# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: zun123 on November 09, 2014, 05:44:05 PM

Title: acid base question
Post by: zun123 on November 09, 2014, 05:44:05 PM
5.   (a)   The graph below shows how the pH changes when 0.12 M NaOH is added to 25.0 cm3 of a solution of a weak monoprotic acid, HA.

Click image for larger version.

(i)   Use the graph to calculate the initial concentration of the acid HA.
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(ii)   Write an expression for the dissociation constant. Ka, of the weak acid HA.
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(iii)   Determine the volume of sodium hydroxide added when [HA] = [ A–] and use the graph to determine the pH at this point.
Volume of NaOH(aq) added .. .. .
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(iv)   Use your answers to part (a)(ii) and part (a)(iii) to determine the value of Ka for the acid HA.
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(9)
(b)   A buffer solution is formed, when approximately half of the original amount of the acid HA(aq) has been neutralised by the base NaOH(aq). Explain how this buffer solution is able to resist change in pH when
(i)   a small amount of NaOH(aq) is added,
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(ii)   a small amount of HCl(aq) is added.
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(4)
(Total 13 marks)
Title: Re: acid base question
Post by: Borek on November 09, 2014, 06:32:18 PM
You have to show your attempts at solving the question to receive help. This is a forum policy.
Title: Re: acid base question
Post by: Arkcon on November 09, 2014, 06:32:37 PM
zun123:, please trouble yourself to read the Forum Rules{click} (http://www.chemicalforums.com/index.php?topic=65859.0).  We don't give complete answers on this forum.  Show us what you've done so far to try and solve this problem yourself. Show us what formula you would use to determine part 2, as given in your text book or class notes, as an example.
Title: Re: acid base question
Post by: zun123 on November 10, 2014, 01:38:39 PM
I was able to do part (ii) but rest i'm clueless about.
Title: Re: acid base question
Post by: Arkcon on November 10, 2014, 02:15:53 PM
Really?  I thought you'd use the answer to find out the rest.  Do tell us then, what did you get?  We'd like to see the result.  We'll start with that, and help with the rest.