Question

How many moles of Unknown weak acid (HA) in a 100L solution are required to produce a solution with PH =2 ? Ka of the Unknown weak acid (HA)=5x10⁻³
a. 3x10⁻³
b. 0.3x10⁻³
c. 3
d. 0.3

Answer 1

The moles of unknown weak acid (HA) in a 100L solution are required to produce a solution with pH 2 is 0.002 moles.

What is the equation of the acid dissociation constant, Ka?

The acid dissociation constant of an acid is given by the formula below:

$K_a= \frac{[A^{-}][H_3O^{-}]}{[HA]}$

Where

• Ka = 5x10⁻³
• pH = 2
• [H3O+] = 10^-2
• Volume of solution = 100 mL or 0.1 L

Since, the Ka is given at equilibrium concentrations, [H3O+] = [A-]

[A-] = 10^-2

Making [HA] subject of the formula:

$[HA]= \frac{[10^{-2}][10^{-2}]}{5 \times 10^{ - 3} } = 0.02$

Moles of acid = 0.1 × 0.02 = 0.002 moles

Therefore, the moles of unknown weak acid (HA) in a 100L solution are required to produce a solution with pH 2 is 0.002 moles.

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