A 0.446g sample of an unknown monoprotic acid was titrated with 0.105M KOH the molar mass is 120g/mol. What is the pka value?
My calculation
0.105M x 0.446g = 0.04683 x 1000 =46.83
46.83 x 120 = 56196 changes 5.6gm
This answer 5.6gm I am not sure answer. Pls help
5.6 g is not a pKa. I don't think there is enough information to solve this problem.
To find the pKa value, we need to first determine the moles of the acid used in the titration and the moles of the base used to reach the equivalence point. Then, we can use these values to calculate the concentration of the acid and the conjugate base at the equivalence point.
Here's how you can go about solving this problem step by step:
1. Convert the mass of the acid sample to moles:
Moles = Mass / Molar mass
Moles = 0.446g / 120g/mol = 0.00372 moles
2. Determine the moles of KOH used to reach the equivalence point using the balanced chemical equation of the reaction:
The balanced chemical equation between the acid and KOH is:
Acid + KOH → Water + Salt
Since the acid is monoprotic, it will react with 1 mole of KOH.
Moles of KOH = Molarity x Volume
Moles of KOH = 0.105M x Volume (which we need to find)
3. At the equivalence point, the moles of acid consumed equal the moles of base added. So,
Moles of KOH = Moles of acid
0.00372 moles (acid) = 0.105M x Volume (KOH)
4. Rearrange the equation to solve for the volume of KOH used:
Volume = Moles of acid / Molarity
Volume = 0.00372 moles / 0.105M = 0.03542 L (35.42 mL)
Now that we have the volume used, we can proceed to calculate the pKa value.
5. The pKa value is determined using the Henderson-Hasselbalch equation:
pKa = -log(Ka)
Since we reached the equivalence point, there is an equal concentration of the acid (A-) and the conjugate base (HA). Therefore, we can calculate the Ka value using the following equation:
Ka = [A-] / [HA]
6. Calculate the concentration of the acid and the conjugate base at the equivalence point:
Concentration of acid (HA) = Moles of acid / Volume of solution
Concentration of acid (HA) = 0.00372 moles / 0.03542 L = 0.105M
Concentration of conjugate base (A-) = Concentration of acid (HA)
Concentration of conjugate base (A-) = 0.105M
7. Substitute the values into the Ka equation:
Ka = [A-] / [HA]
Ka = 0.105M / 0.105M = 1
8. Calculate the pKa using the Ka value:
pKa = -log(1)
pKa = 0
Therefore, the pKa value of the unknown monoprotic acid is 0.
Note: There seems to be an error in your calculation where you multiplied 46.83 by 120. The correct value should be 0.00372 moles, as shown in step 1.