How many grams of $CH_3OH$ should be added to water to prepare $150 \ mL$ solution of $2 \ M \ CH_3OH$?

Define the following terms:
$(i)$ Mole fraction $(ii)$ Molality $(iii)$ Molarity $(iv)$ Mass percentage

When $7.1 \ g$ of $Na_2SO_4$ (molecular mass $142$) dissolves in $100 \ mL$ of $H_2O$,the molarity of the solution is $.........$ $M$.

$A$ solution of sugar is obtained by mixing $200 \ g$ of its $25 \%$ solution and $500 \ g$ of its $40 \%$ solution (both by mass). The mass percentage of the resulting sugar solution is $..........$. (Nearest integer)

Molar concentration $(M)$ of any solution $=$

Density of $3 \ M$ $NaCl$ solution is $1.25 \ g/mL$. The molality of the solution is : (in $m$)