One liter of a 0.1M Tris buffer (pKa=8.3) is adjusted to a pH of 2.0. a. What are the concentrations of the conjugate base and weak acid at this pH? b. What is the pH when 1.5mL of 3.0M HCl is added to this buffer? Is Tris a good buffer at this pH? Why? c. What is the pH when 1.5mL of 3.0M NaOH is added to this buffer?

a. [Weak acid] = 0.099999949M

[Conjugate base = 5.0119x10⁻⁸M

b.  pH = 1.84. Is not a good buffer

c. pH = 6.97

Explanation:

a. To find pH of the buffer we must use Henderson-Hasselbalch formula:

pH = pKa + log [Conjugate base] / [Weak acid]

2.0 = 8.3 + log [Conjugate base] / [Weak acid]

5.01x10⁻⁷ = [Conjugate base] / [Weak acid] (1)

As concentration of the buffer is 0.1M = [Conjugate base] + [Weak acid] (2)

Replacing (2) in (1):

5.01x10⁻⁷ = 0.1M - [Weak acid] / [Weak acid]

5.01x10⁻⁷ [Weak acid] = 0.1M - [Weak acid]

[Weak acid] = 0.099999949M

[Conjugate base] = 5.0119x10⁻⁸M

b. The conjugate base reacts with the HCl. Moles of HCl are:

1.5x10⁻³L * (3.0mol / L) = 4.5x10⁻³ moles HCl

As the conjugate base contains just 5x10⁻⁸ moles. Almost all HCl doesn't react and pH =

[H⁺] = 4.5x10⁻³ moles + 1x10⁻² moles (Initial moles H⁺) / L = 0.0145M

pH = -log [H⁺]

pH = 1.84

As the concentration of the conjugate base is <<< than weak acid. In this pH, Tris is not a good buffer. As general rule a good buffer works pH between pKa ± 1.

c. Now, NaOH reacts with the weak acid producing conjugate base.

The new moles are:

[Weak acid] = [Weak acid] = 0.099999949M - 4.5x10⁻³ = 0.0955

[Conjugate base] = 5.0119x10⁻⁸M + 4.5x10⁻³ = 4.5x10⁻³

pH = pKa + log [Conjugate base] / [Weak acid]

pH = 8.3 + log [4.5x10⁻³] / [0.0955]

pH = 6.97

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.