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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

Molestermannn69 SG

Hi, I have a question for chemistry and read you can post them here?

5.00g of an anhydrous Grp 2 metal nitrate loses 3.29g in mass when heated strongly

2M(NO3)2 => 2MO+4NO2+O2

What is metal M?

Magnesium

Calcium

strongtium

barium

UltimaOnline SG

Hi, I have a question for chemistry and read you can post them here?

5.00g of an anhydrous Grp 2 metal nitrate loses 3.29g in mass when heated strongly

2M(NO3)2 => 2MO+4NO2+O2

What is metal M?

Magnesium

Calcium

strongtium

barium

I'll leave it to Flying Grenade (don't disappoint me) to show you how to solve it.

Flying grenade SG

cs toh didnt notice this ,or at least he didn't update this

pg 188 chem advanced guide

the 'slow' and 'fast' is wrong, and must be cancelled right?

UltimaOnline SG

cs toh didnt notice this ,or at least he didn't update this

pg 188 chem advanced guide

the 'slow' and 'fast' is wrong, and must be cancelled right?

Obviously. Now go solve Molestermannn69's qn.

Flying grenade SG

Originally posted by UltimaOnline :

I'll leave it to Flying Grenade (don't disappoint me) to show you how to solve it.

5.00g of an anhydrous Grp 2 metal nitrate loses 3.29g in mass when heated strongly

2M(NO3)2 (s) => 2MO(s) + 4NO2(g) + O2(g)

What is metal M?

Flying grenade's suggested solution :

Mass MO must be 5.00 - 3.29 = 1.71g

mol = mass/mr

let mr of M be x

ηM(NO3)2 = 5/(x+124)

ηMO=1.71/(x+16)

since stoichiometric ratio of M(NO3)2 to MO is 1:1

we can equate ηM(NO3)2 = ηMO

solving the equations , we obtain x=40.1 (3sf)

Metal M is Calcium.

UltimaOnline SG

Originally posted by UltimaOnline :

I'll leave it to Flying Grenade (don't disappoint me) to show you how to solve it.

5.00g of an anhydrous Grp 2 metal nitrate loses 3.29g in mass when heated strongly

2M(NO3)2 => 2MO+4NO2+O2

What is metal M?

Flying grenade's suggested solution

mol = mass/mr

let mr of M be x

ηM(NO3)2 = 5/(x+124)

ηMO=1.71/(x+16)

since stoichiometric ratio of M(NO3)2 to MO is 1:1

we can equate ηM(NO3)2 = ηMO

solving two equations simultaneously, we obtain x=40.1 (3sf)

Metal M is Calcium.

Correct. Well done, Flying grenade.

Fyi, my own working is [ 5g / [ 3.29g / (molar mass of 4NO2+O2) ] x 2 ] - [ (molar mass of NO3) x 2] = 40.1g

But your working will of course get full marks as well. Cambridge only marks the final answer, as long as your working is reasonable, you've some creative freedom to work it out as you wish.

Flying grenade SG

can write cold, alkaline kmno4 (aq)?

or must write cold, kmno4 (aq), NaOH(aq)?

for formation of diol fron alkene

can write acidified K2Cr2O7 (aq), heat

or must write H2SO4 (aq) , K2Cr2O7 (aq), heat?

my sch say must write the compound out

UltimaOnline SG

can write cold, alkaline kmno4 (aq)?

or must write cold, kmno4 (aq), NaOH(aq)?

for formation of diol fron alkene

can write acidified K2Cr2O7 (aq), heat

or must write H2SO4 (aq) , K2Cr2O7 (aq), heat?

my sch say must write the compound out

In the A level exams, the reagents and conditions for each reaction, should be written in exactly the same way that either CS Toh, or Chan Kim Seng, or George Chong writes them (ie. above & below the equation arrow) in the reaction equations. Cambridge will accept any of these alternative ways of writing the reagents and conditions. All JC students are advised to closely follow these 3 sources, over their own school lecture notes.

Flying grenade SG

why U.O. say H- can't exist? can exist with o.s. -1 as in hydride, but cannot exist with mononegative formal charge is it?

can H have 2 electrons around it?

I'm confused, help !!

Flying grenade SG

for Haemoglobin, two N atoms are ionically bonded to Fe2+ central atom, the other 2 N atoms are datively bonded to Fe2+?

