Question

0.50 M solutions of each weak acid in the table are available. Choose an acid solution and determine what volume, in ml, of a 1.00 M NaOH solution should be added to 1.00 L of the acid solution to create a buffer with pH = 3.94. acid base pairs to use: K på, HCO2H/NaHCO2 H2CO3 NaHCO3 H3B03 NaH2B03 NaHCO3/Na2CO3 1.8*10-4 3.74 4.2x10-7 6.38 7.3x10-10 9.14 4.8x10-11 10.32 HCO2H/NaHCO2 H2CO3 NaHCO3 H3B03 NaH2B03 NaHCO3/Na2CO3 ml NaOH

Answer 1

To prepare the buffer solution at pH =3.94 , the pKa of the acid must be close to the pH.

Hence, H₂CO₂H acid should be used.

The buffer is HCO₂H/NaHCO₂.

Given moles of acid = volume × molarity = 1.00×0.50 = 0.5

Using Hasselbalch Henderson equation

pH = pKa + log ( moles of NaHCO₂ /moles of H₂CO₂H).

3.94 = 3.74 + log (moles of NaHCO₂/moles of H₂CO₂H)

Or, log (moles of NaHCO₂ /moles of HCO₂H) = 0.20

Or, (moles of NaHCO₂/moles of HCO₂H) = 10^0.20= 1.58.

Or, moles of HA/moles of A^- = 1.58 (1)

Now,

moles of acid (HA) × 1.58 = moles conjugate base ( A^-).

BCA table is

	HA	OH-	A-
Before	0.5	x	0
Change	-x	-x	+x
After	0.5 -x	0	x

Now, from Eq.1

moles of A moles of HA
  =
-co
= 1.58

Or, x = 0.79 - 1.58 x

Or, 2.58 x = 0.79

Or, x = 0.306.

Hence, moles of NaOH added = 0.306 .

Now, volume of NaOH

= (Moles/molarity)

= (0.306/1.00)

= 0.306 L

= 306 mL.