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# A-level Chemistry question HELP!!! watch

1. When ammonium dichromate(VI) is added gradually to molten ammonium thiocyanate, Reinecke’s salt is formed. It has the formula NH4[Cr(SCN)x(NH3)y] and the following composition by mass:

Cr - 15.5 %
S - 38.15 %
N - 29.2 %.

(a) Calculate the values of x and y in the above formula.

Thanks
2. can anyone answer, it's very important
3. anyone...
4. You need to work out the ratios of each element you can, being N, S and Cr.

You do this by finding the molar ratios (working out the moles and then dividing the largest by the smallest ones)- kinda like empirical formula

=Percentage given in the question divided by atomic weight
Cr: 15.5 / 51.9 = 0.29
=Divide the moles of both sulphur and nitrogen by 0.29
S: 1.19 / 0.298 = 4
N: 2.08 / 0.298 = 7

This gives us ratios of number of atoms in the WHOLE compound

S: 128 / 32. = 4

N: 97 / 14 = 7

This means there are a total of 4 sulphur and 7 nitrogen atoms in the compund. resen in the complex. SCN has 4 of the 7 nitrogen atoms and NH4 has another, so that leaves 2.

overall= NH4[Cr(SCN)4(NH3)2]

i hope that makes sense
5. (Original post by ionicbond)
You need to work out the ratios of each element you can, being N, S and Cr.

You do this by finding the molar ratios (working out the moles and then dividing the largest by the smallest ones)- kinda like empirical formula

=Percentage given in the question divided by atomic weight
Cr: 15.5 / 51.9 = 0.29
=Divide the moles of both sulphur and nitrogen by 0.29
S: 1.19 / 0.298 = 4
N: 2.08 / 0.298 = 7

This gives us ratios of number of atoms in the WHOLE compound

S: 128 / 32. = 4

N: 97 / 14 = 7

This means there are a total of 4 sulphur and 7 nitrogen atoms in the compund. resen in the complex. SCN has 4 of the 7 nitrogen atoms and NH4 has another, so that leaves 2.

overall= NH4[Cr(SCN)4(NH3)2]

i hope that makes sense
Thanks for responding but i don't really get it, what does the 0.29 represent. Does it mean Cr has 0.29th of an atom in it as Cr has only one atom?
And you already found the moles of s and n ( 4 and 7) then why did you do for e.g. 128/32=4 again? (where did you get 128 and 97 from) I hope you get what I mean.
6. (Original post by GCSE 9)
Thanks for responding but i don't really get it, what does the 0.29 represent. Does it mean Cr has 0.29th of an atom in it as Cr has only one atom?
And you already found the moles of s and n ( 4 and 7) then why did you do for e.g. 128/32=4 again? (where did you get 128 and 97 from) I hope you get what I mean.

I see what you mean. sorry that was unclear. i will work it again:

so: N= 29.2/14= 2.08 moles of N in compound
S: 38.15/32=1.19 moles of S in compound
Cr: 15.5/51=0.29 MOLES OF CR IN COMPOUND

Divide everything by 0.3. That gives you;

N=7
S=4
Cr=1

Therefore, there are 4 of S and 7 of N
7. (Original post by ionicbond)
I see what you mean. sorry that was unclear. i will work it again:

so: N= 29.2/14= 2.08 moles of N in compound
S: 38.15/32=1.19 moles of S in compound
Cr: 15.5/51=0.29 MOLES OF CR IN COMPOUND

Divide everything by 0.3. That gives you;

N=7
S=4
Cr=1

Therefore, there are 4 of S and 7 of N
Thank you so much i get it know ))

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