Homework Help: chemistry

Posted by Kris on Wednesday, August 11, 2010 at 8:43pm.

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?

chemistry - DrBob222, Wednesday, August 11, 2010 at 8:59pm
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.
chemistry - Kris, Wednesday, August 11, 2010 at 9:03pm
This question is a pre-lab assignment, therefore we have not done the titrations yet.
chemistry - Kris, Wednesday, August 11, 2010 at 9:08pm
thank you for the the set up of the problem
chemistry - DrBob222, Wednesday, August 11, 2010 at 9:12pm
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?
chemistry - Kris, Wednesday, August 11, 2010 at 9:29pm
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
orange-red
chemistry - DrBob222, Wednesday, August 11, 2010 at 10:40pm
You have six indicators listed. Probably one of the questions on your pre-lab assignment is to identify the proper indicator to use; therefore, the list doesn't help me.
chemistry - kris, Thursday, August 12, 2010 at 3:59pm
what if the equation is H3PO4 + OH --> H2PO4 + H2O

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