Quick Chem question...

[rickblas]

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So I was doing some questions from Kaplan and this seemingly simple question caught me off gaurd.

it was :

What is the normality of 1M of H3PO4 if titrated with CaCl2?

The answer is 1.5N

Their explanation is that H3PO4 has 3 equivilants of acid so one half of the molecules of CaCl2 is needed to react with it and 3x.5 = 1.5.

Why is it needed to react with one half the molecules of CaCl2?

Thanks!

 

[UndergradGuy7]

Sounds like a bad problem.

http://forums.studentdoctor.net/showthread.php?p=3761904
Read post 17.

I think kaplan just did there are 3H+ in H3PO4 and it reacts with 2 Cl- so 3/2, but if you balance the equation like in that link above... then it should be different?

 

[Vinden]

So I was doing some questions from Kaplan and this seemingly simple question caught me off gaurd.

it was :

What is the normality of 1M of H3PO4 if titrated with CaCl2?

The answer is 1.5N

Their explanation is that H3PO4 has 3 equivilants of acid so one half of the molecules of CaCl2 is needed to react with it and 3x.5 = 1.5.

Why is it needed to react with one half the molecules of CaCl2?

Thanks!

IMO, normality of one substance depends on a particular reaction, and can be understood that the normality is the amount/mole equivelent of that substance to react with 1 mole of another reactant. In this reaction:
3CaCl2(aq) + 2H3PO4(aq) -> (Ca)3(PO4)2(s)+ 6HCl(aq)
3 moles of CaCl2 reacts with 2 moles of H3PO4. So, 1.5 moles of CaCl2 reacts with 1 mole of H3PO4. In another word, 1 mole of H3PO4 is equivelent to 1.5 moles of CaCl2. That's why the normality of H3PO4 with respect to CaCl2 is 1.5N.
Hope this will help.