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2001 - Multiple Choice - Q4 - HSN forum

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2001 - Multiple Choice - Q4


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#1 oddbins

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Posted 28 May 2005 - 02:24 PM

Hey, can anyone show me how to do this question please, thanks.

A mixture of sodium bromide and sodium sulphate is known to contain 5 mol of sodium and 2 mol of bromide ions.

How many moles of sulphate ions are present?

A 1.5

B 2.0

C 2.5

D 3.0

The answer is A, btw.

#2 dfx

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Posted 28 May 2005 - 02:42 PM

Na has a valency of 1, Br has a valency of 1, SO4 a valency of 2 (ignoring the charges)
So:
Na+ + Br- and Na+ + SO4 2-

1 : 1 - - - -1 : 2 (Valencies on this line)

Thus, there's 2 moles of Bromine (Br) so there should be 2 moles of Sodium in Sodium Bromide (valencies are 1:1 ratio).

So in total there are 5 moles of Sodium, 2 used up in Sodium Bromide, which leaves 3 Sodiums in the Sodium Sulphate. Again, use the valency ratio: 1 mole of Sodium REQUIRES 2 moles of Sulphate (So No. of Sulphate is a half of Sodium). Thus a half 3 is 1.5. So 1.5.

I know the wording isn't exactly the best but I'm sure someone will summarise it in a much better way. Hope this helped smile.gif

#3 oddbins

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Posted 28 May 2005 - 02:53 PM

QUOTE(dfx @ May 28 2005, 03:42 PM)
Na has a valency of 1, Br has a valency of 1, SO4 a valency of 2 (ignoring the charges)
So:
Na+  +  Br- and  Na+  +  SO4 2-

  1    :      1 - - - -1      :        2  (Valencies on this line)

Thus, there's 2 moles of Bromine (Br) so there should be 2 moles of Sodium in Sodium Bromide (valencies are 1:1 ratio).

So in total there are 5 moles of Sodium, 2 used up in Sodium Bromide, which leaves 3 Sodiums in the Sodium Sulphate. Again, use the valency ratio: 1 mole of Sodium REQUIRES 2 moles of Sulphate (So No. of Sulphate is a half of Sodium). Thus a half 3 is 1.5. So 1.5.

I know the wording isn't exactly the best but I'm sure someone will summarise it in a much better way. Hope this helped smile.gif

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Kool, that helped alot thanks. biggrin.gif





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