adding indictor is not a chemical reaction, fyi, it's the indictor that changes not the substance.
8) F is fluorine. F60 never heard before.
Well here's something http://en.wikipedia.org/wiki/Fullerene
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Originally posted by Garrick_3658:
2) Both. Or ammonium sulphate IF calcium sulphate is concentrated.
3) One mole of hydrocarbon X reacts with 2 mol of hydrogen gas. Hydrocarbon has two double bonds.
Seems like none of the above leh, anymore choices? Most likely a polyunsaturated hydrocarbon.
4) idiot question go straight to the point PLEASE.
Mass of lead = 5.18g
No of moles of Pb = 5.18/207 = 0.025 mol
Mass of oxide = 5.98 - 5.18 = 0.8g
No of moles of oxide (a.k.a. oxygen) = 0.8/16 (or 32?) = 0.05 mol
Ratio of Pb : O = 1:2
Empirical formula = PbO2
Not sure ah. Wait for UltimaOnline or tintin to check.
EDIT: I think for oxygen it should be 32. PbO2 doesn't seem to make sense. PbO makes much more sense lor...
5) The way you write the formula? Yes.
6) Yes.
7) Chemical properties can only be observed with a chemical reaction, a.k.a. the substance undergoes a chemical change. Physical properties can be observed without the substance undergoing a chemical change. Adding indictor is not a chemical reaction, fyi, it's the indictor that changes not the substance.
8) F is fluorine. F60 never heard before.
Thanks for helping David, Garrick. Really busy now, a day before the 'O' level exam paper, with lots of students at my place the entire day today. I'm still rushing off for dinner now. Be back later tonight, but still with my students.
I take it that Garrick has answered all David's qns, except Q4 which Garrick asked for my input.
Mass of test tube = 21.28g
Mass of test tube + lead oxide = 27.26
Mass of test tube + lead = 26.46
What is the emprical formula of this oxide of lead?
Sample mass of lead oxide = 5.98g
Sample mass of lead = 5.18g
Sample mass of oxide ions = 0.8g
No. of moles of oxide ions = sample mass / molar mass = 0.8 / 16 = 0.05
No. of moles of lead = sample mass / molar mass = 5.18 / 207 = 0.025
Formula of lead oxide = PbO2(s)
Stock name of compound = lead(IV) oxide
Notice that this is not the regular lead(II) oxide, which is PbO(s).
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Yes, I am rushing through my FYS too. I have came online to check on some electrolysis stuffs. Shall put it into a topic later. Dinner time!
David, I am awaiting your input on my doubts. There are still some unanswered questions.
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3) One mole of hydrocarbon X reacts with 2 mol of hydrogen gas. Hydrocarbon has two double bonds.
Seems like none of the above leh, anymore choices? Most likely a polyunsaturated hydrocarbon.
I only provided two choices, one is the answer i wrote which is wrong, and another is the ''correct'' answer. The other 2 choices are C6H6 and C10 H20. Ok ill try to be calm but now that i typed it out it has gotta be C10H20 since its the obvious alkene right? Still must blame the answer keey.
7) Chemical properties can only be observed with a chemical reaction, a.k.a. the substance undergoes a chemical change. Physical properties can be observed without the substance undergoing a chemical change. Adding indictor is not a chemical reaction, fyi, it's the indictor that changes not the substance.
BP and MP is physical properties.
Leads me to more questions, how do you comment on something's chemical properties? Do you say like it produces H+ ions etc? For a more specific example, what are the properties of sodium chloride?
2) An ionic compound forms a white ppt with acidified barium chloride solution, no change observed with aqeuous ammonia. Is the ionic compound ammonium sulphate or calcium sulphate?
Based on the fact that calcium hydroxide is sparingly soluble in water right? Gosh this is tricky.
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I only provided two choices, one is the answer i wrote which is wrong, and another is the ''correct'' answer. The other 2 choices are C6H6 and C10 H20. Ok ill try to be calm but now that i typed it out it has gotta be C10H20 since its the obvious alkene right? Still must blame the answer keey.
AHH, which stupid answer key gave the wrong answer. My thoughts exactly. I think the answer is C10H20. Alkene.
Properties of sodium chloride
Hard, strong, high mp and bp, regular-shaped... what else?
They would only ask about the physical properties and wanna you to link it to its structure, for NaCl.
Why not you ask, properties of acids?
Physical
Acids have a sour taste (don't try it)
Acids change the colours of indictors. For example, blue litmus red.
Chemical
Acids produce a salt and hydrogen gas when reacted with a metal of "reactable" nature.
