From the information available in the question, the concentration of the acid is 0.21 M.
H3PO4(aq) + 3NaOH(aq) -----> Na3PO4(aq) + 3H2O(l)
Volume of acid(VA) = 51.0 mL
Concentration of acid (CA) = ?
Volume of base (VB) = 26.2 mL
Concentration of base (CB) = 1.25 M
Number of moles of acid (NA) = 1
Number of moles of base (NB) = 3
Using the formula;
CAVA/CBVB = NA/NB
CAVANB = CBVBNA
CA = CBVBNA/VANB
CA = 1.25 M × 26.2 mL × 1/51.0 mL × 3
CA = 0.21 M
The concentration of the acid is 0.21 M.
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