What is the sum of the mole fractions of all the components in a solution?

The volume of water added to $500 \ mL$ of $0.5 \ M \ NaOH$ solution so that its strength becomes $10 \ mg \ mL^{-1}$ will be .......... $mL$.

Two solutions of a substance are mixed as follows: $480 \ mL$ of $1.5 \ M$ first solution + $520 \ mL$ of $1.2 \ M$ second solution. The molarity of the final solution will be ............. $M$.

The volume of ethyl alcohol (density $1.15 \text{ g/cc}$) that has to be added to prepare $100 \text{ cc}$ of $0.5 \text{ M}$ ethyl alcohol solution in water is: (in $\text{ cc}$)

$5.0 \ g$ of sodium hydroxide (Molar mass $40 \ g \ mol^{-1}$) is dissolved in a small quantity of water and the solution is diluted up to $100 \ mL$. What is the molarity of the resulting solution?

$A$ sample of drinking water is found to be severely contaminated with chloroform $(CHCl_3)$,which is a suspected carcinogen. If the level of contamination is $15 \ ppm$ (by mass),express this in percentage by mass.