Normality?

[DoctreDrey]

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#171 on 2010 verion of DAT destroyer

30ml of 0.2M barium hydroxide, Ba(OH)2, is required to neutralize 25ml of citric acid, H3C6H5O7. What is the molarity of the citric acid solution?

Answer is 0.167M but I keep got 0.25M and I don't understand the solution...

I calculated moles of OH- then converted it to moles of H+ and then divided by 25ml:

0.2M x 2 = 0.4M x 0.03 L = 0.012 moles OH-

then divided 0.012 by 2 and multiplied by 3 to get moles of H+= 0.018 moles H+

then divided 0.018 by 3 to get moles citric acid? = 0.006 moles. then divided this by the volume of citric acid = 0.25M

 

[UndergradGuy7]

I remember this one. Anyway I would have never got it because I had no idea citric acid is triprotic. Something you either know, or need given, or have to look up.

Anyway 0.2M x 2 = 0.4M x 0.03 L = 0.012 moles OH-
is right. You need 0.012 moles of OH- = moles of acid
So if the acid was monoprotic all you would do is 0.012 / 0.025 L = Molarity of acid = 0.48M

Since the acid is triprotic 0.48M is the normality of the acid. The acid gives off 3 hydrogens. So its molarity is 0.48M/3 which is 0.16M.