Which of the following fluorides does not exist?

The locus of a point such that the sum of its distances from the points $(0, 2)$ and $(0, -2)$ is $6$,is

$A$ pendulum suspended from the ceiling of a train oscillates with a time period $2\,s$,when the train is accelerating at $10\,m/s^2$. What will be its time period when the train retards at $10\,m/s^2$?

Which of the following compounds lacks iron?

If the pair of lines represented by $3 x^2 - 5 x y + P y^2 = 0$ and $6 x^2 - x y - 5 y^2 = 0$ have one line in common,then the sum of all possible values of $P$ is

Given that $K_w$ for water is $10^{-13} \ M^2$ at $62 \ ^oC$,compute the sum of $pOH$ and $pH$ for a neutral aqueous solution at $62 \ ^oC$.