Which of the following formulas is correctly represented according to Stock notation?

Which of the following contain the same number of atoms?
$A$. $2 \text{ g}$ of $O_2$ gas
$B$. $4 \text{ g}$ of $SO_2$ gas
$C$. $1400 \text{ mL}$ of $O_2$ at $STP$
$D$. $0.05 \text{ L}$ of $He$ at $STP$
$E$. $0.0625 \text{ mol}$ of $H_2$ gas
Choose the correct answer from the options given below:

$10 \ g$ $NaHCO_{3(s)}$ sample is heated in open to constant weight,the residue is titrated against $0.1 \ M \ HCl$ in presence of phenolphthalein as an indicator,$500 \ mL$ of acid was required to reach end point hence $PERCENT$ purity of sample is $............... \%$.

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$2AgF + MgCl_2 \longrightarrow MgF_2 \downarrow + 2AgCl \downarrow$

Iron sulphide is heated in air to form $A$,an oxide of sulphur. $A$ is dissolved in water to give an acid. The basicity of this acid is

What quantity (in $mL$) of a $45\%$ acid solution of a mono-protic strong acid must be mixed with a $20\%$ solution of the same acid to produce $800 \ mL$ of a $29.875\%$ acid solution?