What is the reason for passing dry air through the combustion tube during the estimation of $C$ and $H$?

In an estimation of bromine by Carius method,$1.6 \ g$ of an organic compound gave $1.88 \ g$ of $AgBr$. The mass percentage of bromine in the compound is..........
(Atomic mass,$Ag = 108, Br = 80 \ g \ mol^{-1}$)

$A$ sample of $0.50 \, g$ of an organic compound was treated according to Kjeldahl's method. The ammonia evolved was absorbed in $50 \, mL$ of $0.5 \, M \, H_{2}SO_{4}$. The residual acid required $60 \, mL$ of $0.5 \, M \, NaOH$ solution for neutralisation. Find the percentage composition of nitrogen in the compound. (in $\%$)

In the sulphur estimation,$0.471 \, g$ of an organic compound gave $1.44 \, g$ of barium sulphate. The percentage of sulphur in the compound is $...... \%$. (Nearest integer)
(Atomic Mass of $Ba=137 \, u$)

Dumas method involves the determination of nitrogen content in the organic compound in the form of

How much $\%$ sulphur is present in an organic compound if on analysis $0.53 \ g$ of this compound gives $1.158 \ g$ of $BaSO_4$ (in $\%$)?