$A$ mixture of gases contains $H_2$ and $O_2$ gases in the ratio of $1 : 4$ $(w/w)$. What is the molar ratio of the two gases in the mixture?

$20 \ g$ of iron pyrite,$FeS_2$,is roasted completely and the $SO_2$ gas produced is absorbed completely in $400 \ mL$ of $NaOH$ solution. If only $50\%$ of the $NaOH$ is used in the reaction,what was the molarity of the $NaOH$ solution?

$5 \, mL$ of $N$ $HCl$,$20 \, mL$ of $N/2$ $H_2SO_4$,and $30 \, mL$ of $N/3$ $HNO_3$ are mixed together and the volume is made up to $1 \, L$. The normality of the resulting solution is:

$A$ $1.85 \ g$ sample of an arsenic containing pesticide was chemically converted to $AsO_4^{3-}$ (atomic mass of $As = 74.9$) and titrated with $Pb^{2+}$ to form $Pb_3(AsO_4)_2$. If $20 \ mL$ of $0.1 \ M \ Pb^{2+}$ is required to reach the equivalence point,the mass percentage of arsenic in the pesticide sample is closest to

What is the volume in $dm^3$ occupied by $3 \ mol$ of ammonia gas at $STP$?

Aluminium reduces manganese dioxide to manganese at high temperature. The amount of aluminium required to reduce one $g \ mol$ of manganese dioxide is $.....$