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    • Thread Starter

    I am about to start A Level Chemistry in september and our teacher has given us some preparatory work involving back-titration to do over the Summer which I have tried but have now become very stuck on... any help as to what I am supposed to do (and the meaning of 1M and 0.1M) would be very helpful!

    The Experiment
    - Measure 10cm³ of the Milk of Magnesia (active ingrediant is Mg(OH)₂ ) into a 250cm³ volumetric flask
    - Pipette 50cm³ of 1M hydrolchloric acid into the volumetric flask and shake the contents
    - Top up with distilled water to the 250cm³ mark
    - Pipette 25cm³ of this solution into a conical flask and add a few drops of methyl-orange indicator
    - Prepare a burette with 0.1M sodium hydroxide solution and add this alkali to the contents of the conical flask and record how much needs to be added until the end-point
    - Repeat to obtain concordant results

    - after repeating the experiment and taking an average, our result was 25.3 cm³ of sodium hydroxide solution needed to be added to neutralise the solution in the conical flask

    Calculate/ Find:
    - the no. of moles of HCl added to the M' of M' in 50cm³ of 1M HCl
    - the no. of moles in the mean titre of 0.1M NaOH
    - a symbol eq. for the neutralisation of NaOH with HCl
    - how many moles of excess HCl are present in the 25cm³ aliquot and therefore the 250cm³ sample
    - the no. of moles of HCl which reacted with the M' of M' (reacted=total-excess)
    - an eq. for the neutralisation of Mg(OH)₂ with HCl
    - the no. of moles and mass in grammes of Mg(OH)₂ in the 10ml sample

    Any help would be very much appriciated!
    Thank you,
    Katie x
    • PS Helper

    PS Helper
    moles of HCL added = 1 x (50/1000)
    mean titre = 1.0 x (25.3/1000)
    Equation = you should be able to do that, its a 1:1 ratio btw

    i can do the rest but cant be bothered sorry, generally remember its a 1:1 ratio so finding out number moles of NaOH used will give you pretty much everything you need.

    symbol eq- NaOH + HCl -> NaCl + H2O
    • Thread Starter

    Thank you! So what does the 1M HCl and 0.1M NaOH actually mean? x

    1 M = 1 molar = 1 mole per litre = 1 mol dm-3
    • Thread Starter

    ahhh okkk! that makes so much more sense now - thats where you get the concentrations from then! thank youuuu! x
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