Question

It's been a while since I've done this type of chemistry so I'm just very lost trying to figure out these problems. Could someone please help and if you could write out the equations or give an explanation as to how you came up with your answers. Thank you in advance!

1. You have 50 mL of a buffer solution that is 0.15 M in HA and 0.25 M in A^-. Calculate the pH of the solution after you add 100 mL of 0.01 M HCl to the solution. The pKa of HA is 4.75.

2. You wish to prepare 100 mL of a buffer solution that is 0.025 M in carbonic acid (H₂CO₃).   The pKa of H₂CO₃ is 6.351. Calculate the amount of H₂CO₃, in grams, that you will need to weigh out.

3.You wish to prepare 100 mL (total volume) of a buffer solution that is 0.025 M in carbonic acid, pH 7.25. You have solid H₂CO₃ and solid NaHCO₃. Calculate how many grams of H₂CO₃ and HCO₃^- you need to weigh out to prepare this solution.

Answer 1

1.

millimoles of HA = 50 x 0.15 = 7.5

millimoles of A- = 12.5

addition of HCl = 100 x 0.01 = 1

on addition of ’ C’ moles of acid to acidic buffer salt moles decreases and acid moles increases

so

pH = pKa + log [Salt –C/acid + C]

pH = pKa + log [12.5 - 1 / 7.5 + 1]

      = 4.75 + log 11.5/8.5

      = 4.88

pH = 4.88

3.

millimoles of carbonic acid = 100 x 0.025 = 2.5

millimoles of NaHCO3 = 100 x M

H2CO3 pKa = 6.35

pH = pKa + log [Salt /acid]

7.25 = 6.35 + log [100 M / 2.5]

   100 M/2.5 = 7.94

M = 0.199

Molarity of NaHCO3 = 0.199 M

volume = 100mL

moles = 0.199 x 0.1 = 0.0199

mass of NaHCO3 = moles x molar mass

                              = 0.0199 x 84

                              = 1.67 g

mass of NaHCO3   = 1.67 g

mass of H2CO3 = 2.5 x 10^-3 x 62 = 0.155 g