$A$ mixture of $O_2$ and gas $Y$ (molecular weight $80$) in the mole ratio $a : b$ has a mean molecular weight of $40$. What would be the mean molecular weight if the gases are mixed in the ratio $b : a$ under identical conditions? (The gases are non-reacting)

Which of the following should be done in order to prepare $0.40 \ M \ NaCl$ starting with $100 \ mL$ of $0.30 \ M \ NaCl$ (mol.wt. of $NaCl = 58.5$)?

State whether the following statements are true or false:
$(i)$ $^\circ F = \frac{9}{5} (^\circ C) + 32$
$(ii)$ In $1808$,Dalton published the law of chemical combination in the journal "New System of Chemical Philosophy".
$(iii)$ If the molar mass of a dibasic acid is $100$,its equivalent mass is $200$.

The correct order of the total number of atoms present in
$(A)$ $2$ moles of cyclohexane
$(B)$ $684 \text{ g}$ of sucrose
$(C)$ $90.8 \text{ L}$ of dihydrogen at $STP$
is:

If one liter of air is repeatedly passed over heated copper and magnesium until no further reduction in volume occurs,the final volume of the air will be approximately ......... $mL$.

$A$ mixture of hydrogen and oxygen contains $20\%$ by weight of hydrogen. Find the total number of molecules per gram of the mixture.