Section4_Solutions2

Section4_Solutions2 - Solutions 2 By the end of this...

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Page 1 of 13 Solutions 2 By the end of this section, you will be able to: (1) Understand titrations , endpoints and indicators (2) Identify redox reactions, calculate oxidation numbers and balance redox equations
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Page 2 of 13 Acid-Base Titrations A titration is a method to determine the concentration of a basic (or acidic) solution. Two possibilities: an acid of known concentration is added to neutralize a base of unknown concentration – or – a base of known concentration is added to neutralize an acid of unknown concentration In both cases, the key reaction is: H + + OH H 2 O The endpoint (or, equivalence point) of a titration is the point at which equal numbers of moles of acid and base have reacted. The endpoint is marked by a change in colour of an added pH indicator . The indicator is typically an organic compound that is itself a weak acid; it is usually designated as Hin (see MH5, Ch. 9).
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Page 3 of 13 Different pH indicators change colors at different values of pH. Examples: MH5, Fig. 14.4: (a) Methyl red (b) Bromothymol (c) Phenolphthalein blue red yellow colourless orange blue pink (pH 5) (pH 7) (pH 9) HIn + H 2 O H 3 O + + In Acidic solution Basic solution Low pH High pH “Acid” colour “Base” colour The color changes at a pH where [HIn] = [In - ] This point is called the endpoint of the indicator.
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Page 4 of 13 Experimental Set-up An Erlenmeyer flask contains a solution of a strong acid of an uknown concentration and a very small quantity of pH indicator (a).
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