Which of the following is the weakest base?

$A$ metal is burnt in air and the ash on moistening smells of $NH_3$. The metal is

Two gases $A$ and $B$ having the same volume,effuse through an orifice in $30 \ min$ and $10 \ min$ respectively. The molar mass of $A$ is $162$. The molar mass of $B$ will be

The wavelength of the first line in the Balmer series in the hydrogen spectrum is $\lambda$. What is the wavelength of the second line?

In a circuit,the current lags behind the voltage by a phase difference of $\pi / 2$. The circuit contains which of the following?

$A$ converging beam of rays is incident on a diverging lens. Having passed through the lens,the rays intersect at a point $15 \, cm$ from the lens on the opposite side. If the lens is removed,the point where the rays meet will move $5 \, cm$ closer to the lens. The focal length of the lens is.....$cm$