Acids react with carbonates to produce a salt, CO2 and H2O
Acids react with bases/alkalis to produce a salt and water.
Based on the fact that calcium hydroxide is sparingly soluble in water right? Gosh this is tricky.
Actually it's based on that CALCIUM cations do produce a slightly white ppt with aqueous ammonia IF, IF, AND ONLY IF, the calcium cations are in high concentrations, which is why I asked in my first post.
And of course, when you react ammonium ions(in aq ammonia) with ammonium ions(ammonium sulphate) you still get ammonium ions, no brainer, thus no reaction.
EDIT: I think you got confused with this.BP and MP is physical properties.
Yes, they ARE physical properties. You are asking why I said it is a chemical property right? No, I didn't. When boiling or melting a substance, they merely undergo a change in PHYSICAL state, not a chemical change. Like what I said previously,
Chemical properties can only be observed with a chemical reaction, a.k.a. the substance undergoes a chemical change. Physical properties can be observed without the substance undergoing a chemical change. Adding indictor is not a chemical reaction, fyi, it's the indictor that changes not the substance.
A chemical reaction for example, is something like this:NaOH + HCl --> NaCl + H2O
The reactants undergo a chemical change to form a completely new product. THAT IS A CHEMICAL CHANGE.
Ok? :D:D:D GL FOR TMR!!! CHIONG ARRRRRRR
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Garrick, David, can liao hor? Any more qns? (coz u 2 discussing with each other, so I'm not sure if there's anything u want to ask me in particular).
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Does your lack of comments mean that my points were accurate? :D
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Originally posted by Garrick_3658:
Does your lack of comments mean that my points were accurate? :D
Yes, your points are correct.Just to point out though, the fullerenes are allotropes of carbon (not fluorine). Hence you have to write them as (there are other fullerenes, but the most common one is) C60 (and not F60).
If there's any point you want to clarify further or anything else to ask, feel free.
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C subscript 60 has gotta be 60 C atoms in one molecule right? And i typed it wrongly not F lol. My fault on tat.
Lastly, and its the highlight of this thread, is that it cant be calcium sulphate since it is insoluble. LOLOLOL
PS: i honestly did not realise this. My chem teacher is just too pro.
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oh no i got owned on solubility of compounds. shoot.
C subscript 60 has gotta be 60 C atoms in one molecule right
Yes.
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Originally posted by davidche:
the 60 means 60 Ar or 60 atoms that make up one molecule?
Lastly, and its the highlight of this thread, is that it cant be calcium sulphate since it is insoluble. LOLOLOL
PS: i honestly did not realise this. My chem teacher is just too pro.
60 carbon atoms in one giant fullerene molecule. Looks like a soccer ball.Correct, calcium sulfate (solid) is insoluble, and will not "form" a new ppt, but will simply remain as a solid, when placed in aqueous barium chloride solution.
Here is a qn for you (I'll reveal the answer at 11pm, if no one gets it by then. Simple qn lah, you all 'O' level students better know how to answer correctly hor.).
Explain (not just state) how the brightness of a bulb will change over time, when connected to a setup (ie. got battery included, the bulb is glowing brightly at the beginning of the reaction), in which the electrolyte is aqueous barium hydroxide, and you (continuously but slowly) add aqueous copper(II) sulfate, until there is no futher change in the brightness (or lack thereof) of the bulb. After which, you continuously add aqueous ammonia until there is no futher change in the brightness (or lack thereof) of the bulb. Rest assured the battery is functioning fine (this is not a trick riddle in which the battery is dead). Also, for the purpose of this question, disregard any electrode reactions (ignore redox reactions at the cathode and anode, if any).
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Explain (not just state) how the brightness of a bulb will change over time, when connected to a setup (ie. got battery included, the bulb is glowing brightly at the beginning of the reaction), in which the electrolyte is aqueous barium hydroxide, and you (continuously but slowly) add aqueous copper(II) sulfate, until there is no futher change in the brightness (or lack thereof) of the bulb. After which, you continuously add aqueous ammonia until there is no futher change in the brightness (or lack thereof) of the bulb. Rest assured the battery is functioning fine (this is not a trick riddle in which the battery is dead). Also, for the purpose of this question, disregard any electrode reactions (ignore redox reactions at the cathode and anode, if any).
First, Insoluble barium sulphate is formed in the reaction between barium and sulphate ions in the electrolyte. This decreases the number of mobile ions available to carry electrical charges, thus decreases the brightness of the bulb.
As aqeuous ammonia is added, a blue ppt(which we do not need to know the name or formula for it as it is a complex ammonium salt) is formed, which further decreases the number of mobile ions in the solution causing a further decrese in brightness. After a while, as more ammonia is added, the ppt dissoves in excess, causing the brightness to increase.
