What is the product formed when sodium is reacted with an excess of oxygen/air?

An auditorium has a volume of $10^5 \ m^3$ and a total absorption area of $2 \times 10^4 \ m^2$. Its average absorption coefficient is $0.2$. The reverberation time of the auditorium in seconds is:

If $\log _{0.3}(x - 1) < \log _{0.09}(x - 1),$ then $x$ lies in the interval

If $x^2+y^2=t-\frac{1}{t}$ and $x^4+y^4=t^2+\frac{1}{t^2}$,then $\frac{dy}{dx}=$

If $f(x) = x^3 - 6x^2 + 9x + 3$ is a decreasing function,then $x$ lies in

Water is considered to be polluted if the dissolved oxygen $(D.O.)$ content is less than ... $ppm$ :