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1. 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?

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?

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 %
2. (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?

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?

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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