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Thread: another URGENT titration question

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    another URGENT titration question

    I recently took part in the nsw titration competition and in the first part, we put unknown concentration of NaOH in burette, and 25mL of 0.1292mol/L of HCL in the conical flask. Then we put 3 drops of phenolphthalein indicator into the conical flask, and let the NaOH run into the conical flask until the solution in the flask turned a light pink colour. From this we recorded the volume and found out the concentration of the NaOH solution.

    But my question is how come we can you can still use PHENOLPHTHALEIN (meant to be used when equivalence point is between 8.3 - 10 pH), when we are using a STRONG ACID and a STRONG BASE, which means it will have an equivalence point of 7.

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    Re: another URGENT titration question

    Quote Originally Posted by Mr_Kap View Post
    I recently took part in the nsw titration competition and in the first part, we put unknown concentration of NaOH in burette, and 25mL of 0.1292mol/L of HCL in the conical flask. Then we put 3 drops of phenolphthalein indicator into the conical flask, and let the NaOH run into the conical flask until the solution in the flask turned a light pink colour. From this we recorded the volume and found out the concentration of the NaOH solution.

    But my question is how come we can you can still use PHENOLPHTHALEIN (meant to be used when equivalence point is between 8.3 - 10 pH), when we are using a STRONG ACID and a STRONG BASE, which means it will have an equivalence point of 7.
    You're assuming that they 100% neutralise which they wouldn't... the pH wouldn't be 7

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    Executive Member Ekman's Avatar
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    Re: another URGENT titration question

    Quote Originally Posted by Drsoccerball View Post
    You're assuming that they 100% neutralise which they wouldn't... the pH wouldn't be 7
    As a matter of fact the whole idea of the equivalence point is that they do 100% neutralise one another. The pH of the solution would be 7 since a neutralisation between a strong acid and base gives a solution of pH of 7.

    In terms of the question, the reason why we can use methyl orange, bromothymol blue and phenolphthalein as indictors for this particular titration is because the change in pH is so drastic, it doesn't matter which of the three we use. So the colour will drastically turn from colourless to pink as you put more NaOH in. So the transition range of indicators doesn't necessarily have to be where the equivalence point is.
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    Re: another URGENT titration question

    Quote Originally Posted by Ekman View Post
    As a matter of fact the whole idea of the equivalence point is that they do 100% neutralise one another. The pH of the solution would be 7 since a neutralisation between a strong acid and base gives a solution of pH of 7.

    In terms of the question, the reason why we can use methyl orange, bromothymol blue and phenolphthalein as indictors for this particular titration is because the change in pH is so drastic, it doesn't matter which of the three we use. So the colour will drastically turn from colourless to pink as you put more NaOH in. So the transition range of indicators doesn't necessarily have to be where the equivalence point is.
    How do you know all of this -.- it's not like we have a different teacher or anything ?...

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    Re: another URGENT titration question

    Quote Originally Posted by Drsoccerball View Post
    How do you know all of this -.- it's not like we have a different teacher or anything ?...
    Its called studying/making notes on the syllabus...
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    Re: another URGENT titration question

    ^ QUOTE (Ekman)
    "As a matter of fact the whole idea of the equivalence point is that they do 100% neutralise one another. The pH of the solution would be 7 since a neutralisation between a strong acid and base gives a solution of pH of 7.

    In terms of the question, the reason why we can use methyl orange, bromothymol blue and phenolphthalein as indictors for this particular titration is because the change in pH is so drastic, it doesn't matter which of the three we use. So the colour will drastically turn from colourless to pink as you put more NaOH in. So the transition range of indicators doesn't necessarily have to be where the equivalence point is. "

    with phenolphthalein it is slightly easier to notice the change.
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    Re: another URGENT titration question

    how urgent is URGENT?
    2014 HSC
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