## volumetric analysis

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

second question:
A 1 g sample of a mixture of sodium carbonate and sodium hydrogencarbonate was titrated with $0.2 mol/dm^3$ HCl using phenolphthalein as indicator. If $17.5 cm^3$ 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.