$4 \ g$ equimolar mixture of $NaOH$ and $Na_{2}CO_{3}$ contains $x \ g$ of $NaOH$ and $y \ g$ of $Na_{2}CO_{3}$. The value of $x$ is $..... \ g$. (Nearest integer)

$(i)$ Calculate the total number of electrons present in one mole of methane.
$(ii)$ Find $(a)$ the total number and $(b)$ the total mass of neutrons in $7 \, mg$ of $^{14}C$. (Assume that mass of a neutron $= 1.675 \times 10^{-27} \, kg$).
$(iii)$ Find $(a)$ the total number and $(b)$ the total mass of protons in $34 \, mg$ of $NH_3$. Will the answer change if the temperature and pressure are changed?

$100 \ g$ of a mixture of $NaOH$ and $Na_2SO_4$ is neutralized by $100 \ mL$ of $0.5 \ M \ H_2SO_4$. What is the amount of $Na_2SO_4$ present in the mixture (in $g$)?

$20 \ mL$ of $2 \ M$ $NaOH$ solution is added to $400 \ mL$ of $0.5 \ M$ $NaOH$ solution. The final concentration of the solution is $.......... \times 10^{-2} \ M$. $(Nearest \ integer)$.

Which one of the following is the lightest?

$A$ gas has a density of $2.68 \ g/L$ at $STP$. Identify the gas.