As there are additional ammonium ions in the solution, the incremement of brightness is more than that before ammonia was added.
Side note: challenge that i faced was in explaining that the brightness after the ppt dissoved is more than the initial brightness before the ppt was formed, since ammonia is added in.
In hindsight, i just did. XD
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Originally posted by davidche:
First, Insoluble barium sulphate is formed in the reaction between barium and sulphate ions in the electrolyte. This decreases the number of mobile ions available to carry electrical charges, thus decreases the brightness of the bulb.
As aqeuous ammonia is added, a blue ppt(which we do not need to know the name or formula for it as it is a complex ammonium salt) is formed, which further decreases the number of mobile ions in the solution causing a further decrese in brightness. After a while, as more ammonia is added, the ppt dissoves in excess, causing the brightness to increase.
As there are additional ammonium ions in the solution, the incremement of brightness is more than that before ammonia was added.
Side note: challenge that i faced was in explaining that the brightness after the ppt dissoved is more than the initial brightness before the ppt was formed, since ammonia is added in.
In hindsight, i just did. XD
Not bad, pretty well done... BUT you made a couple of errors (and/or lack of key points) and thus cannot get full marks.
Do you (or Garrick) want to try to spot your errors (and/or lack of key points)?
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Solution :
The light bulb will become dimmer over time, as Ba2+(aq) and OH-(aq) ions are removed as BaSO4(s) and Cu(OH)2(s) respectively. There are therefore increasingly less mobile ions left in solution to conduct electricity. The light bulb will be turned off totally when, at a particular point in time, there are only covalently bonded water molecules left. As additional CuSO4(aq) is added, the light bulb will become brighter again. When aqueous ammonia is added, a blue ppt of Cu(OH)2(s) is formed, thereby removing Cu2+(aq) ions, so the bulb becomes dimmer again.
However, the bulb does not go off (as it did previously), because there are still mobile NH4+(aq) and SO4 2-(aq) ions left in solution. As excess aqueous ammonia is added, the Cu(OH)2(s) becomes a soluble complex ion [Cu(NH3)4]2+(aq) giving the solution a deep/dark blue colour, which (the tetraaminecopper(II) ion) is a mobile ion and conducts electricity, and so the bulb becomes even brighter.
(Note : at 'O' levels you don't need to know the name or formula of the complex ion.)
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3) One mole of hydrocarbon X reacts with 2 mol of hydrogen gas. Hydrocarbon has two double bonds.
Seems like none of the above leh, anymore choices? Most likely a polyunsaturated hydrocarbon.
I only provided two choices, one is the answer i wrote which is wrong, and another is the ''correct'' answer. The other 2 choices are C6H6 and C10 H20. Ok ill try to be calm but now that i typed it out it has gotta be C10H20 since its the obvious alkene right? Still must blame the answer keey.
Huh? Isn't C5H8 the answer? Only C5H8 has 2 C=C what. C10H20 has only a double bond, and would only react with one mole of H2 gas.
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Ya hor. Should have drawn it out. Thanks for clarifying. :D
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Originally posted by UltimaOnline:
Solution :
The light bulb will become dimmer over time, as Ba2+(aq) and OH-(aq) ions are removed as BaSO4(s) and Cu(OH)2(s) respectively. There are therefore increasingly less mobile ions left in solution to conduct electricity. The light bulb will be turned off totally when, at a particular point in time, there are only covalently bonded water molecules left. As additional CuSO4(aq) is added, the light bulb will become brighter again. When aqueous ammonia is added, a blue ppt of Cu(OH)2(s) is formed, thereby removing Cu2+(aq) ions, so the bulb becomes dimmer again.
However, the bulb does not go off (as it did previously), because there are still mobile NH4+(aq) and SO4 2-(aq) ions left in solution. As excess aqueous ammonia is added, the Cu(OH)2(s) becomes a soluble complex ion [Cu(NH3)4]2+(aq) giving the solution a deep/dark blue colour, which (the tetraaminecopper(II) ion) is a mobile ion and conducts electricity, and so the bulb becomes even brighter.
(Note : at 'O' levels you don't need to know the name or formula of the complex ion.)
i missed out copper hydroxide cos i forgot it is insoluble. And i didnt know the ppt was simple Cu(OH)2, so the complex salt is only formed after it dissolves in excess cool.
and i also missed this out
The light bulb will be turned off totally when, at a particular point in time, there are only covalently bonded water molecules left.
Actually today should practise long answering since it is paper 2