Question

1) Use the Henderson-Hasselbalch equation to calculate the pH of a buffer solution prepared by mixing equal volumes of 0.230 M NaHCO3 and 9.00×10⁻² M Na2CO3. (use Ka values given on Wiki)

2) A volume of 100 mL of a 0.440 M HNO3 solution is titrated with 0.440 M KOH. Calculate the volume of KOH required to reach the equivalence point.

Answer 1

Solution:

Lets assume volume of each acid and its conjugate base = 1.0 L

We find moles by using volume and molarity

Mol NaHCO3 =mol HCO3- =   1.0 L * 0.230M = 0.230 mol

Mol Na2CO3 = CO3 2- = 1.0 L* 9.0E-2 M = 9.00 E -2 mol

Henderson-Hasselbalch :

pH = pka + log ([conj. Base] / [Acid])

Reaction :

    HCO3- (aq) + H2O (l) --- > H3O+ (aq) + CO3^2- (aq)

(acid)                                                 (conj. Base)

We use concentration ration total volume is same in both the numerator and denominator so we can use mole ratio instead of concentration ratio

Ka of HCO3^- = 4.8E-11

pka = -log ka = -log 4.8E-11

= 10.32

pH = 10.32 + log ( 9.00E-2/ 0.230)

= 9.91

pH = 9.91                                           

2).        Lets write the reaction :

HNO3 (aq) + KOH (aq) ---- > KNO3 (aq) + H2O (l)

From the reaction we say that mole ratio of acid : base is 1:1

At equivalence point mole of acid = moles of base

Lets calculate moles of HNO3

Mol HNO3 = volume in L * molarity

= 0.100 L * 0.440 M

= 0.0440 mol

Mol of HNO3 = mole of KOH = 0.0440 mol

We calculate volume of KOH :

Volume of KOH = mol KOH / molarity

= 0.0440 mol KOH / 0.440 M

= 0.100 L

Volume of KOH in mL = 100.0 mL