What is the number of moles of solute dissolved in $1 \ L$ of solution called?

Which of the following statements is correct about an aqueous solution of an acid and of a base?
$(i)$ Higher the $pH$,stronger the acid
$(ii)$ Higher the $pH$,weaker the acid
$(iii)$ Lower the $pH$,stronger the base
$(iv)$ Lower the $pH$,weaker the base

What type of substance is $NaOH$?

How is an aqueous solution with $pH = 2$ compared to an aqueous solution with $pH = 4$?

Sodium carbonate is a basic salt because it is a salt of :

Write the chemical name and chemical formula of washing soda.