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.