t &t; Buffers on pH Do all of the buffer solutions show the same change in the s

Question

t &t; Buffers on pH Do all of the buffer solutions show the same change in the same change in pli on the addition of an identical 5. 6. What buffering capacity ratio provides the best buffer solution? 7. Calculate the pH of the buffered solution of 0.50M C,H,NH 0.25M C,H,NH,a. 8. 8. Calculate the pH of a hypochlorite buffer containing 0.150 M HCIO and 0.100 M NaCIO The bufer has a pH of 4.74. What is the pH after 0.008 mol HCl is added to 1.00 Lof this buffer? 9. 9. A buffer solution is prepared from 0.40 M HA and 0.40 M NaA (its conjugate base),

Explanation / Answer

pH of all the buffer solution is calculated by the equation known as Henderson–Hasselbalch equation.

Henderson–Hasselbalch equation:-   pH = pKa + log10 {[A-]/ [HA]} ------- for acidic buffer and pH = pKb + log10 {[A-]/ [HA]} ------- for basic buffer -------1

A) No, all the buffer do not show same change in pH on the addition of the same amount of acid or base because according to the above equation of pH it depends on the pKa or pKb of acid, base, and acid or base to salt ratio. As all the acids and bases have different pKa or Pkb value so the pH will be different on adding acid or base but next it also depends on the ratio of acid or base to salt ratio so the difference means change in pH ( delta pH ) will be same on adding the same amount of acid or base but addition of acid decreases the pH by the same number as base increases the pH.

pH = pKa + log10 {[A-]/ [HA]} change in pH ( delta pH) = pKa - Pka + log10 ( initial ratio / final ratio)

change in pH ( delta pH ) = 0 + log10 ( initial ratio / final ratio) so the change in pH depends only on the ratio of acid and salt so on adding same acid or base pH of every buffer will be different but the amount of change will be same.

B) Buffer capacity:- buffer capacity measures the capacity of any buffer to resist the change in pH on adding 1 g equivalent of acid or base.

when the salt to acid or base ratio is 1 that buffer can resist maximum change in PH. so the buffer capacity is maximum when salt to acid or base ratio is one. and this is the best buffer solution,

C) B+ = C2H5NH3Cl= 0.25 M and HB = C2H5NH2 = 0.50 M pKb = 3.193 put all the value in equation 1

pH = 3.193 + log[0.25/0.50] = 2.89    pH = 2.89  

D) A- = NaClO = 0.100 M and HA = HClO = 0.150 M pKa = 7.53 put all the value in equation 1

pH = 7.53 + log[0.100/0.150] = 7.354    pH = 7.354  

E) From equation 1, pKa = pH + log10 {[A-]/ [HA]} so pKa = 4.74 + log [0.40/0.40] = 4.74

we have calculated pKa = 4.74

now mole of HA = molarity* volume ( in liters) = 0.40*1 = 0.40 HCl will react with NaA to give NaCl and HA

HCl + NaA = NaCl + HA

so mole of HA after adding HCl = ( mole of HA + mole of HCl) = 0.40+ 0.008 = 0.408 mol

now mole NaA = ( mole of NaA - mole of HCl ) = 0.40-0.008 = 0.392 mole

pH = pKa + log10 {[A-]/ [HA]} put all value in this

pH = 4.74 + log [ 0.392/0.480] = 4.65    pH = 4.65

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