UltimaOnline SG

why U.O. say H- can't exist? can exist with o.s. -1 as in hydride, but cannot exist with mononegative charge is it?

can H have 2 electrons around it?

I'm confused, help !!

Don't be silly. I never said H- can't exist.

UltimaOnline SG

for Haemoglobin, two N atoms are ionically bonded to Fe2+ central atom, the other 2 N atoms are datively bonded to Fe2+?

Obviously.

Flying grenade SG

Don't be silly. I never said H- can't exist.

why mechanism the H atom cannot take a lone pair of e- uh?

H cannot have uninegative formal charge uh?

UltimaOnline SG

why mechanism the H atom cannot take a lone pair of e- uh?

Because your mistake (a common error for Singapore JC students) is that you wrongly eliminated a H- when the reaction required the elimination of a H+. H- is extremely unstable, and thus will attack right back. It is therefore a non-viable leaving group. H- does exist, but must always be carried by a more stable, larger, lower charge density species, the carrier species known as the reducing agent which delivers the H- ion.

Flying grenade SG

THANKSSSS ULTIMAAA

UltimaOnline SG

THANKSSSS ULTIMAAA

Also, if you're observant, you'll realize that whenever you mistakenly eliminate a H- instead of a H+, the O or N or C atom that was bonded to the H atom will consequently lack a stable octet, which will alert you to the error. When H+ is eliminated, it leaves its electrons behind, allowing the O or N or C atom to keep its stable octet.

Flying grenade SG

page 130 cs toh advanced guide

for electrolysis of brine, why when Hg is used as cathode, Na(s) is liberated, instead of H2(g)?

redn potential of H+/H2 =0.00V shld be more +ve than Na+/Na

from data booklet, no Na+/Na redn potential value, but have for Li+/Li, which is very -ve.

is it becos if there is no electrode(as in without the Hg electrode) , the Na(s) want to deposit but no where to deposit? (and hence H2 is liberated instead? lolol)

for electrolysis of Na2SO4, the H+ at the cathode come from water H2O uh?

Flying grenade SG

pg 130

for electrolysis of H2SO4, why H2SO4 slowly become more concentrated?

for the anode reaction, Cs toh provided 2 equations

1st equation, more water form as product

2nd eqn, water as a reactant, then form O2 as product

UltimaOnline SG

page 130 cs toh advanced guide

for electrolysis of brine, why when Hg is used as cathode, Na(s) is liberated, instead of H2(g)?

redn potential of H+/H2 =0.00V shld be more +ve than Na+/Na

from data booklet, no Na+/Na redn potential value, but have for Li+/Li, which is very -ve.

is it becos if there is no electrode(as in without the Hg electrode) , the Na(s) want to deposit but no where to deposit? (and hence H2 is liberated instead? lolol)

for electrolysis of Na2SO4, the H+ at the cathode come from water H2O uh?

Mercury thermodynamically (both enthalpically and entropically) stabilizes the reduced sodium metal in amalgam, and also on further condition that a concentrated (not dilute) solution is electrolyzed.

UltimaOnline SG

pg 130

for electrolysis of H2SO4, why H2SO4 slowly become more concentrated?

for the anode reaction, Cs toh provided 2 equations

1st equation, more water form as product

2nd eqn, water as a reactant, then form O2 as product

At cathode, 4 H2O used (to supply the 4 H+), and O2 + 2 H2O + 4 OH- byproduct generated (not written in CS Toh's eqn, since he used 4 H+ instead of 4 H2O as reactant).

At anode, 4 H2O used (to supply the 4 OH-), and 2 H2 + 4 H+ byproduct generated (not written in CS Toh's eqn, since he used 4 OH- instead of 4 H2O as reactant).

Hence, total H2O used = 8, but total H2O generated = 2 + 4 = 6.

Therefore, overall balanced redox equation is 2 H2O(l) --> 2 H2(g) + O2(g), and the solution becomes more concentrated (as H2O is electrolyzed away).

Note that Singapore JCs will insist you use H2O as the reactant for both cathode & anode, instead of CS Toh's H+ and OH- respectively.

Cambridge will accept both versions, unless the solution being electrolyzed is strongly acidic or strongly alkaline; which is why Singapore JCs have decided it's safer to instruct JC students to just always use H2O as the reactant.

And if you can't figure out why, or which cathode & anode equations are acceptable or not acceptable in such electrolytic setups of strong acidity or alkalinity, go ask your school teacher or private tutor.

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