Answer
The pH value of fresh-brewed coffee is about $4.9$.
Work Step by Step
First, note that the hydrogen ion concentration of fresh brewed coffee is about $1.3\times {{10}^{-5}}$moles per liter.
Therefore, $\left[ {{\text{H}}^{\text{+}}} \right]=1.3\times {{10}^{-5}}$.
Substitute it in the formula $\text{pH}=-\log \left[ {{\text{H}}^{\text{+}}} \right]$as follows:
$\begin{align}
& \text{pH}=-\log \left[ {{\text{H}}^{\text{+}}} \right] \\
& \text{pH}=-\log \left[ 1.3\times {{10}^{-5}} \right] \\
\end{align}$
Therefore,
$\begin{align}
& \text{pH}=-\log \left[ 1.3\times {{10}^{-5}} \right] \\
& \text{pH}\approx -\left( -4.8860 \right) \\
& \text{pH}\approx 4.8860
\end{align}$
