this video is a walkthrough of how to answer an exam question writing a method for a simple acid-base titration the like of which you would have completed for the second part of the first required practical in AQA a-level chemistry before I walk you through how to tackle this question pause the video and have a go at writing an answer of your own remember for a six mark question like this you should be spending about six minutes making sure that you name all of your equipment and justify the steps of the method where you can by this point you've probably done more than a dozen titrations and are fairly happy with how to complete them but remember there is a lot of detail that we need to be including here and we do need to be talking about why we're doing each step that we're doing so before we even start we're going to make sure that our buret is free of contamination so we're going to rinse it through with sodium hydroxide now the reason that it's the sodium hydroxide for this question is that I do know the concentration of the sodium hydroxide so with your titration for the two different solutions you're working with there's always one thing you do know and one thing you don't know about each solution so whatever's going in the buret I don't know at the start of the reaction how much I'm going to add that's going to be determined by when the end point happens so therefore that's the one I do know the concentration of so it's not the case that it's always The Alkali or anything like that now we need to rinse that buret in order to remove any contamination or any impurities that are in there and I can't rinse it with water because if I did there'd be some water left behind afterwards and that would dilute the sodium hydroxide that was going to add so the concentration wouldn't be what I thought it was and all of my calculations would be out so you're always rinsing with the solution that you are going to use once we've done that we're going to make sure that the buret is overfilled so it's gone over the zero and that's going to allow me to let some through into a waste Beaker and I'm doing that that the jet space is filled because if that jet space is empty when I start titrating then the first couple of centimeters cubed that I sort of think of adding is not actually going to go into the conical flask with acid it's just going to be filling up that jet space so that's going to give me a really inaccurate reading and therefore um all of my calculations would be out next I need to measure out my hydrochloric acid using a volumetric pipette and pipette filler you can get away with not calling it a volumetric perpet but you absolutely can't be mentioning a dropping pipette at any point so we're going to use that volumetric pipette to transfer exactly 25 centimeters cubed of hydrochloric acid to a conical flask and the reason it's a conical flask is because a conical flask is going to avoid any splashing while I'm swirling later once I've done that I'm going to add just a couple of drops of um a suitable indicator so here I'm using phenolphthalein because that's a really really good one for this titration but you don't actually need to name a specific indicator unless it's a question where they've given you in the year 13 acids and bases topic they've given you some information about indicators and they're asking you to pick the appropriate one so now I'm set up I've got my solution with the known concentration in the buret I've got my solution with the unknown concentration in the conical flask and also in that conical flask I have just one or two drops of this suitable indicator and I don't want to have any more than that because if I did then that could react with the reactant in the conical flask and that would affect the results of the titration as well so now I'm ready to start titrating I'm ready to start adding The Alkali from the buret to the conical flask where it can react with the acid and as I'm doing that I'm going to be swirling consistently to homogenize that solution and make sure that everything is reacting together and I'm also going to rinse down the end of the buret and also the sides of the conical flask using a wash bottle of deionized water I'm going to keep going until I see my first permanent color change so it's important that even after you've swirled that color change remains and therefore we know that we found the end point and when we find that end point we don't just shout hooray and stop we write down the volume of The Alkali that's been added to the acid to achieve that end point you're then obviously going to do this more than once your first titration is probably a rough titration that you throw away anyway but even then you're going to continue doing these titrations until you have results that are concordant so remember that concordant data our volumes are within 0.1 of each other we're going to use those concordant data to calculate a mean tighter and we're then going to use the formula moles is concentration multiplied by volume to calculate how many moles of sodium hydroxide we've added now we need to look at the ratio with which The Alkali and the acid react together which here would just be one to one so because they react in a one-to-one ratio however many moles of sodium hydroxide I've added that is the same as the moles of hydrochloric acid and then finally we're going to use those moles of hydrochloric acid divided by the volume of the hydrochloric acid in order to calculate what the concentration of that original solution is thank you very much for watching and I hope you're now feeling slightly more confident about your ability to write a method for this second half of the first required practical of AQA a-level chemistry if you are finding these videos useful then don't forget to like And subscribe for more a level chemistry content coming soon