How to align text to the right in math mode?

You can use \tag* from the amsmath package.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\mathbf{P} = \int_{t_i}^{t_f}\mathbf{F}\,dt = \Delta \mathbf{p}\tag*{(Impulse) }
\]
\[
\frac{dA}{dt} = \frac{l}{2\mu}=cte \tag*{(Kepler's Second Law) }
\]
\end{document}

Note that \mathbf is preferable to \bf; see this question for more details.

Here's another approach, currently set up to use the full \textwidth.

\documentclass{article}
\newcommand\lreqn[2]{\noindent\makebox[\textwidth]{$\displaystyle#1$\hfill(#2)}\vspace{2ex}}
\begin{document}
\lreqn{\textbf{P} = \int_{t_i}^{t_f}\textbf{F}\,dt = \Delta\textbf{p}}{Impulse}
\lreqn{\frac{dA}{dt} = \frac{l}{2\mu}=cte }{Kepler's Second Law}

This is the paragraph that follow the list
\end{document}

You could put it in a tabular environment

\documentclass{article}
\usepackage{array}
\def\extrarowheight{1.5em}
\let\ds\displaystyle
\begin{document}
\begin{tabular}{lr}
\( \textbf{P} = \ds\int_{t_i}^{t_f}\textbf{F}\,dt = \Delta\textbf{p}\) & (Impulse) \\
\( \ds\frac{dA}{dt} = \ds\frac{l}{2\mu}=cte \) & (Kepler's Second Law) \\
\end{tabular}
\end{document}