We will abbreviate malonic acid CH2(CO2H)2, a diprotic acid, as H2A (pK1 = 2.847 and pK2 = 5.696). Find the pH in (a) 0.200 M H2A; and (b) 0.200 M NaHA.

a. pH  → 1.77

b. pH → 4.27

Explanation:

Malonic acid is a dyprotic acid. It releases two protons:

H₂A  +  H₂O →  H₃O⁺   +  HA⁻     Ka1

HA⁻   +  H₂O →  H₃O⁺   +  A⁻²          Ka2

Let's find the first pH:

We expose the mass balance:

Ca = [HA]  +  [HA⁻] + [A⁻²] = 0.2 M

We can not consider the [A⁻²] so → Ca =  [HA]  +  [HA⁻] = 0.2M

As the acid is so concentrated, we can not consider the HA- so:

Ca = [HA] = 0.2 M

Charge balance →  [H⁺]  =  [HA⁻]  + [OH⁻]

H₂A  +  H₂O →  H₃O⁺   +  HA⁻     Ka1

Ka = H₃O⁺ . HA⁻ / H₂A

We need the HA⁻ value to put on the charge balance. We re order the Ka expression:

HA⁻ = Ka . H₂A / H₃O⁺           (notice that H₃O⁺ = H⁺)

We replace:  H⁺¨ = Ka . H₂A / H⁺

(H⁺)² = Ka . Ca

H⁺ = √(Ka . Ca)         We determine Ka from pKa → 10²'⁸⁴⁷ = 1.42×10⁻³

H⁺ = √(1.42×10⁻³ . 0.2)  =  0.016866

- log [H⁺] = pH  → - log 0.016866 = 1.77

b. NaHA is the salt from the weak acid, where the HA⁻ works as an amphoterous, this means that can be an acid or a base:

HA⁻  +  H₂O  ⇄  A⁻²  +  H₃O⁺     Ka₂

HA⁻  +  H₂O ⇄  H₂A  +  OH⁻      Kb2

There is a formula than can predict the pH, so now we need to compare the Ka₂ and Kb₂ data.

Ka₂ = 10⁻⁵'⁶⁹⁶ = 2.01×10⁻⁶

Kb₂ = 1×10⁻¹⁴ / 1.42×10⁻³  = 7.04×10⁻¹²

So Ka₂ > Kb₂. In conclussion the pH will be acid.

[H⁺] = √(Ka1 .  Ka2)  →  [H⁺] = √ 1.42×10⁻³ . 2.01×10⁻⁶ = 5.34×10⁻⁵

- log 5.34×10⁻⁵  = pH → 4.27

answer: c. the experiment shows that the red substance experienced a chemical change.

explanation: chemical change is the change in which chemical composition of the substance changes. physical change is the change in which there is no change in chemical composition.

chemical changes are accompanied by following changes:

a) precipitate formation

b) energy is released or absorbed

c) formation of bubbles

d) change in color

as it is given that the red color has disappeared and there is production of gas, it means the red substance has undergone a chemical change.

e

b) nuclear ive learned that in a pedtrodic table