chemistry
posted by Kris on .
The cola sample is 50. mL, the equivalence point volume is 13.4 mL, and the NaOH solution concentration is 0.025 M, what mass of H3PO4 was in the sample?

If done right, the titration of H3PO4 with NaOH has three equivalence points. Which equivalence point is the 13.4 mL?
1. Write the equation and balance it.
2. moles NaOH = M x L
3. Use the balanced equation to determine moles H3PO4. moles NaOH x (?moles H3PO4/moles NaOH) = moles H3PO4.
4. grams H3PO4 = moles x molar mass. 
This question is a prelab assignment, therefore we have not done the titrations yet.

thank you for the the set up of the problem

The third H is almost impossible to titrate so it must be the first or second equivalence point. Judging from the volume my best guess it's the first one. The indicator can tell us which H is titrated. Do you have an indicator listed? Methyl Orange? phenolphthalein?

yes we have a list  it ranges from 2.2 to 12.0
Indicator
benzopurpurine (BP)
violet
2.2 – 4.2
red
bromocresol green (BG)
yellow
3.8 – 5.4
blue
methyl red (MR)
red
4.8 – 6.0
yellow
bromothymol blue (BB)
yellow
6.0 – 7.6
blue
thymol blue (TB)
yellow
8.0 – 9.2
blue
alizarin yellow (AY)
yellow
10.1 – 12.0
orangered 
You have six indicators listed. Probably one of the questions on your prelab assignment is to identify the proper indicator to use; therefore, the list doesn't help me.

what if the equation is H3PO4 + OH > H2PO4 + H2O