A 0.446g sample of an unknown monoprotic acid was titirated with 0.105M KOH the molar mass is 120g/mol. What is the PKA value?
Who helps me to solve it for me. Step by step.
There must be more to the problem than what you've posted.
To solve for the pKa value, we need to determine the concentration of the acid and its conjugate base at the equivalence point of the titration. From there, we can use the Henderson-Hasselbalch equation to calculate the pKa value.
Step 1: Calculate the number of moles of the acid
Given the mass of the acid (0.446g) and its molar mass (120g/mol), we can calculate the number of moles using the formula:
moles = mass / molar mass
moles = 0.446g / 120g/mol
moles ≈ 0.00372 mol
Step 2: Calculate the concentration of the acid
We know that the volume of the KOH solution is not provided, so we can't directly calculate the concentration of the acid. However, by knowing the concentration of the base (0.105M) and the volume used at the equivalence point, we can determine the number of moles of the base used.
Step 3: Determine the number of moles of the base used
Since the acid is monoprotic, the stoichiometry between the acid and the base is 1:1. Therefore, the number of moles of the base used will be the same as the number of moles of the acid.
moles of base used = moles of acid = 0.00372 mol
Step 4: Calculate the concentration of the acid
Given that the concentration of the base (KOH) is 0.105M and the volume of base used is not provided, let us assume that we used a volume of V mL of KOH to reach the equivalence point.
moles of base used = concentration of base × volume of base used
0.00372 mol = 0.105M × (V mL / 1000 mL/ L)
Solving for V, we get:
V = (0.00372 mol / (0.105M)) × (1000 mL/L)
V ≈ 35.43 mL
Step 5: Calculate the concentration of the acid
Given that the initial mass of the acid is 0.446g and we dissolved it in a volume of V mL, we can calculate the concentration of the acid in mol/L using the formula:
concentration of acid (mol/L) = moles acid / volume of solution (L)
concentration of acid = 0.00372 mol / (V mL / 1000 mL/L)
concentration of acid ≈ 0.00372 mol / (35.43 mL / 1000 mL/L)
concentration of acid ≈ 0.1049 M
Step 6: Calculate the concentration of the conjugate base
Since the stoichiometry of the reaction is 1:1, the concentration of the conjugate base (K+ and the conjugate of the acid) will be the same as the concentration of the base (KOH) at the equivalence point.
concentration of conjugate base = 0.105 M
Step 7: Calculate the pKa value
Now that we have the concentration of the acid (0.1049 M) and the concentration of the conjugate base (0.105 M), we can substitute these values into the Henderson-Hasselbalch equation:
pKa = -log10 (concentration of conjugate base / concentration of acid)
pKa = -log10 (0.105 M / 0.1049 M)
pKa ≈ -log10 (1.000)
pKa ≈ 0
Therefore, the pKa value for the unknown monoprotic acid is approximately 0.