$2 \ g$ of substance $A$ is added to $18 \ g$ of water to form a solution. Calculate the mass percentage of the solute. (in $\%$)

$50 \ mL$ of an aqueous solution of glucose contains $6.02 \times 10^{22}$ molecules. The concentration of the solution is? ........... $M$

The molality of $90\%$ ${H_2SO_4}$ solution is [density $= 1.8 \ g/mL$].

The molarity of the solution obtained by mixing $50 \ mL$ of concentrated $H_2SO_4$ $(36 \ N)$ with $50 \ mL$ of water is .......... $M$.

$200 \ g$ of $20 \% \text{ } w/w$ urea solution is mixed with $400 \ g$ of $40 \% \text{ } w/w$ urea solution. What is the weight percentage $(w/w \%)$ of the resultant solution (in $.33$)?

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