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#1 william

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Posted 04 April 2007 - 08:23 PM

I was doing a past paper today and i found a question that i have absolutely no idea how to tackle (2003 paper 1 Q5). the same type of question seems to come up nearly every year so i figured that it was a mark to bag.

A mixture of sodium chloride and sodium sulphate is known to contain 0.6 mol of chloride ions and 0.2 mol of sulphate ions. how many moles of sodium ions are present?
A 0.4
B 0.5
C 0.8
D 1.0


"millis times ten to the power of minus 3 yeah?"

#2 maximus

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Posted 04 April 2007 - 09:22 PM

Erm u've got 

\begin{align*}Na^+\end{align*}



\begin{align*}Cl^-\end{align*}

n u've got 0.6 mol of chloride ions, so you'll have 0.4 mol of sodium ions. i think it's something to do with how the ionic lattice is neutral because there's equal numbers of ions.



\begin{align*}Na_2^+ SO_4^2-\end{align*}

. i just did the same here, 0.2 mol of sulphate, so in that compound there must be two lots of 0.3 mol of sodium to make up the lattice cos there has to be equal numbers of them i think. So adding them up u get 1.0 . hope that helps smile.gif




#3 ginneswatson

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Posted 05 April 2007 - 01:11 PM

QUOTE(maximus @ Apr 4 2007, 10:22 PM) View Post
Erm u've got 

\begin{align*}Na^+\end{align*}



\begin{align*}Cl^-\end{align*}

n u've got 0.6 mol of chloride ions, so you'll have 0.4 mol of sodium ions. i think it's something to do with how the ionic lattice is neutral because there's equal numbers of ions.



\begin{align*}Na_2^+ SO_4^2-\end{align*}

. i just did the same here, 0.2 mol of sulphate, so in that compound there must be two lots of 0.3 mol of sodium to make up the lattice cos there has to be equal numbers of them i think. So adding them up u get 1.0 . hope that helps smile.gif


Maximus is basically doing the right thing, but I think a couple of numbers are wrong.

These questions are really testing your ability to write correct ionic formulae. You should start off by writing the formulae (using the data booklet to help). In this case Na+Cl and (Na+)2SO42.

Then it is an exercise in logic. In Na+Cl the ratio of ions is 1:1, so 0.6 moles of Na+ to match up with 0.6 moles Cl.

In (Na+)2SO42 the ratio of ions is 2:1, so 0.4 moles of Na+ to match up with 0.2 moles SO42.

A total of 1.0 mole of Na+ ions.

Hope this helps dry.gif

#4 maximus

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Posted 05 April 2007 - 05:57 PM

QUOTE(ginneswatson @ Apr 5 2007, 02:11 PM) View Post
QUOTE(maximus @ Apr 4 2007, 10:22 PM) View Post
Erm u've got 

\begin{align*}Na^+\end{align*}



\begin{align*}Cl^-\end{align*}

n u've got 0.6 mol of chloride ions, so you'll have 0.4 mol of sodium ions. i think it's something to do with how the ionic lattice is neutral because there's equal numbers of ions.



\begin{align*}Na_2^+ SO_4^2-\end{align*}

. i just did the same here, 0.2 mol of sulphate, so in that compound there must be two lots of 0.3 mol of sodium to make up the lattice cos there has to be equal numbers of them i think. So adding them up u get 1.0 . hope that helps smile.gif


Maximus is basically doing the right thing, but I think a couple of numbers are wrong.

These questions are really testing your ability to write correct ionic formulae. You should start off by writing the formulae (using the data booklet to help). In this case Na+Cl and (Na+)2SO42.

Then it is an exercise in logic. In Na+Cl the ratio of ions is 1:1, so 0.6 moles of Na+ to match up with 0.6 moles Cl.

In (Na+)2SO42 the ratio of ions is 2:1, so 0.4 moles of Na+ to match up with 0.2 moles SO42.

A total of 1.0 mole of Na+ ions.

Hope this helps dry.gif



Ahh.. that makes it a whole lot simpler, thanks biggrin.gif

#5 william

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Posted 05 April 2007 - 06:29 PM

thanks for the help because this one had me stumped and it was basically standard grade stuff!
"millis times ten to the power of minus 3 yeah?"





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