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docbextravolcalcs1 updated April 9th 2008 |
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Advanced Level Chemistry Revision on Titrations GCE-AS-A2-IB Acid-base and other non-redox volumetric titration calculation QUESTIONS ANSWERS to the titration Q'S below * Redox titration Q's * EMAIL query?comment Q1-8 and Q13-14 & 19 based on acid-base titrations (acid-alkali/oxide/hydroxide/carbonate/hydrogencarbonate) and Q15-18 based on NaOH-organic acid titrations, Q9 includes useful exemplars for coursework on how much to use in titrations including EDTA, Q10-12 on silver nitrate-chloride ion titrations, further Q's will be added in the future. Appendix 1. information on EDTA structure and function in titrations. The non-redox titration Questions Q1 A solution of sodium hydroxide contained 0.25 mol dm-3. Using phenolphthalein indicator, titration of 25.0 cm3 of this solution required 22.5 cm3 of a hydrochloric acid solution for complete neutralisation.
Q3 4.90g of pure sulphuric acid was dissolved in water, the resulting total volume was 200 cm3. 20.7 cm3 of this solution was found on titration, to completely neutralise 10.0 cm3 of a sodium hydroxide solution. [atomic masses: S = 32, O = 16, H = 1)
Q4 100 cm3 of a magnesium hydroxide solution required 4.5 cm3 of sulphuric acid (of concentration 0.1 mol dm-3) for complete neutralisation. [atomic masses: Mg = 24.3, O = 16, H = 1)
Q5 Magnesium oxide is not very soluble in water, and is difficult to titrate directly. Its purity can be determined by use of a 'back titration' method. 4.06 g of impure magnesium oxide was completely dissolved in 100 cm3 of hydrochloric acid, of concentration 2.0 mol dm-3 (in excess). The excess acid required 19.7 cm3 of sodium hydroxide (0.20 mol dm-3) for neutralisation. This 2nd titration is called a 'back-titration', and is used to determine the unreacted acid. [atomic masses: Mg = 24.3, O = 16)
Q6 2 dm3 of concentrated hydrochloric acid (10 M) was spilt onto a laboratory floor. It can be neutralised with limestone powder. [atomic masses: Ca = 40, C = 12, O = 16)
Q7 A 50.0 cm3 sample of sulphuric acid was diluted to 1.00 dm3. A sample of the diluted sulphuric acid was analysed by titrating with aqueous sodium hydroxide. In the titration, 25.00 cm3 of 1.00 mol dm-3 aqueous sodium hydroxide required 20.0 cm3 of the diluted sulphuric acid for neutralisation.
Q8 A sample of sodium hydrogencarbonate was tested for purity using the following method. 0.40g of the solid was dissolved in 100 cm3 of water and titrated with 0.20 mol dm-3 hydrochloric acid using methyl orange indicator. 23.75 cm3 of acid was required for complete neutralisation. [Ar's: Na = 23, H = 1, C = 12, O = 16]
Q9 This question involves theoretical calculations to do with 'how much to weigh out' for titrations and a common requirement to show development in coursework projects. They involve reagents such as pure anhydrous sodium carbonate, standardised hydrochloric acid and EDTA titrations (theory). Atomic masses: O = 16, H = 1, Na = 23, C = 12, Ca = 40, P = 31.0 9(a)(i) Write out the equation, complete with state symbols for the reaction between hydrochloric acid and sodium carbonate.
9(b)(i) The simplified molecular structure of 2-ethanoylhydroxybenzoic acid ('Aspirin') is CH3COOC6H4COOH.
9(c) Pure calcium carbonate can be used to make a standard calcium ion solution to practice a complexometric titration of calcium ions with EDTA or determine the molarity of the EDTA reagent. See Appendix 1. for theoretical information on EDTA structure and function in titrations (advisable to read).
Q10 25 cm3 of seawater was diluted to 250 cm3 in a graduated volumetric flask. A 25 cm3 aliquot of the diluted seawater was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added. On titration with 0.1 mol dm-3 silver nitrate solution, 13.8 cm3 was required to precipitate all the chloride ion. [Atomic masses: Na = 23, Cl = 35.5]
Q11 0.12 g of rock salt was dissolved in water and titrated with 0.1 mol dm-3 silver nitrate until the first permanent brown precipitate of silver chromate is seen.
