1. Analytical chemistry

2. Qualitative Analysis What substances are present in the product? Testing for the presence of pesticide and herbicide residues in milk Actual oils used in a margarine Ethanol in petrol Arsenic in food 3. Quantitative Analysis How much of a particular substance is present in a product? Determination of the actual concentration of pesticide and herbicide residues in milk Percentage of canola oil used in margarine Analysis uses modern instrumentation or the old fashioned reactions 4. Analysis InstrumentsChemistryspectroscop yGravimetric Volumetricchromatography HPL AAUV Acid baseRedoxTLCGLC IRC 5. Balance equations Na2CO3(aq) + HCl(aq) aq) + HNO3(aq) NaOH(aq) + HCN(aq) HCOOH(l) + NaOH(aq) 6. Ionic EquationsFull Equation NaOH(aq) + HCN(aq)NaCN(aq) + H2O(l)Ionic EquationNa+(aq) + OH-(aq) + H+(aq) + CN-(aq) Na+(aq) + CN-(aq) + H2O(l)Partial Ionic equation Just shows the substances that undergo change OH-(aq) + H+(aq) H2O(l) 7. Units Convert 125oC to K 23mL to L 2.4atm to Pa 740mmHg to Pa 8. Relative atomic mass Mass of an atom or compound relative to 12C having a mass of 12 MR of Mg = MR of MgO = MR of Al2O3 = 9. Examples Calculate the volume of 3.5 mole of helium at STP 12g of hydrogen at SLC 12g of hydrogen at 110oC and 200000Pa 10. Percentage Composition % mass = MR of element x number present x 100 MR of compound Examples % S in SO2 % Al in Al2O3 11. Empirical Formulae A calculation frequently asked for is the determination of an empiricalformula given the percentage composition of the atoms or just a massratio 12. Example Determine the empirical formula for cholesterol given that thepercentage composition of cholesterol is 83.938% carbon, 11.917%hydrogen and 4.145% oxygen. The empirical formula is therefore C27H46O 13. Example When 0.864g of nitrogen burns, it forms 2.839g of oxide. Find theempirical formula of the oxide. If the molar mass of the oxide is 92g, determine its molecular formulaas well. 14. Example When a 0.995g sample of an organic molecule containingcarbon, hydrogen and oxygen is burnt in air, the only products are1.468g of carbon dioxide and 0.602g of water. What is the empirical formula? 15. Example 10.848g hydrated copper(II) sulfate is dried until there is no furtherchange in mass. After drying, the anhydrous salt has a mass of6.935g. Determine the degree of hydration of the salt. 16. The value of MR The value of molar mass can sometimes be provided in a round aboutway. A 51g sample of a compound occupied the same volume as 16g ofoxygen at the same temperature and pressure. n(O2) = 16 / 32 = 0.5 mol Therefore the number of mole of the unknown compound must also be 0.5 mol. If 0.5 mol has a mass of 51, then the molar mass must be 102g 17. Mole 1 mole is the amount of substance that contains the same number ofparticles as there are in 12g of 12C The number of particles in 1 mole = 6.02 x 1023. This is Avagadros number Examples 1 mole of aluminium = 26.9g 1 mole of copper = 1 mole of Al2O3 = 18. Mole calculationsmn = MR Example Calculate the number of mole in 200g of aluminium 0.34g of Al2O3 19. Significant figures Your answer must only have the same number of significant figuresas the least acurate data given The zeros before and immediately after a decimal point are notcounted as significant 20. Masses of solids Take special care with questions that give mass in kilograms ormilligrams. Moles are calculated using grams so you must convert kgand mg to grams before you start. 21. Stoichiometry Calculate the number of mole of each substance for the followingreactions 2FeCl32FeCl2 + Cl2 3 mole_____ ___ ______0.45mole ____ 4Al + 3O22Al2O3 10mole _________ _____ 0.45mole ______ 22. Excess 20g of magnesium is added to 200mL of 2M hydrochloric acid.Which chemical is in excess? Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) The amount of product will be determined by the reactant which isscarce, as it all reacts. The volume of H2(g) must be calculated from the limiting reactant. 23. Gravimetric Analysis Is a method of analysis that involves accurate measurement of massesin a precipitation reaction An ion in solution is caused to precipitate out of solution. Theprecipitate is filtered, then dried. Its mass is related to theconcentration of the original ion. Gravimetric analysis relies on stoichiometry Gravimetric analysis will work if either of the ions present can beprecipitated. 24. Simplified solubility table All nitrates, acetates (ethanoates), group 1 and ammoniumcompounds are soluble All chlorides, bromides and iodides are soluble except Ag+and Pb2+ compounds All sulfates are soluble except BaSO4 and PbSO4 All carbonate compounds are of low solubility except group1 carbonates and (NH4)2CO3 All hydroxide compounds are of low solubility exceptgroup 1 hydroxides and NH4OH, Sr(OH)2 and Ba(OH)2 25. Gravimetric Analysis A chemist determined that the salt (NaCl) content of food byprecipitating chloride ions as silver chloride. A 8.45g sample of foodyielded 0.636g of precipitate. 