[simon_v]

I'm having trouble getting my head around this problem...

The question is:

"How many moles of;

a. MgCl₂
b. Mg²⁺ ions
c. Cl^- ions

is there in 16.4mL of 0.117mol L^-1 of MgCl₂"

Now the first one is easy if I just use C=n/V and works out to be 1.92x10^-3 mol. This is where I come unstuck. Why is the answer to (b) the same as (a)? And how do you come up with an answer of 3.84 x 10^-3 mol of Cl^- ions.

My logic was that if 1mol of MgCl₂ has a molecular weight of 95.3g/mol, then it will be 25.5% Mg and 74.5% Cl, and therefore 1.92x10^-3mol of MgCl₂ will be 25.5% Mg²⁺ ions and 74.5% Cl^- ions so I could work it out from these percentages based on the chemical formula. Obviously this is wrong but I can't find an explanation of why it is wrong.

If anyone could explain this I'd very much appreciate it.

regards,

Simon

[Mitch]

For every molecule of MgCl2 how many atoms of Mg2+ do you have?

[simon_v]

1

[simon_v]

No - wait -  

the chemical formula states the number of MOLES of Mg²⁺ ions : Cl^- ions - so this formula states that there is 1 mole of Mg²⁺ ions to 2 moles of Cl^- ions so if there is 1.92 x 10^-3 mole of MgCl - then, stoichiometrically speaking there is 1.92x10^-3 mole of Mg²⁺ AND 2 x 1.92x10^-3mol of Cl^- ions which gives you 3.84x10^-3mol Cl^- ions.

GOT IT

Thanks

[Borek]

atoms of Mg2+

Huh?

But from the educational point of view - 100% success  

[Mitch]

Thankyou, thats years of chemical education experience at work. Sometimes grammar is a victim.