The indicator used in the titration of $Na_2CO_3$ with $H_2SO_4$ is

While titrating a dilute $HCl$ solution with an aqueous $NaOH$ solution,which of the following will not be required?

Consider the titration of $NaOH$ solution versus $1.25\, M$ oxalic acid solution. At the end point,the following burette readings were obtained:
$(i)$ $4.5\, mL$ $\quad (ii)$ $4.5\, mL$ $\quad (iii)$ $4.4\, mL$
$(iv)$ $4.4\, mL$ $\quad (v)$ $4.4\, mL$
If the volume of oxalic acid taken was $10.0\, mL$,then the molarity of the $NaOH$ solution is .... $M$ (Rounded-off to the nearest integer).

Solution $X$ contains $Na_2CO_3$ and $NaHCO_3$. $20 \ mL$ of $X$ when titrated using methyl orange indicator consumed $60 \ mL$ of $0.1 \ M \ HCl$ solution. In another experiment,$20 \ mL$ of $X$ solution when titrated using phenolphthalein consumed $20 \ mL$ of $0.1 \ M \ HCl$ solution. The concentrations (in $mol \ L^{-1}$) of $Na_2CO_3$ and $NaHCO_3$ in $X$ are respectively:

Which reagent is commonly used in titration to determine the hardness of water?

For standardizing $NaOH$ solution,which of the following is used as a primary standard?