mole concept(ICSE -2006)

Active member

GIVEN THAT HE MOLECULAR MASS OF THE COPPER OXIDE IS 80, WHAT VOLUME OF AMMONIA
(measured at STP) IS REQUIRED TIO COMPLETLY REDUCE 120 g OF CuO?

EQUATION= 3CuO+2NH3 = 3Cu +3(WATER) +N2
Volume occupied by 1 mole of gas at STP =22.4 litres

PLEASE SOLVE THIS
I KNOW THE ANSWER BUT DONT KNOW THE CONCEPT BEHIND IT

ANSWER=22.4 dm3

Active member

3CuO+2NH3=3Cu+3H2O+N2
MOLECULAR MASS OF CUO=80
MOLECULAR MASS OF 3 CUO =3*80=240g
MOECULAR MASS OF NH3=17g
MOLECULAR MASS OF 2 NHE=2*17=34g
240g of cuo is reduced by 34g of NH3
SO 120G OF CUO IS REDUCED BY (34*120)/240=17g of NH3

THER4 17g OF NH3 OCCOPIES 22.4dm3
so 17g OF NH3 OCCUPIES (22.4*17)/17=22.4dm3
by the way dm3 & litre r the same

Active member

thanks a lot !

Subject: Re: mole concept(ICSE -2006) Sun Feb 24, 2008 10:14 pm

the molecular mass of amonia is not given in the question, how did you get it?

Active member

molecular wt of nitrogen=14
molecular wt of hydrogen =3*1=3
14+3=17
there4 of 2NH3=2*17
=34g

Active member

SINCE THE MOLECULAR MASS IS NOT GIVEN IN THE QUESTION
CAN WE SOLVE LIKE THIS ( I MEAN WHETHER THE METHOD IS CORRECT)

3CuO+2NH3=3Cu+3H2O+N2
MOLECULAR MASS OF CUO=80
MOLECULAR MASS OF 3 CUO =3*80=240g
1 mole 0f NH3 at STP =22.4dm3(given)
since in the equation the volume of NH3 is double
there4 volume of NH3= 44.8dm3
240g of cuo is reduced by 44.8dm3of NH3
SO 120G OF CUO IS REDUCED BY (44.8*120)/240=22.4dm3 of NH3

this way we can directly get the answer without finding the molecular
mass of NH3
and I suppose thats the reason that no molecular mass of N,O or
NH3 is given becoz we dont require it !

Active member

i m not sure............but i dont think dat the method is correct.i feel.........it is always better to find the molecular mass in such type of sums!!!!!!!!!!

Active member

Since in the question the molecular mass of H,N or NH3 is not given
then we have to learn the molecular mass of elements ?
I mean there so many of them and we dont know which compounds
may ICSE give.

and far as I know molecular mass of elements are given in the
question if required.

Active member

dat's crazy i'll die lern'in all da mol.masses. i'm just learn'in da imp.1's like O2,N,H,Cl,etc. not those rare 1's

Active member

just learn the imp 1s.molecular mass of elements r given in the questions if required.don't worry!
ALL THE BEST 4 UR EXAMS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Active member

good problem