What is obtained when sodium metal is heated in a stream of dry ammonia?

The number of atoms/molecules contained in one face-centred cubic $(FCC)$ unit cell of a monoatomic substance is:

What are $X$ and $Y$ in the following reactions?

$\mathop {\lim}\limits_{x \to 0} \frac{{\sin \left( {\pi {{\cos }^2}x} \right)}}{{{x^2}}} = $

The potential energy for a force field $\vec{F}$ is given by $U(x, y) = \cos(x + y)$. The force acting on a particle at the position given by coordinates $(0, \pi/4)$ is:

If one of the lines $2x^2 - xy + by^2 = 0$ passes through the point $(-4, -2)$,then $b^2 =$