o lvl chem qns
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1)Which of the following suggests that a metal oxide MO has amphoteric properties?
Ans is this eqn: MO(s) + H2O(l) => M(2+) (aq) + 2OH(-) (aq)
No idea how to tell from this eqn. Can anyone help?
2)An isotope of element Z has 18neutrons and 17 protons. Wich of the following symbols is correct for the ion of Z?
a)18(mass no), 17(proton no), Z (-1)
b)35, 17, Z (+1)
c)35, 17, Z (-1)
d)35, 18, Z (+1)
Ans is C but i dunno y cant be B
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Originally posted by ultimatenolifer:
1)Which of the following suggests that a metal oxide MO has amphoteric properties?
Ans is this eqn: MO(s) + H2O(l) => M(2+) (aq) + 2OH(-) (aq)
No idea how to tell from this eqn. Can anyone help?
2)An isotope of element Z has 18neutrons and 17 protons. Wich of the following symbols is correct for the ion of Z?
a)18(mass no), 17(proton no), Z (-1)
b)35, 17, Z (+1)
c)35, 17, Z (-1)
d)35, 18, Z (+1)
Ans is C but i dunno y cant be B
Q1. The equation which you gave as the answer, does not suggest the oxide is amphoteric, merely that the metal is in Grp II and that its oxide is soluble. The correct answer should be an equation which shows the metal oxide reacting with a base or alkali (eg. NaOH) to generate a complex salt. Eg. Al2O3 + 3H2O + 2NaOH ---> 2NaAl(OH)4
Q2. Because you should familiar with the fact that chlorine is a non-metal and readily gains electrons to be reduced to chloride anions, instead of losing electrons to form Cl+ cations. Also note that Cl- has a stable octet while Cl+ does not.
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hi with regards to qn1, the 4 choices are as below
a)MO + 2H(+)(aq) => M(2+)(aq) + H2O(l)
b)MO + 20H(-)(aq) => MO2 (aq) + H2O(l)
c) as above
d)MO(s) + 2NH4Cl(s) => MCl2(s) + H2O(l) + 2NH3(g)
So may i kindly know wich is the correct answer coz answer sheet indicated C. Thks.
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I stared at the question, and realized what the setter trying to say....a) reaction with positive ion species, implying hydrogen ions
b) reaction with negative ion species, implying hydroxide ions
c) reaction with both positive and negative ion species, water contains both hydrogen and hydroxide ions in equal.
d) You need not know.
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Either he/she is newbie from nie, or some top school question paper...
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Originally posted by ultimatenolifer:
hi with regards to qn1, the 4 choices are as below
a)MO + 2H(+)(aq) => M(2+)(aq) + H2O(l)
b)MO + 20H(-)(aq) => MO2 (aq) + H2O(l)
c) as above
d)MO(s) + 2NH4Cl(s) => MCl2(s) + H2O(l) + 2NH3(g)
So may i kindly know wich is the correct answer coz answer sheet indicated C. Thks.
Metal oxides are usually basic. Therefore, for its oxide to display amphoteric properties, look for an equation in which it behaves as an acid, rather than a base. Eg. Al2O3 + 3H2O + 2NaOH ---> 2NaAl(OH)4
Option A shows the metal oxide behaving as a regular base (in accepting protons), ie. doesn't suggest amphoteric properties.
Option B is faulty/errorneous because the charges are not balanced. If they added water as a reactant and balanced it correctly (see my equation in previous paragraph), then option B would be the answer.Option C shows the hydrolysis and hydration of a soluble Grp II metal oxide (transition metal oxides are usually insoluble), which therefore implies it is a basic oxide, ie. doesn't suggest amphoteric properties.
Option D shows the metal oxide behavnig as a base (in accepting protons from the acidic salt NH4+Cl- which for 'O' levels you're not required to know is acidic), ie. doesn't suggest amphoteric properties.
So for this faulty question with these faulty options, all 4 options are wrong. As Darkness_hacker99 said, this question setter jia-lat / jiak-lak ("eat strength").
