Step 1: Volume of tetrahedron.
The volume of a tetrahedron with vertices \( P(x_1, y_1, z_1) \), \( Q(x_2, y_2, z_2) \), \( R(x_3, y_3, z_3) \), and \( S(x_4, y_4, z_4) \) can be found using the determinant formula.
\[
V = \frac{1}{6} \left| \begin{vmatrix} x_1 & y_1 & z_1 & 1
x_2 & y_2 & z_2 & 1
x_3 & y_3 & z_3 & 1
x_4 & y_4 & z_4 & 1 \end{vmatrix} \right|
\]
Step 2: Conclusion.
Thus, the volume of the tetrahedron is \( \frac{2}{3} \).