What volume of 0.200M of aqueous solution of formic acid, a weak monoprotic acid (KA =
1.78x10-4) and 0.200M aqueous solution of NaOH would you mix to prepare a 500mL of a
buffer solution of pH = 4.0.
I know I need to use the H-H equation, I'm just not sure what I plug in and where. I am also not sure if I need to use an ICE chart or not.
4.0 =( 1.78x10^-4) + log ([HCOO-]/0.2)
with the 0.2 coming from the concentration of formic acid.
I am also unsure of if I need to use the MaVa=MbVb equation...
Any guidance on this question would be very helpful.
Thank you for answering this below Dr Bob
I responded at your post below. In fact I probably did more than you needed but take a look.
To prepare a buffer solution of pH 4.0 using formic acid and sodium hydroxide, you can follow these steps:
1. Determine the Henderson-Hasselbalch equation for a buffer solution, which is given by pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant Ka, and [A-] and [HA] are the concentrations of the conjugate base and the acid, respectively.
2. In this case, formic acid (HCOOH) is the weak monoprotic acid, and the conjugate base is formate (HCOO-). The Ka value for formic acid is given as 1.78x10^-4. Plug this value into the Henderson-Hasselbalch equation: pH = -log(1.78x10^-4) + log([HCOO-]/[HCOOH]).
3. Rearrange the Henderson-Hasselbalch equation to solve for the ratio of [HCOO-]/[HCOOH]: [HCOO-]/[HCOOH] = 10^(pH - pKa).
4. Since you want a buffer solution at pH 4.0, plug this value into the equation: [HCOO-]/[HCOOH] = 10^(4.0 - (-log(1.78x10^-4))).
5. Simplify the equation to find the ratio of [HCOO-]/[HCOOH].
Now, let's address the second part of your question regarding the MaVa = MbVb equation. In this case, you don't need to use that equation. The MaVa = MbVb equation is typically used for acid-base neutralizations, where the amount of moles of acid (A) equals the amount of moles of base (B). However, in this scenario, you are simply calculating the ratio of the two solutions to make a buffer solution, and not performing a neutralization reaction.
Therefore, using the Henderson-Hasselbalch equation, as explained above, will give you the desired ratio of [HCOO-]/[HCOOH]. Once you determine this ratio, you can use it to find the actual concentrations of formic acid and sodium formate needed to make a 500 mL buffer solution.
Note: If you have access to a reliable pH calculator, you can input the values of pKa and pH directly to calculate the required ratio of [HCOO-]/[HCOOH].