Chemistry titration a level help

I don’t get part B. Can someone show me a diagram or explain what’s going on. Also it mentions in mark scheme acid and HCL. Is that referring to propanoic acid or HCL. Someone please explain to me this experiment and how it works with the mark scheme

Hi! The mark scheme complicates it a little, but I hope this sounds simpler (I've referred to propan-1-ol as propanol throughout this explanation)

You have added propanol and propanoic acid. Remember that a carboxylic acid + an alcohol -----> always gives an ester. In this case, the propanol + propanoic acid -----> propyl propanoate (the ester) + water. This is the equation given in the question.

Now, also recall that esters can be hydrolysed by an acid, for ex. HCl. So, you initially added unknown moles of propanol and unknown moles of propanoic acid, which formed unknown moles of an ester. But you have also added HCl. This HCl will hydrolyse the ester. But say, you have added more than required HCl. So only some moles of HCl will react with the ester, and the remaining moles of HCl can be titrated against to find the moles of HCl remaining.

If you initially knew how many moles of HCl you've added, then you can subtract the initial moles of HCl (before any reaction) and the final moles of HCl (after the reaction) to calculate how many moles actually reacted with the ester. Once you've calculated this, it is very easy to calculate the moles of ester from simple stoichiometery.

so basically it’s 3 steps. tirarte the initial HCL to see how much started with. then tirarte the mixture after to see HCL left over . take away to see how much HCL reacted. then i don’t get the second mark

Exactly! To me, the mark scheme seems a little wrong (although that is impossible). It should be subtract the total number of moles of HCl at equilibrium from the number of moles of HCl added inititally