PCl5 + 4H2O -> H3PO4 + 5HCl
If $\displaystyle 5 * 10^{-3}$ mol of PCl5 is reacted completely with water and the solution made up to $\displaystyle 100 cm^3$, what is the volume of $\displaystyle 0.1 mol/dm^3$ sodium hydroxide required to react with $\displaystyle 25 cm^3$ of this solution? (ans:$\displaystyle 100cm^3$)

second question:
A 1 g sample of a mixture of sodium carbonate and sodium hydrogencarbonate was titrated with $\displaystyle 0.2 mol/dm^3$ HCl using phenolphthalein as indicator. If $\displaystyle 17.5 cm^3$ HCl were required, what is the percentage by mass of sodium hydrogencarbonate in the mixture? (ans: 62.9%)

how do i go about solving such questions where they're mixtures reacting with HCl? my ans differ a lot from the suggested answers for both.