If salicylic acid $(138 \text{ u})$ reacts with acetic anhydride $(102 \text{ u})$ to form aspirin $(180 \text{ u})$ and acetic acid $(60 \text{ u})$,calculate the percentage atom economy. (in $\%$)

Amount of oxygen required for combustion of $1 \, kg$ of a mixture of butane and isobutane is ............. $kg$.

$60 \ mL$ $NaOH$ solution is required for complete neutralization of $0.98 \ g$ orthophosphoric acid. The concentration of $NaOH$ solution is ............. $M$.

How many millilitres of $6.0 \ M$ hydrochloric acid should be used to prepare $150 \ mL$ of a solution which is $0.30 \ M$ in hydrogen ion?

The molarity of a $NaOH$ solution prepared by dissolving $4 \ g$ of it in $250 \ mL$ of water is: (in $M$)

Sodium oxide reacts with water to produce sodium hydroxide. $20.0 \ g$ of sodium oxide is dissolved in $500 \ mL$ of water. Neglecting the change in volume,the concentration of the resulting $NaOH$ solution is $........ \times 10^{-1} \ M.$ (Nearest integer)
[Atomic mass $: Na=23.0, O=16.0, H=1.0]$