Summary :
ultimatenolifer, for 'O' levels, when they ask you to write/choose an equation to show that a metal oxide is amphoteric, just choose an equation in which the metal oxide behaves as an acidic oxide to react with bases/alkalis (since non-amphoteric metal oxides only behave as basic oxides to react with acids).
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Additional Note :
Beryllium oxide, a Grp II oxide, is indeed an amphoteric oxide. (For an explanation why, see the end of this post. For 'O' levels, you don't need to explain why; just memorize that 3 common amphoteric oxides are the oxides of Pb, Zn, and Al. There are actually several more, which you'll encounter at 'A' levels.)
However, BeO does not dissolve (ie. hydrolyse and hydrate) in water; as implied by option C of your question (see 1st post of this thread). (The reason why BeO is insoluble in water, and has no reaction with water, is due to the large magnitude of the endothermic lattice dissociation process. Again, an understanding of this is not required for 'O' levels.)
BeO does however, dissolve (ie. react to form soluble products) in aqueous acids and aqueous alkalis, demonstrating its amphoteric properties.
BeO reacts with acids as follows :
BeO(s) + 2H+(aq) --> Be2+(aq) + H2O(l)
BeO reacts with alkalis as follows :
BeO(s) + 2OH- + H2O(l) --> [Be(OH)4]2-(aq)
I could go into deeper detail and explanation why certain metal oxides are amphoteric (ie. due to the high charge density of the metal cations, they have a propensity to accept dative bonds from hydroxide ion ligands to generate complex ions; their oxide anions undergo hydrolylsis to generate hydroxide ions, but overall the metal oxide accepts more hydroxide ion ligands than it generates; hence when the metal oxide removes from solution hydroxide ions as an overall net effect, it is considered to have acidic capabilities and and hence the metal oxide is regarded as amphoteric), but an understanding of such is not required for 'O' levels.
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1) Three metals (B,C,D) are dropped into water. D does not react but B and C do, liberating a gas that gives a pop sound with a burning splint. When C is dropped into aqueous B sulphate, B is not displaced. What is the increasing order of reactivity of these metals.
A) D,B,C
B) D,C,B
C) B,C,D
D)B,D,C
2) Elements E, G, H belong to the same period of the periodic table. The properties of the oxides of E, G, H are given in the table below.
Oxide of E - Soluble in both HCL and aqueous NaOH
Oxide of G- Decolourises acidified aqueous potassium manganate (VII)
Oxide of H- Insoluble in aqueous NaOH but soluble in dilute HCL
Which of the following gives the correct sequence of E,G and H in order of increasing proton number in the periodic table?
A) E,G,H
B)E,H,G
C) H, E, G
D) G,E,H
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Originally posted by ultimatenolifer:
1) Three metals (B,C,D) are dropped into water. D does not react but B and C do, liberating a gas that gives a pop sound with a burning splint. When C is dropped into aqueous B sulphate, B is not displaced. What is the increasing order of reactivity of these metals.
A) D,B,C
B) D,C,B
C) B,C,D
D)B,D,C
2) Elements E, G, H belong to the same period of the periodic table. The properties of the oxides of E, G, H are given in the table below.
Oxide of E - Soluble in both HCL and aqueous NaOH
Oxide of G- Decolourises acidified aqueous potassium manganate (VII)
Oxide of H- Insoluble in aqueous NaOH but soluble in dilute HCL
Which of the following gives the correct sequence of E,G and H in order of increasing proton number in the periodic table?
A) E,G,H
B)E,H,G
C) H, E, G
D) G,E,H
Q1. B.
Q2. C
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How many of the electrons in a molecule of CF4 are not involved in bonding?
A) 0
B) 2
C) 26
D) 34
Ans is D but i dont know the reason. Anyone can help?
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Originally posted by ultimatenolifer:
How many of the electrons in a molecule of CF4 are not involved in bonding?
