Problem: A 31.00 mL sample of an unknown H3PO4 solution is titrated with a 0.120 M NaOH solution. The equivalence point is reached when 26.43 mL of NaOH solution is added. What is the concentration of the unknown H3PO4 solution? The neutralization reaction is H3PO4 (aq) + 3 NaOH(aq) → 3 H2O(l) + Na3PO4(aq)

FREE Expert Solution

Recall that at the equivalence point of a titration:


moles acid=moles base


Also, recall that moles = molarity × volume


This means:


MVacid=MVbase


Correct the concentration depending on the number of acidic or basic ions present:

For acids: Multiply by the number of H+ ions

For bases: Multiply by the number of OH, H, O2–, or NH2 ions


Solving for Macid:

  • There are 3 H+ ions in H3PO4:


3·Macid31.00 mL=0.120 M NaOH26.43 mL

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Problem Details

A 31.00 mL sample of an unknown H3PO4 solution is titrated with a 0.120 M NaOH solution. The equivalence point is reached when 26.43 mL of NaOH solution is added. What is the concentration of the unknown H3PO4 solution? The neutralization reaction is H3PO4 (aq) + 3 NaOH(aq) → 3 H2O(l) + Na3PO4(aq)


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What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Equivalence Point concept. If you need more Equivalence Point practice, you can also practice Equivalence Point practice problems.