Q12 5.0 g of a solid mixture of anhydrous calcium chloride(CaCl2) and sodium nitrate (NaNO3) was dissolved in 250 cm3 of deionised water in a graduated volumetric flask. A 25 cm3 aliquot of the solution was pipetted into a conical flask and a few drops of potassium chromate(VI) indicator solution was added.
Q13 A bulk solution of hydrochloric acid was standardised using pure anhydrous sodium carbonate (Na2CO3, a primary standard). 13.25 g of sodium carbonate was dissolved in about 150 cm3 of deionised water in a beaker. The solution was then transferred, with appropriate washings, into a graduated flask, and the volume of water made up to 250 cm3, and thoroughly shaken (with stopper on!) to ensure complete mixing.
Q14 For this question relevant formula mass and equation are in the answers to Q13.
Q15 (a) Describe a procedure that can used to determine the molecular mass of an organic acid by titration with standardised sodium hydroxide solution. Indicate any points of the procedure that help obtain an accurate result and explain your choice of indicator. 0.279g of an organic monobasic aromatic carboxylic acid, containing only the elements C, H and O, was dissolved in aqueous ethanol. A few drops of phenolphthalein indicator were added and the mixture titrated with 0.100 mol dm-3 sodium hydroxide solution. It took 20.50 cm3 of the alkali to obtain the first permanent pink. [at. masses: C = 12, H = 1 and O = 16]
Q16 Using the method outlined in Q15(a), 0.103g of a dibasic/diprotic non-aromatic carboxylic acid required 19.85 cm3 of a standardised sodium hydroxide solution for complete neutralisation. If the concentration of the alkali was 0.0995 mol dm-3. [at. masses: C = 12, H = 1 and O = 16] Calculate ...
Q17 The % purity of an organic acid can be determined by the procedure outlined in Q15(a).
Q18 Using the method described in Q15(a), sodium hydroxide solution can be standardised. 0.25g of very pure benzoic acid (C6H5COOH) was titrated with a solution of sodium hydroxide of unknown molarity. If 22.50 cm3 of the alkali was required for neutralisation, calculate ...
Q19 The solubility of calcium hydroxide in water can be measured reasonably accurately to 3sf by titrating the saturated solution with standard hydrochloric acid.
EDTA is an acronym abbreviation for the old name EthyleneDiamineTetraAcetic acid and is used in equations. It is a hexadentate ligand i.e. it can donate 6 electron pairs to form 6 dative-covalent bonds and binds strongly with many metal cations Mn+ where n is usually 2 or 3. Multi-dentate ligands are called chelating* agents, because the two ligand bonds become part of a five membered ring system. (*The chelate is from the Greek word meaning a crab's claw) The solid EDTA 'off the shelf' used in analysis is usually the disodium dihydrate salt, which has the structure (Mr = 372.2) The full unionised structure is (HOOCCH2)2NCH2CH2N(CH2COOH)2 which we could abbreviate to H4EDTA since theoretically four hydrogens from the four carboxylic acid groups are ionisable.
of which H2EDTA2- is the most prominent chelating species in solutions of pH10 in titrating calcium ions though the complex is actually formed by the combination of a metal ion and the EDTA4- ion. The theory behind the titration of calcium ions with EDTA reagent is a bit complicated and the titration should be carried out in the presence of magnesium ions, usually included in the EDTA volumetric reagent, but if not, they must be in the mixture being titrated. This may seem to prelude an incorrect titration for calcium since magnesium ions reacting with EDTA, but it doesn't (see explanation later). Hydrated/hexa-aqua metal ions like aqueous calcium and magnesium ions (M2+) give the following reaction with EDTA reagent,
which, for theoretical explanation of the titration and subsequent calculations, is best simplified to
Both calcium and magnesium EDTA complexes are strongly formed i.e. virtually 100% to the right BUT the Kstab for the formation of the EDTA-calcium ion complex is greater than that for the EDTA-magnesium ion complex i.e. the calcium ion complex is more stable and calcium ions will displace magnesium ions from their EDTA complex. The indicators used e.g. Eriochrome Black T (represented in a 'free' anionic form as HIn2-) weakly complexes with ions such as the magnesium ion.
