# What is a mesure of the acidity of a compound?

Apr 20, 2017

pH. It measures the concentration of hydrogen ions in a solution.

#### Explanation:

pH is a measure of the hydrogen ion concentration in a solution.

The higher the concentration, the more acidic the solution.

You can calculate pH by using the formula: $\text{pH" = -"log} \left[{H}^{+}\right]$
=> Where $\text{log}$ is the logarthmic sequence for exponents (you don't need to worry about this if you didn't learn this in math - just use it correctly in this equation).
=> Where ${H}^{+}$ is the concentration of hydrogen ions in the solution, in $\frac{\text{mol}}{L}$.

Hope this helps :)

Apr 20, 2017

Why, the $p H$ of its aqueous solution......

#### Explanation:

When we assess the acidity of a compound we interrogate the completion of the following equilibrium.........

$H X + {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

As with any equilibrium, we can write the ${K}_{a}$:

${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{X}^{-}\right]}{\left[H X\right]}$

This is an equation, that we can treat as we would any other equation.......We can take ${\log}_{10}$ of both sides to give.......

${\log}_{10} {K}_{a} = {\log}_{10} \left[{H}_{3} {O}^{+}\right] + {\log}_{10} \left\{\frac{\left[{X}^{-}\right]}{\left[H X\right]}\right\}$

On rearrangement..............

$- {\log}_{10} \left[{H}_{3} {O}^{+}\right] = - {\log}_{10} {K}_{a} + {\log}_{10} \left\{\frac{\left[{X}^{-}\right]}{\left[H X\right]}\right\}$

But by definition, $p H = - {\log}_{10} \left[{H}_{3} {O}^{+}\right]$ AND

$p {K}_{a} = - {\log}_{10} {K}_{a}$

And so, finally...............

$p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{X}^{-}\right]}{\left[H X\right]}\right\}$

So given the $p {K}_{a}$ of an acid, and a measurement of $p H$, the concentration of the weak acid, and its conjugate base, may be accessed. The final equation is a form of the $\text{buffer equation}$ that you will use extensively.