A) 0
B) 2
C) 26
D) 34
Ans is D but i dont know the reason. Anyone can help?
Total no of electrons = total no of protons (since CF4 is a neutral molecule) = 6 + 4(9) = 42No of bonding electrons = 8 (since there are 4 bond pairs)
No of electrons not involved in bonding = total electrons - bonding electrons = 42 - 8 = 34
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1) Substance Q, upon addition of aq NH3 forms a white ppt which is insoluble in excess. Besides, upon addition of dilute HNO3 and aq AGNO3, a white ppt is formed. What is Q?
A) Aluminium chloride B)Aluminium sulfate C)Lead(II) chloride D)Lead(II) sulfate
Ans is C but i wanna know why A is not possible.
2) Which of the following statements cannot explain why the temperature at the bottom of the furnace for extracting iron is higher than the temp at the top?
A)Combustion of coke is an exothermic process which occurs where hot air enters at the btm of the furnace
B)Decomposition of limestone is an endothermic process which occurs where it is fed in at the top of the furnace
C)Hot air is fed in at the btm of the furnace is at a higher temp than the waste gases leaving frm the top of the furnace
D) Reduction of iron ore to iron is an exothermic process which occurs where the iron ore is fed in at the top of the furnace
Ans is D but i would also be interested to know how do A to C explain the question.
3)What effect does a higher temp have on its activation energy and enthalpy change?
EA Enthalpy change
A decreases increases
B decreases no change
C no change increases
D no change no change
Ans is D but would like to know why its not B
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Q1.
You're right. The ans should be A not C.
Q2.
Regard this as a simple logic-type question, ie. knowledge of content of 4 options not required; in other words, assuming the info stated by each of the options is true, which option is logically inconsistent? D, because if the reduction of iron ore to iron is exothermic and takes place near the top, then logically the top should be warmer, not cooler.
Q3.
Increasing the temperature does not change the activation energy required for the reaction; instead what happens is increasing the temperature increases the percentage of reactant molecules whose energy equals or exceeds the activation energy required for the reaction, and therefore the rate of reaction increases.
Originally posted by ultimatenolifer:1) Substance Q, upon addition of aq NH3 forms a white ppt which is insoluble in excess. Besides, upon addition of dilute HNO3 and aq AGNO3, a white ppt is formed. What is Q?
A) Aluminium chloride B)Aluminium sulfate C)Lead(II) chloride D)Lead(II) sulfate
Ans is C but i wanna know why A is not possible.
2) Which of the following statements cannot explain why the temperature at the bottom of the furnace for extracting iron is higher than the temp at the top?
A)Combustion of coke is an exothermic process which occurs where hot air enters at the btm of the furnace
B)Decomposition of limestone is an endothermic process which occurs where it is fed in at the top of the furnace
C)Hot air is fed in at the btm of the furnace is at a higher temp than the waste gases leaving frm the top of the furnace
D) Reduction of iron ore to iron is an exothermic process which occurs where the iron ore is fed in at the top of the furnace
Ans is D but i would also be interested to know how do A to C explain the question.
3)What effect does a higher temp have on its activation energy and enthalpy change?
EA Enthalpy change
A decreases increases
B decreases no change
C no change increases
D no change no change
Ans is D but would like to know why its not B
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thx ultimaonline, but i have 2 more qns here :)
1)in which electrolysis expt would there be no change in the conc of the electrolyte over time?
electrolyte electrodes
a aq AgNO3 silver
b aq CuSO4 graphite
c conc NaCl platinum
d Dilute HCl platinum
No idea what the qn is asking for...zzz
2) during the electrolysis of water, which of the following shows the vol of hydrogen and oxygen gases collected when 3x10^24 electrons pass thru the circuit?
Vol of H2(dm3) Vol of O2(dm3)
a) 30 15
b) 30 60
c) 60 30
d) 60 60
dunno how to calculate also....
btw wif regards to the first qn i asked in the previous thread, why is a not accepted. Coz i tot both alcl3 and pbcl2 are accepted since both yield same results??
