3 NaOH (aq) + 1 H3PO4 (aq) → Na3PO4 (aq) + 3 H2O (ℓ)

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Stoichiometry

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NaOH           Mass: g  or Solution Volume: mL of Concentration: mol/L
H3PO4          Mass: g  or Solution Volume: mL of Concentration: mol/L
Na3PO4         Mass: g
H2O            Mass: g
Heat Released: kJ

Enthalpy of Reaction

[1ΔHf(Na3PO4 (aq)) + 3ΔHf(H2O (ℓ))] - [3ΔHf(NaOH (aq)) + 1ΔHf(H3PO4 (aq))]
[1(-1997.68) + 3(-285.83)] - [3(-470.09) + 1(-1277.38)] = -167.52 kJ
-167.52 kJ     (exothermic)

Entropy Change

[1ΔSf(Na3PO4 (aq)) + 3ΔSf(H2O (ℓ))] - [3ΔSf(NaOH (aq)) + 1ΔSf(H3PO4 (aq))]
[1(-44.75) + 3(69.91)] - [3(48.25) + 1(-221.75)] = 241.98 J/K
241.98 J/K     (increase in entropy)

Free Energy of Reaction (at 298.15 K)

From ΔGf° values:
[1ΔGf(Na3PO4 (aq)) + 3ΔGf(H2O (ℓ))] - [3ΔGf(NaOH (aq)) + 1ΔGf(H3PO4 (aq))]
[1(-1804.5) + 3(-237.18)] - [3(-419.18) + 1(-1018.8)] = -239.7 kJ
-239.70 kJ     (spontaneous)

From ΔG = ΔH - TΔS:
-239.67 kJ     (spontaneous)

Equilibrium Constant, K (at 298.15 K)

9.9074026803e+041
This process is favorable at 25°C.

Reference(s):

Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; p 140.

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