# A level chemistry help ions!

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#1
Hi
Please could someone explain this question
I got up to 4.818 x 10^23 particles of FeCl3, but then I don’t understand how the got the final answer as and what did they do to get it as: 1.445 x 10^24 chloride ions
Question b

Last edited by Cherrygrape1234; 10 months ago
0
10 months ago
#2
Ok, so the Mr of FeCl3 is 55.8 + (35.5 x 3) = 162.3
moles = mass ÷ Mr
= 129.9 ÷ 162.3 = 0.800… moles of FeCl3

In 1 mole of a species there are 6.02 x 10^23 particles
so in 0.800… moles there must be 0.800… x (6.022 x 10^23) particles
=4.818 x 10^23 particles /units of FeCl3
(which you got up to)

The Fe3+ ions and the Cl- ions are in a ratio of 1:3
if we had 2 units of FeCl3, there would be 2 Fe3+ and 6 Cl- ions present
ie there are three times as many Cl- ions than there are Fe3+ ions
so we multiply the 4.818 x 10^23 by 3
= 1.445 x 10^24 chloride ions
0
#3
(Original post by bl0bf1sh)
Ok, so the Mr of FeCl3 is 55.8 + (35.5 x 3) = 162.3
moles = mass ÷ Mr
= 129.9 ÷ 162.3 = 0.800… moles of FeCl3

In 1 mole of a species there are 6.02 x 10^23 particles
so in 0.800… moles there must be 0.800… x (6.022 x 10^23) particles
=4.818 x 10^23 particles /units of FeCl3
(which you got up to)

The Fe3+ ions and the Cl- ions are in a ratio of 1:3
if we had 2 units of FeCl3, there would be 2 Fe3+ and 6 Cl- ions present
ie there are three times as many Cl- ions than there are Fe3+ ions
so we multiply the 4.818 x 10^23 by 3
= 1.445 x 10^24 chloride ions
Thank you so much this makes so much sense! Also

For this question is the mark scheme is incorrect as I got the answer to be 1.81 x10^23??
And
Do you know how to workout 4a) and 4b)?

I’ve done the first one wrong as I got 3.27 x 10^31 and I also I don’t understand what to do for the second one…

Thank you!
Last edited by Cherrygrape1234; 10 months ago
0
10 months ago
#4
(Original post by Cherrygrape1234)

For this question is the mark scheme is incorrect as I got the answer to be 1.81 x10^23??
The mark scheme looked right when I checked on my calculator, make sure your last step is multiplying by 5 and all you numbers are correct.

Do you know how to workout 4a) and 4b)?

I’ve done the first one wrong as I got 3.27 x 10^31 and I also I don’t understand what to do for the second one…

Thank you!
a) remember mass= Ar x moles
Moles is also in the equation for Avogadros constant -> moles x Avogadros constant = number of atoms therefore to find moles for the first equation you divide number of atoms by Avogadros constant. The final step is then put that into the first equation and multiply by the Ar which is 197.

b) same two equations except instead of Ar and atoms your dealing with Mr and molecules.
Mass =Mr x moles
moles x Avogadros constant = number of molecules

The number of molecules is 1 so you divide 1 by Avogadros constant to get the moles. Then multiply by the Mr which is 18.
0
#5
(Original post by Hellllpppp)
The mark scheme looked right when I checked on my calculator, make sure your last step is multiplying by 5 and all you numbers are correct.

a) remember mass= Ar x moles
Moles is also in the equation for Avogadros constant -> moles x Avogadros constant = number of atoms therefore to find moles for the first equation you divide number of atoms by Avogadros constant. The final step is then put that into the first equation and multiply by the Ar which is 197.

b) same two equations except instead of Ar and atoms your dealing with Mr and molecules.
Mass =Mr x moles
moles x Avogadros constant = number of molecules

The number of molecules is 1 so you divide 1 by Avogadros constant to get the moles. Then multiply by the Mr which is 18.
Thank you so much Hellllpppp you explained all the questions so well I finally understand them all. Thanks again! Are you an A- level student?
0
10 months ago
#6
(Original post by Cherrygrape1234)
Thank you so much Hellllpppp you explained all the questions so well I finally understand them all. Thanks again! Are you an A- level student?
Just finished my A-levels and going to do sciences at uni in September
1
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