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>>> Coz i tot both alcl3 and pbcl2 are accepted since both yield same results?? <<<
True, that.
Q1.
Ans is (a) aq AgNO3, silver electrodes.
Anode : Ag ---> Ag+ + e-
Cathode : Ag+ + e- ---> Ag
Q2.
Cathode :
4H2O + 4e- ---> 2H2 + 4OH-
Anode :
2H2O ---> O2 + 4H+ + 4e-
Redox :
6 H2O + 4e- ---> 2H2 + O2 + 4H2O + 4e-
Simplyfing, we get :
2 H2O ---> 2H2 + O2
Since 3x10^24 e- = 5 moles of e-
No of moles of H2 gas generated at cathode = 0.5 x 5 = 2.5 moles = 60dm3
No of moles of O2 gas generated at cathode = 0.25 x 5 = 1.25 moles = 30dm3
Originally posted by ultimatenolifer:thx ultimaonline, but i have 2 more qns here :)
1)in which electrolysis expt would there be no change in the conc of the electrolyte over time?
electrolyte electrodes
a aq AgNO3 silver
b aq CuSO4 graphite
c conc NaCl platinum
d Dilute HCl platinum
No idea what the qn is asking for...zzz
2) during the electrolysis of water, which of the following shows the vol of hydrogen and oxygen gases collected when 3x10^24 electrons pass thru the circuit?
Vol of H2(dm3) Vol of O2(dm3)
a) 30 15
b) 30 60
c) 60 30
d) 60 60
dunno how to calculate also....
btw wif regards to the first qn i asked in the previous thread, why is a not accepted. Coz i tot both alcl3 and pbcl2 are accepted since both yield same results??
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0.1 mol of metal X (Ar=27) was burned in oxygen to give an oxide with mass of 5.1g. What is the formula of the metal oxide if it has a relative molecular mass of 30?
a)XO
B)XO2
c)X203
d)X20
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The question is in error (all options' molar masses exceed 30g, since X = 27g and O = 16g). Tell your student to ignore this qn. An error-free, similar question, that you can give your student for stoichiometry practice would be : Titanium(IV) oxide TiO2, is heated in hydrogen has to give water and a new titanium oxide. TixOy. if 1.598g of TiO2 produced 1.438g of TixOy, what is the formula of the new oxide? Solution : No of moles of Ti before redox = No of moles of Ti after redox this implies : 1 (No of moles of TiO2) = x (No of moles of TixOy) this implies : 1 (1.598 / 79.9) = x (1.438 / (47.9x + 16y)) this implies : x / y = 2 / 3 Therefore, formula of titanium oxide = Ti2O3
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Actually the qn can be solved if the last statement is ignored
0.1 mol X = 2.7 g since Ar of X = 27
Mass of oxide formed= 5.1g
Then mass of O in X oxide = 5.1 - 2.7 g = 2.4g
Number of mol of X = 0.1
Amt of O in oxide = 2.4/16 = 0.15 mol
X : O = 0.1 : 0.15 = 2 :3
Taking the ratio of X: O to be mol. formula, formula of oxide of X is X2O3
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thx for the replies guys. btw i would like to clarify some stuff on the topic of reactions due to some varying answers given in different assessment papers.
Am i right to say that an endothermic rxn is as follows? (correct me if im wrong)
1) the products have more energy than the reactants
2) the total energy change in bond formation is less than that in bond breaking
3)the enthalpy change of a rxn is calculated by : energy released by products - energy taken in by reactants
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Originally posted by ultimatenolifer:
thx for the replies guys. btw i would like to clarify some stuff on the topic of reactions due to some varying answers given in different assessment papers.