Both metal ions form a weak complex with the indicator at the start of the titration, but the indicator is displaced by the stronger binding EDTA (ligand displacement reactions), but much more slowly from the calcium complex than the magnesium complex i.e.
and this means without the presence of magnesium ions, the end-point is sluggish giving an inaccurate with just the calcium ions present, because (eq 6) is too slow. Therefore the order of complex ion stability is [CaEDTA]2-(aq) > [MgEDTA]2-(aq) > [MgIn]-(aq) and this order of stability is crucial to the success of the titration as the ensuing argument will show. In the EDTA solution are the Mg-EDTA complex ion plus excess uncomplexed EDTA ions. As the EDTA reagent is run into the calcium ion solution the calcium ion-EDTA complex is formed by reactions (eq 7) or (eq 8).
In (eq 8) the magnesium ion is displaced from its EDTA complex on a 1 : 1 molar basis by the calcium ion and then the free magnesium ions form a red complex with the blue indicator (eq 4) below. This continues as long as there are still Ca2+ ions to titrate and magnesium ions to be displaced i.e. no blue colour is seen yet.
Continued addition of EDTA eventually converts all the 'free' calcium ions into their EDTA complex ion via (eq 7 or 8), BUT, the 1st drop of excess EDTA after the calcium ions are all complexed then releases the blue form of the indicator via the fast reaction (eq 5),
so giving the sharp end point from red to blue at pH10. In the case of analysing a mixture of calcium and magnesium ions in the same mixture, one method is to analyse for Ca2+ as above (VCa). Obviously, you do NOT add magnesium ions to the EDTA or the mixture being titrated if you wish to estimate the total (Mg2+ + Ca2+). You add a known excess of standardised EDTA solution (Vexcess) and then back titrate with another standardized M2+ ion solution (Vback) and the end point is blue to red.
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BUT please note that my on-line revision notes and quizzes are no substitute for good classroom teaching-lecturing and thorough studying of your own notes and textbooks, practicing past papers and a copy of the syllabus which are readily downloaded from the examination board sites, but I hope here and there they will lend a tutoring hand on some topic, unit, module etc. For final revision you have to be intellectually honest about what you don't know or follow, YOU have to take the stuff to pieces, analyse what you do/do not understand and reconstruct it so it all makes sense in the end. There is no other way, there are no magic secrets on how to revise and learn, its mainly down to hard work and just good old fashioned study and employing teach-yourself strategies without the need for extra tutors and tutoring lessons. I also think there is too much hit and miss revision using past papers (which I do NOT supply) and not enough systematic revision. I also hope it will help teachers in planning lessons and developing schemes of work for science-chemistry. There are no lesson plans on the site but there are plenty of quizzes to incorporate into classroom activities whether photocopied or on electronic whiteboard projector for use as self-tuition-assessment purposes and a variety of teaching and learning styles and the images may be used in Microsoft Word documents and powerpoint projections. The site seems to be used by a large number of home study tutors, particularly the revision notes. An individual tutor may print out the notes for science-chemistry learning teaching-tuition purposes and for background material for assignments and projects. I have no interest or time in producing WORD.doc or xxxx.pdf files of the notes at the moment. Neither have I time to write up many practical laboratory experiments ('lab'-'labs') at the moment, but the notes contain lots of background information of chemical reactions in terms of observations-balanced equations-reactants-products-theory etc. I also find it difficult to recommend specific exam websites or syllabus textbooks, it depends exactly on what you need, what you have time for, and there are so many of them to choose from and I do not supply past examination papers for classes. The sites resources include revision notes, quizzes and worksheets which provide support for home study or tuition for homework and coursework help e.g. science investigations for any of the key stage courses indicated, but I do not supply lesson plans. 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docbextravolcalcs1 updated April 9th 2008 |