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    I am having a problem with these 2 questions. the answer is provided in bold but i don't understand it. why is the base the ionised one and why can we conclude it is yellow ??

    Methyl red is a common acid-base indicator. It has a Ka equal to 6.3 x 10–6. Its unionised form is red and its anionic form is yellow. What color would a methyl red solution have at
    pH = 7.8?


    Correct Answer: yellow
    Feedback: pH = pKa + log10([Base]/[Acid])
    ([Base]/[Acid]) = 10(pH – pKa)
    = 2.6
    > 1
    [Base] > [Acid]
    Ionised > unioinised (anionic)



    Question 2. Also I don't understand the answer in bold for this next question, where has the [10/(100 + 10)] come from
    Appreciate your help. I'm slightly confused thanks!

    What is the percentage dissociation of glycine if the solution has a pH = 8.60 and a pKa = 9.60?

    Correct Answer: 9 %
    Feedback: pH = pKa + log10([Base]/[Acid])
    [Base]/[Acid] = 10(pH – pKa)
    = 10(8.60 – 9.60)
    = 0.1 = 10/100
    % Dissociation = [10/(100 + 10)] × 100
    = 9 %
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    (Original post by groovyangel2008)
    I am having a problem with these 2 questions. the answer is provided in bold but i don't understand it. why is the base the ionised one and why can we conclude it is yellow ??

    Methyl red is a common acid-base indicator. It has a Ka equal to 6.3 x 10–6. Its unionised form is red and its anionic form is yellow. What color would a methyl red solution have at
    pH = 7.8?


    Correct Answer: yellow
    Feedback: pH = pKa + log10([Base]/[Acid])
    ([Base]/[Acid]) = 10(pH – pKa)
    = 2.6
    > 1
    [Base] > [Acid]
    Ionised > unioinised (anionic)



    Question 2. Also I don't understand the answer in bold for this next question, where has the [10/(100 + 10)] come from
    Appreciate your help. I'm slightly confused thanks!

    What is the percentage dissociation of glycine if the solution has a pH = 8.60 and a pKa = 9.60?

    Correct Answer: 9 %
    Feedback: pH = pKa + log10([Base]/[Acid])
    [Base]/[Acid] = 10(pH – pKa)
    = 10(8.60 – 9.60)
    = 0.1 = 10/100
    % Dissociation = [10/(100 + 10)] × 100
    = 9 %
    Indicators have pH = pKa at the midpoint because there is equal concentration of both forms.

    Because Ka = [H+][A-]/[HA]

    therefore if [A-]=[HA] then ka = [H+]

    So if Ka equal to 6.3 x 10–6 then pKa = 5.2
    Therefore at pH 7.8 it has gone into the basic colour - i.e. yellow

    you take it from here...
 
 
 
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