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A 25.0-mL sample of 0.150 M butanoic acid is titrated with a 0.150 M NaOH solution. What is the pH before any base is added? The Ka of butanoic acid is 1.5 × 10-5.

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HA ⇄ H⁺ + A⁻ 
so:
[tex] \frac{[H^+][A^-]}{[HA]} = 1.5 x 10^{-5} [/tex]
and now:
[tex] \frac{(x)(x)}{(0.150-x)} [/tex] = 1.5 x 10⁻⁵
x is considered very small compared to 0.15
x² = 2.25 x 10⁻⁶
x = 1.5 x 10⁻³
So [H⁺] = 1.5 x 10⁻³
pH = - log [H⁺] = - log (1.5 x 10⁻³) = 2.83

pH is the negative log of the hydrogen or the proton released in the aqueous solution by a substance. The pH before the addition of the base is 2.83.

What is titration?

Titration is the quantitative analytical method that determines the concentration of the unknown solution by the known solution.

The dissociation can be shown as,

[tex]\rm HA \leftrightharpoons H^{+} + A^{-}[/tex]

Given, the Ka of the acid is [tex]1.5 \times 10^{-5}.[/tex]

[tex]\begin{aligned} 1.5 \times 10^{-5} &=\rm \dfrac{[H^{+}][A^{-}]}{[HA]}\\\\&= \rm \dfrac{(x)(x)}{0.150 - x}\end{aligned}[/tex]

The value of x is calculated as:

[tex]\begin{aligned} \rm x ^{2} &= 2.25 \times 10^{-6}\\\\\rm x &= 1.5 \times 10^{-3}\end{aligned}[/tex]

The concentration of hydrogen ions is [tex]1.5 \times 10^{-3}.[/tex]

The pH can be calculated as:

[tex]\begin{aligned} \rm pH &= - \rm log [H^{+}] \\\\&= \rm - log (1.5 \times 10^{-3}) \\\\&= 2.83\end{aligned}[/tex]

Therefore, 2.83 is the pH.

Learn more about pH here:

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