8303171-57-redox-V - Chem Factsheet...

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Answering Questions on Redox Titrations I Number 57 1 C hem F actsheet www.curriculumpress.co.uk To succeed with this topic you need to: Know and understand how to do AS titration calculations ( Factsheets No. 7 (‘Moles and Volumetric Analysis’) and No. 23 (‘How to Answer Questions on Titration Calculations’)) Understand the whole concept of Redox Equilibria (Factsheet No.37, No.45, and No.50). Understand the basics of redox titrations (Factsheet No.51) After working through this Factsheet you will have met more examples of redox titrations seen the ‘method’ being used on a range of examples You may be given a problem that looks very different from ones you have seen before. The way to deal with it is to remember the following method: (1) Write the EQUATION for the titration reaction (you may need to combine half equations using numbers of e to write the full equation). (2) Write down the REACTING RATIO from the balanced equation. (3) Identify which of the reactants you can find the NUMBER OF MOLES of using moles = volume (cm 3 ) × M(mol dm -3 ) 1000 (4) Combine answers from (2) + (3) to find the NUMBER OF MOLES OF THE OTHER REACTANT. (5) Look at the question – WHAT DO YOU NEED TO FIND? This will dictate which equation to use next i.e. moles = volume (cm 3 ) × M(mol dm -3 ) 1000 or moles = grams A r / M r or % purity = mass of pure × 100 mass of impure Summary - you could remember the 5 stages as EQ RR Moles ‘other moles’ Q A (1) (2) (3) (4) (5) (6) In the following examples the questions are underlined at key places so the method can be explained e.g. U2 = the second underlined section. Underlining key points is often helpful in examination questions – and not just for the topic we are dealing with now - as it helps you focus your attention on the key facts and figures. Before you start – remember this topic requires problem solving - so the method is a way of thinking, not just a recipe sheet. It's worth taking your time to make sure you really are happy with what is going on. Example 1 Ammonium iron(II) sulphate crystals have the formula: (NH 4 ) 2 Fe(SO 4 ) 2 . x H 2 O. In an experiment to find the value of x , 4.25 g of crystals were dissolved in water and dilute sulphuric acid and made up to 250 cm 3 . A
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This note was uploaded on 03/08/2011 for the course CHEM 101 taught by Professor Hard during the Spring '11 term at UT Arlington.

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8303171-57-redox-V - Chem Factsheet...

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