تركيزات الحمض والقاعدة
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Q1: How do you prepare a 0.1 M NaOH solution from a 50% w/w stock solution?
Calculate the molarity of the 50% w/w NaOH stock solution using its density (1.53 g/mL) and molar mass (39.998 g/mol). Apply the dilution formula to determine the volume of stock solution needed to prepare 500 mL of ~0.1 M NaOH. Transfer the calculated volume to a labeled polyethylene bottle, add the appropriate volume of water, cap, and invert several times to mix thoroughly.
Q2: What is the purpose of standardization in acid-base titration?
Standardization determines the exact concentration of a prepared solution, such as NaOH, which may differ from the target concentration. Using a known acid like potassium hydrogen phthalate (KHP), you perform a neutralization reaction where one mole of base neutralizes one mole of acid. This allows you to calculate the actual molarity of NaOH based on the volume required to reach the endpoint indicated by phenolphthalein color change.
Q3: Why does the actual NaOH concentration differ from the prepared 0.1 M concentration?
NaOH is hygroscopic, meaning it readily absorbs moisture from the air, making it difficult to weigh accurately. This property causes the actual concentration to be lower than the expected 0.1 molarity. Standardization using KHP corrects for this discrepancy by determining the true concentration through titration.
Q4: How does the half-equivalence point relate to pKa determination?
At the half-equivalence point, the concentrations of undissociated acid and its conjugate base are equal, making the pH equal to the pKa value. To find the first half-equivalence point, divide the volume of NaOH at the first equivalence point by two. Look up the pH at this volume from your titration data to obtain a precise pKa value for comparison with theoretical values.
Q5: What does it mean that phosphoric acid is a triprotic acid?
A triprotic acid can donate three protons per molecule when it dissociates in aqueous solution. Phosphoric acid has three pKa values, each corresponding to the dissociation of one proton. In this experiment, you measure two of the three Ka values by observing two sigmoidal curves and two equivalence points on the titration curve.
Q6: How do you identify equivalence points on a titration curve?
Equivalence points appear as the maximum peaks on the first derivative plot of the titration curve. Each peak corresponds to a dissociation constant (Ka) of the acid being titrated. For phosphoric acid, two equivalence points are visible, representing the first two pKa values. The experiment stops at pH 12, so only two of the three Ka values are measured.
Q7: What role does phenolphthalein play in standardizing NaOH with KHP?
Phenolphthalein is an acid-base indicator that is colorless in acidic conditions and turns pink in basic conditions. Added to the KHP solution before titration, it signals the endpoint when enough NaOH has been added to make the solution basic. The persistent pink color indicates that the neutralization reaction is complete, allowing you to record the volume of NaOH used to calculate its molarity.