One has a solution Sa of ethanoic acid of concentration Ca=1×10^-2 mol/l and pH=3.4

One mixes a volume Va=60ml of the solution Sa of ethanoic acid with a volume Vb1=20ml of the Sb1 solution of ammonia of concentration Cb1=2×10^-2 mol/l. The pH is 5
One has a soutions of weak bases :
Sb1 solution of ammnia
Sb2 of mythelamine .Cb2=1.5×10^-2
Is ammonia a weaker base than methylamine. Justify

To determine if ammonia (NH3) is a weaker base than methylamine (CH3NH2), we need to compare their respective equilibrium constants for the protonation reactions. The equilibrium constant (Kb) for a weak base is a measure of its strength as a base.

The protonation reactions for ammonia and methylamine can be represented as follows:

NH3 + H2O ⇌ NH4+ + OH-
CH3NH2 + H2O ⇌ CH3NH3+ + OH-

The Kb expression for each reaction is as follows:

Kb(NH3) = [NH4+][OH-] / [NH3]
Kb(CH3NH2) = [CH3NH3+][OH-] / [CH3NH2]

Comparing the two equilibrium constants can help us determine which base is stronger.

To calculate Kb(NH3) and Kb(CH3NH2), we can use the pH values provided for the mixtures of the solutions and the known equilibrium between H+ and OH- ions in water, which is given by:

[H+][OH-] = 1.0 x 10^-14 (at 25°C)

For the first mixture of solutions (Sa ethanoic acid and Sb1 ammonia), we can calculate the concentration of OH- ions using the pH value:

pH = -log[H+]
10^(-pH) = [H+]
[H+][OH-] = 1.0 x 10^-14
[OH-] = (1.0 x 10^-14) / [H+]

Using the given pH value of 5, we can calculate the OH- concentration:

[H+] = 10^(-5) = 1.0 x 10^-5 M
[OH-] = (1.0 x 10^-14) / (1.0 x 10^-5) = 1.0 x 10^-9 M

Now, let's consider the second mixture (Sa ethanoic acid and Sb2 methylamine). Since the pH is not provided for this mixture, we cannot directly calculate the OH- concentration. However, we can compare the pH values of the two mixtures to make an inference about the strength of the base.

In the first mixture, with ammonia, the pH is 5. In the second mixture, with methylamine, the pH is not provided but is likely higher than 5 since the solution is less acidic.

Since the pH is higher in the second mixture, we can infer that the hydroxide ion (OH-) concentration is lower than 1.0 x 10^-9 M.

Comparing the OH- concentrations in the two mixtures, we can conclude that ammonia (NH3) is a weaker base than methylamine (CH3NH2). The lower OH- concentration in the second mixture suggests lower Kb(CH3NH2) compared to Kb(NH3), indicating that methylamine is a stronger base than ammonia.

Therefore, ammonia is weaker base than methylamine, and this is justified by comparing their respective OH- concentrations or equilibrium constants (Kb).