In a titration of malic acid, what is the pH after:
a. 0.75 molar equivalents of NaOH have been added?
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b. 1.30 molar equivalents of NaOH have been added?
To determine the pH after a certain amount of NaOH is added in a titration of malic acid, you need to consider the reaction that occurs between malic acid and NaOH. Malic acid is a weak acid and NaOH is a strong base. The reaction between them can be represented by the following balanced equation:
C4H6O5 (malic acid) + 2NaOH (sodium hydroxide) = 2H2O (water) + Na2C4H4O5 (sodium malate)
a. If 0.75 molar equivalents of NaOH have been added, it means that 0.75 moles of NaOH have reacted with every mole of malic acid present. Since the stoichiometric ratio between malic acid and NaOH is 1:2, it implies that half of the malic acid has reacted.
To determine the pH after this point, we need to consider the dissociation of malic acid in water. Malic acid is a diprotic acid, meaning it can donate two protons. The first dissociation reaction can be represented as follows:
H2C4H4O5 (malic acid) ↔ H+ + HC4H4O5-
The equilibrium constant (Ka1) for this reaction can be found in a table or calculated using the concentration values.
From here, you can use the Henderson-Hasselbalch equation to calculate the pH. The Henderson-Hasselbalch equation is given by:
pH = pKa + log([A-]/[HA])
Where pKa is the negative logarithm of the acid dissociation constant (Ka), [A-] is the concentration of the conjugate base (HC4H4O5-), and [HA] is the concentration of the acid (H2C4H4O5) before any reaction.
b. Similarly, if 1.30 molar equivalents of NaOH have been added, it means that 1.30 moles of NaOH have reacted with every mole of malic acid. Now, the malic acid is fully neutralized, and the pH will be determined by the concentration of the resulting solution, sodium malate (Na2C4H4O5).
To determine the pH after this point, you need to consider the dissociation of sodium malate, which is a salt. You can look up the dissociation constant (Kw) for sodium malate in a table to calculate the pH.
In summary, to determine the pH at a specific point in a titration of malic acid with NaOH, you need to consider the stoichiometric ratio between the acid and base, the dissociation of the acid, and potentially the dissociation of the resulting salt.