Compounds that should not be used as primary standards in titrimetric analysis are $:$
$A.$ $Na_2Cr_2O_7$
$B.$ Oxalic acid
$C.$ $NaOH$
$D.$ $FeSO_4 \cdot 6H_2O$
$E.$ Sodium tetraborate
Choose the most appropriate answer from the options given below $:$

Temporary hardness in tap water is measured in terms of parts of $CaCO_3$ per $10^6$ parts of water. If $500 \ mL$ of hard water containing $Ca(HCO_3)_2$ was titrated against $\frac{1}{20} \ M \ H_2SO_4$ solution in the presence of a methyl orange indicator,and it required $40 \ mL$ of titrant,then the temporary hardness in the tap water is .............. $ppm$.

$50 \ cm^{3}$ of $0.2 \ N \ HCl$ is titrated against $0.1 \ N \ NaOH$ solution. The titration was discontinued after adding $50 \ cm^{3}$ of $NaOH$. The remaining titration is completed by adding $0.5 \ N \ KOH$. The volume of $KOH$ required for completing the titration is (in $cm^{3}$)

When $10 \ mL$ of an aqueous solution of $Fe^{2+}$ ions was titrated in the presence of dil $H_{2}SO_{4}$ using diphenylamine indicator,$15 \ mL$ of $0.02 \ M$ solution of $K_{2}Cr_{2}O_{7}$ was required to get the end point. The molarity of the solution containing $Fe^{2+}$ ions is $X \times 10^{-2} \ M$. The value of $x$ is $....$ (Nearest integer)

$25 \ mL$ of the given $HCl$ solution requires $30 \ mL$ of $0.1 \ M$ sodium carbonate solution. What is the volume of this $HCl$ solution required to titrate $30 \ mL$ of $0.2 \ M$ aqueous $NaOH$ solution? .............. $mL$

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: