Given that 20 ml of a weak acid (HA) was titrated with NaOH and the titration curve of pH vs titrant volume in ml is given, find the initial concentration of the acid?
The problem gives the full titration curve starting at pH=2.2 at 0 ml added and with the equivalance pt at pH=7 at 18 ml added.
I'm at a distinct disadvantage since I can't see the titration curve but I think this is what you do.
The equivalence point is 7 (which I really don't understand since weak acids and strong bases are supposed to be somewhere in the 8.3 range for the eq pt but this is probably just a made up problem). However, with the eq pt at 7.0 and 18 mL., take 1/2 of the 18 and look up on the curve for the pH at 9.0 mL. That will give you the pKa value. Convert that to Ka by pKa = -log Ka.
Then convert pH 2.2 to (H^+) and plug that into the Ka equation of
Ka = (H^+)(A^-)/[(HA)-(H^+)] and solve for HA.
To find the initial concentration of the weak acid (HA), we can use the concept of the titration curve and the equivalence point.
Here's how you can approach this problem:
1. Identify the equivalence point: From the information given, we know that the equivalence point occurs at pH = 7 when 18 ml of the titrant (NaOH) is added. The equivalence point represents the point at which equal moles of acid and base have reacted.
2. Determine the volume of NaOH required to reach the equivalence point: From the titration curve, locate the point where the pH reaches 7. Note down the corresponding volume of NaOH added. In this case, it is 18 ml.
3. Calculate the moles of NaOH used: Since NaOH reacts with HA in a 1:1 ratio, the number of moles of NaOH used is equal to the number of moles of HA initially present.
Moles of NaOH = Molarity of NaOH * Volume of NaOH in liters
Since 18 ml = 0.018 L, and assuming the concentration of NaOH is known (let's say it's C_MOH), then Moles of NaOH = C_MOH * 0.018
4. Calculate the volume of HA: Since the weak acid HA is initially present in a volume of 20 ml, we can calculate the moles of HA using the equation:
Moles of HA = Moles of NaOH (from step 3)
To convert the moles of HA to its initial concentration:
Concentration of HA = Moles of HA / Volume of HA in liters
Volume of HA = 20 ml = 0.020 L
5. Substitute the known values into the equation: Concentration of HA = Moles of HA / Volume of HA
Concentration of HA = Moles of NaOH (from step 3) / 0.020 L.
By following these steps, you should be able to find the initial concentration of the weak acid (HA) based on the information given in the problem.