Lab4 Potentiometric titrations.pdf - Lab 4 Potentiometric...

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[1] Lab 4: Potentiometric titrations of a weak acid with a strong base Objective To use potentiometric titration data to obtain a titration curve and Gran Plot to determine the equivalence point and pKa value via either plot; utilize the information to determine if the acid is monoprotic or diprotic, calculate the molar mass (g/mol) of the unknown acid, and identify the acid based on your determined value and those given in the table. Introduction Potentiometric titrations are a reliable and rapid method of determining the equivalence point. A titration curve (pH versus volume of titrant e.g., NaOH) is used to estimate the equivalence point and to determine if the acid is monoprotic or diprotic. Fig. 1A typical titration curve 1.Construct a titration curve by plotting pH vs. mL NaOH (Fig. 1). Point D is the equivalence point, and Points A-C are 1/4, 2/4 and 3/4 of Point D (mL). 2.Determine the number of protons/mole of unknown acid by calculating the change in pH from point A to point C. If ΔpH < 1, this indicates a monoprotic acid; if ΔpH > 1, this indicates a di- or triprotic acid. 3.Calculate the molar concentration of the acid solution: Molar concentration of HA (mol/L) = moles of HA Liter of HA solution Monoprotic acid: one mole of NaOH neutralize one mole of acid, therefore moles of OH-= moles of HA (acid solution).

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