Am i right to say that an endothermic rxn is as follows? (correct me if im wrong)
1) the products have more energy than the reactants
2) the total energy change in bond formation is less than that in bond breaking
3)the enthalpy change of a rxn is calculated by : energy released by products - energy taken in by reactants
Statements #1 and #2 are fine. Statement #3 is problematic and ambiguous. It is better rephrased to :The enthalpy change of a reaction is calculated by : the sum of the endothermic (ie. positive value) enthalpy/energy* change or heat absorbed to break the (relevant or total) bonds in the reactants to reach the transition state, and the exothermic (ie. negative value) enthalpy/energy* change or heat released by the transition state species as it is converted into the products.
(* technically, energy and enthalpy are not exactly the same, but for 'O' level and even 'A' level purposes, they may be regarded as approximately the same)
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Good, I see you making use of this forum to clear your doubts. Keep it up!
Best Regards,
Chin Seng
Homework Forum Moderator -
hi would like to ask
1) if equal masses of magnesium carbonate was added to 100cm3 of 2mol/dm3 of hcl(beaker a) and h2so4(beaker b) repectively, will the rate of rxn be the same?
2) Why is a iron block with tin coated on its surface and placed in a beaker of water rust faster than another intact iron block in the same beaker?
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Originally posted by ultimatenolifer:
hi would like to ask
1) if equal masses of magnesium carbonate was added to 100cm3 of 2mol/dm3 of hcl(beaker a) and h2so4(beaker b) repectively, will the rate of rxn be the same?
2) Why is a iron block with tin coated on its surface and placed in a beaker of water rust faster than another intact iron block in the same beaker?
Qn1.
Molarity (ie. molar concentration) of protons (ie. H+ ions) are greater in the solution of H2SO4 (which is a strong monoprotic acid and a weak diprotic acid) compared to in HCl (which is a strong monoprotic acid only).
Qn2.
Since iron is more reactive than tin (see redox potentials below), thus when iron is coated with tin, the iron acts as the anode while the tin acts as the cathode. Any tin which is naturally oxidized (eg. by contact with atmospheric oxygen and water) will be immediately reduced by the iron, and the iron consequently experiences an accelerated oxidation / corrosion / rusting (because it is being oxidized by both the tin cations as well as atmospheric oxygen and water). This is also the basis for 'sacrificial protection' when galvanizing iron with zinc.
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1) Electrolysis of concentrated nacl was carried out. If the gas collected at the anode is 25cm3, what is the vol of gas collected at the cathode, assuming all gases are not soluble in water?
2)Why is it that the ph of dilute sulphuric acid will increase during electrolysis (stated in answer sheet)? I thought it stays the same since oh- and h+ are discharged.
3)Which of these produces the best yield of calcium sulfate?
a) Add dilute sulphuric acid to calcium oxide
b) Add dilute nitric acid to calcium carbonate, followed by dilute sulphuric acid
Ans is given to be a but why not b?
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Originally posted by ultimatenolifer:
1) Electrolysis of concentrated nacl was carried out. If the gas collected at the anode is 25cm3, what is the vol of gas collected at the cathode, assuming all gases are not soluble in water?
2)Why is it that the ph of dilute sulphuric acid will increase during electrolysis (stated in answer sheet)? I thought it stays the same since oh- and h+ are discharged.
3)Which of these produces the best yield of calcium sulfate?
a) Add dilute sulphuric acid to calcium oxide
b) Add dilute nitric acid to calcium carbonate, followed by dilute sulphuric acid
Ans is given to be a but why not b?
Q1.
Write half equations at electrodes :
H+ reduced to H2 at cathode
OR
H2O reduced to H2 at cathode.
AND
Cl- oxidized to Cl2 at anode.
Write balanced redox equation. Apply stoichiometry.
Q2.
If both H+ and OH- (ie. H2O) is being removed from solution, then the molarity of the acid will increase, which means the pH will *decrease* (not increase!) as the electrolysis is being carried out.
Q3.
Correct answer is B, *not* A. Method A sucks (ie. lower yield) because some of the insoluble CaSO4 ppt coating the CaO reactant will prevent complete reaction. Method B is better (ie. higher yield) because both reactants (in the final step) are aqueous, and thus will not encounter the difficulty described in Method A.