# How do you calculate the pH of a strong acid or base?

Feb 16, 2017

You calculate it normally -> find the concentration of the acid, transfer the concentration and insert the value in a logarithmic equation.

#### Explanation:

To calculate pH in general, you need the concentration of hydrogen ions in the compound.

Strong acids are acids with a pH of less than 4-ish while weak acids have a pH of 4-ish to 7.0.

In order to find mols, we typically use stoichiometry and the concentration formula $\left(n = c v\right)$.

When you find concentration of an acid, you need to transfer the concentration to each individual element.

E.g.

${H}_{\text{2"SO_"4" (aq) -> 2H^+ (aq) + SO_"4}}^{2 -} \left(a q\right)$

${H}_{\text{2"SO_"4}}$ has a concentration of $0.25$ mol/L. You transfer the concentration just like how you would transfer mols in stoichiometry. THIS IS THE ONLY TIME YOU CAN TRANSFER CONCENTRATION

As a result, your hydrogen concentration is $0.5$ mol/L.

Use the logarithm function to find pH:

$p H = - \log \left[{H}^{+}\right]$

$= - \log \left[0.5\right]$

$= 0.301029995$

$= 0.3$

In this example, the pH of ${H}_{\text{2"SO_"4}}$ is $0.3$. This is a very small value, indicating its strong acidity.

Hope this helps :)