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Purity by weight (percentage purity ) question...help please? :( watch

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    A summary of the question is this.... 1g of boric acid has 50ml of water added to it as well as 15ml of glycerol. After boiling and cooling it was titrated with 1M NaOH using phenolphthalein as indicator and the titre value was 20.60 cm3.

    The question asks for the percentage purity.
    I tried working out by first finding the concentration of naoh and then using the equation B(OH)3 + NaOH--> NaB(OH)4 to find moles of boric acid and a whole bunch of stuff I've lost track of...I keep getting 1.27g as my answer for the supposedly pure mass which cant be as I had 1g in the beginning?? I'm definitely in need of help ...could someone kindly offer a solution please? Thank you!
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    (Original post by chem_nutella)
    A summary of the question is this.... 1g of boric acid has 50ml of water added to it as well as 15ml of glycerol. After boiling and cooling it was titrated with 1M NaOH using phenolphthalein as indicator and the titre value was 20.60 cm3.

    The question asks for the percentage purity.
    I tried working out by first finding the concentration of naoh and then using the equation B(OH)3 + NaOH--> NaB(OH)4 to find moles of boric acid and a whole bunch of stuff I've lost track of...I keep getting 1.27g as my answer for the supposedly pure mass which cant be as I had 1g in the beginning?? I'm definitely in need of help ...could someone kindly offer a solution please? Thank you!
    With the information provided the answer is 1.27g

    You must provide more detailed information, i.e. the full method or we cannot help ...
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    That's the method really...boric acid is complexed with glycerol to create borospiranic acid which is a stronger acid by adding 1g of boric acid, 50mL of water and 15L of glycerol. Then that whole thing is titrated with 1M NaOH and the amount required is a mean of 20.60cm3. I need to find the 'mass percentage(ie purity by weight)' as quoted in the question.

    glycerine
    The equation is B(OH)3 + NaOH ----------> NaB(OH)4
    (glycerine should be on arrow to indicate its addition)
    Is this what you were referring to?
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    (Original post by chem_nutella)
    That's the method really...boric acid is complexed with glycerol to create borospiranic acid which is a stronger acid by adding 1g of boric acid, 50mL of water and 15L of glycerol. Then that whole thing is titrated with 1M NaOH and the amount required is a mean of 20.60cm3. I need to find the 'mass percentage(ie purity by weight)' as quoted in the question.

    glycerine
    The equation is B(OH)3 + NaOH ----------> NaB(OH)4
    (glycerine should be on arrow to indicate its addition)
    Is this what you were referring to?
    My only suggestion is that you missed the end point and added too much NaOH ...
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    (Original post by charco)
    Where did you get that equation from?

    It was given in my lab sheet to show the stoichiometry
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    (Original post by chem_nutella)
    It was given in my lab sheet to show the stoichiometry
    Please look at the edited post above ...
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    (Original post by charco)
    My only suggestion is that you missed the end point and added too much NaOH ...
    I don't know , could be but the end point was perfect cos I was very careful and attentive but it's possible after all :/ Maybe NaOH was not really 1M...clueless:confused:

    Thank you for replying by the way
 
 
 
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