1. The problem statement, all variables and given/known data The phosphoric acid in a 100.00 mL sample of a cola drink was titrated with 0.1029 M NaOH. The first equivalence point was detected after 13.34 mL of base added, and the scond equivalence point after 28.24 mL. Calculate the concentration of H2PO4- in mol/L. (Hint: if only H3PO4 were present, where would the second equivalence point be?) 2. Relevant equations H3PO4 + 2OH --> 2H2O + HPO4 mole ratio 1 :2 :2 : 1 3. The attempt at a solution I subtracted the first eq Volume from the second: 28.24-13.34 ml and got 14.9 mL=0.0149L I times this Volume by [NaOH] and got NaOH moles= 0.0149 x 0.1029= 0.001532 Since 2 moles NaOH=1 mol biphosphate--> 0.001532 x 1/2=7.666 x 10-4 I divide this by 0.1 L to get [H2PO4]=7.666 x 10-3 This is not right correct answer is 1.605 x 10-3 I have been stuck on this for many hours. If someone can tell me how to do this I will really appreciate it. I already got the answer wrong on the quiz, but I have to study for exam. Thanks a lot.