Question

a. If 200. mL of 2.00 M H3PO4 solution is added to 600. mL of 2.00 M NaOH solution, what is the molarity of the resulting solution of Na3PO4.

b. In the above solution, which of the reactants remains in excess? What is its concentration in the resultant solution?

Please explain. Thanks

Answer 1

you have .2*2 moles H2PO4
you have .6*2 moles of NaOH

balanced equation:
H3PO4 + 3NaOH>> Na3PO4 + H2O

so for each mole of acid, you need three times that amount of base.
.4moles acid, need 1.2 moles of base, but you have 1.2, which is magically just right. So, you should get .4 moles of trisodiumphosphate.

Molarity= .4 moles/(.800L+moleswater made)

moleswater made is .4 moles, or 18*.4grams, or 7.2 ml water or

molarity= .4moles/(.8072) then reduce that to three significant digits.