In the volumetric analysis of $HCl$,if we use phenolphthalein as an indicator,which base is unsuitable for titration?

Which of the following reactions occurs during iodometric titration in the laboratory?

$A$ solution containing $Na_2CO_3$ and $NaOH$ requires $300 \ mL$ of $0.1 \ N$ $HCl$ using phenolphthalein as an indicator. Methyl orange is then added to the above titrated solution when a further $25 \ mL$ of $0.2 \ N$ $HCl$ is required. The amount of $NaOH$ present in the solution is ................. $g$ $(NaOH = 40, Na_2CO_3 = 106)$.

The indicator used in the titration of iodine against sodium thiosulphate is

The amount of oxalic acid present in a solution can be determined by its titration with $KMnO_4$ solution in the presence of $H_2SO_4$. The titration gives an unsatisfactory result when carried out in the presence of $HCl$,because $HCl$:

$20 \, mL$ of calcium hydroxide was consumed when it was reacted with $10 \, mL$ of an unknown solution of $H_2SO_4$. Also,$20 \, mL$ of a standard solution of $0.5 \, M$ $HCl$ containing $2$ drops of phenolphthalein was titrated with calcium hydroxide. The mixture showed a pink color when the burette displayed the value of $35.5 \, mL$,whereas the burette showed $25.5 \, mL$ initially. The concentration of $H_2SO_4$ is $..........M$ (Nearest integer).