26. An example of the steps involved in gravimetric analysis The equation needs to be written down first Find the molar mass of the precipitate and salt Find the number of moles of the precipitate and equate this to thenumber moles of the salt Find the mass in grams of the salt from n = m/MR Determine the percentage of salt 27. What can go wrong in gravimetric analysis? Although precipitates used are of low solubility, a verysmall amount will remain in solution Other insoluble compounds may also be precipitated The precipitate must be washed to prevent any otherchemicals such as solute particles from crystallising outduring the drying process. The washing must belimited, however, so as not to re-dissolve any precipitate The balance has an uncertainty that may be tiny for veryprecise instruments or quite large for simpler models. Forthis reason the mass of precipitate should be reasonablylarge. 28. Test yourself The formation of an insoluble salt in solution is known as__________________. Insoluble salts can be removed from solutionby ________________. When two soluble salts, sodium chloride and silver nitrate, are reactedtogether they form a white precipitate of silver chloride. The overallequation is: NaCl(aq) + AgNO3(aq)AgCl(s) + NaNO3(aq) Theprecipitate is ______________ The ionic equation is: Ag+(aq) + Cl-(aq) AgCl(s) The spectator ions are: _______________ +________________ 29. Test yourself An analytical procedure in which the masses of solids are measured inorder to determine the concentration of a particular substance in amixture is called _________________ Steps involved in gravimetric analysis are: (i) add an _____________of a reactant to form a precipitate, (ii) ______________ theprecipitate and wash it, (iii) dry the precipitate to ____________mass. 30. Chicken soup worksheet 31. Example A 5.40g sample of potato chips is crushed and mixed in water todissolve the potassium chloride that is used instead of sodiumchloride. To analyse the KCl, excess silver nitrate solution is added.The precipitate is filtered, dried and weighed. Its mass is 0.192g.Calculate the % mass of potassium chloride in the chips (assume noNaCl is used.) 32. Tips Set out each question carefully Write the equation out, with each piece of data placed under thechemical it refers to Include the units Where to start will often be evident from what you have written 33. Example A 50.0mL solution containing iron(III) nitrate, Fe(NO3)3, has excesssodium hydroxide solution, NaOH, added to it to precipitate the ironas Fe(OH)3. After heating, the hydroxide decomposes to iron oxide,Fe2O3. The mass of precipitate obtained was 0.533g. Calculate theiron concentration in the original solution. 34. Fertiliser (Exam question) A soluble fertiliser contains phosphorus in the form of phosphate ions(PO43-) content by gravimetric analysis, 5.97g of the fertiliser powderwas completely dissolved in water to make a volume of 250.0mL. A20.00mL volume of this solution was pipetted into a conical flask andthe PO43- ions in the solution were precipitated as MgNH4PO4. Theprecipitate was filtered, washed with water and then converted byheating into Mg2P2O7. The mass of Mg2P2O7 was 0.0352g. 35. Questions Calculate the amount, in mole, of Mg2P2O7. Calculate the amount, in mole, of phosphorus in the 20.00mL volumeof solution Calculate the amount, in mole, of phosphorus in 5.97g of fertiliser Calculate the percentage of phosphate ions by mass in the fertiliser.Ensure you express your answer to an appropriate numberof significant figures. 36. Continued Several actions which could occur during this analyticalprocedure are listed below. For each action indicate the likelyeffect on the calculated percentage of phosphate ions in thefertiliser. The MgNH4PO4 precipitate was not washed with water The conical flask had been previously washed with water but not dried A 25.00mL pipette was unknowingly used instead of a 20.00mL pipette The mass of the fertiliser was recorded incorrectly. The recorded mass was 0.2g higher than the actual mass 37. Method again Step 1: A known mass of the sample is dissolved in a suitable solvent Polar substances are dissolved in Non polar substances are dissolved in Alloys are dissolved in Step 2: the substance to be analysed is precipitated by the addition of an appropriate chemical species. The chosen solution is one that exclusively precipitates the ion of interest.If the precipitating solution co-precipitates other ions, the weighed mass will beand therefore, the calculated percentage by mass will be 38. Step 3: the precipitate is collected by filtration andthoroughly washed to remove substances that wouldotherwise contribute to the mass of the precipitate.Washing is usually performed using deionised water.If the washing step is omitted, the weighed mass will be and the calculated percentage by mass ofprecipitate will be Step 4: the precipitate is then dried in an oven at110oC.If the drying step is incomplete, water will contributeto the mass; the weighed mass will beand the calculated percentage by mass of theprecipitate will be 39. Step 5: the sample is cooled in a dessicator andweighed. The dessicator removes moisture from theatmosphere and minimised the amount of moistureabsorbed by the sample during cooling Step 6: steps 4 and 5 are repeated until there is nosignificant change in mass. This ensures that all thewater has been evaporated. 40. The precipitate formed mustexhibit the following properties: The precipitate must have a low solubility so that it doesnot go back into solutionIf the precipitate has a relatively high solubility, theweighed mass will be and the calculatedpercentage by mass of precipitate will be The precipitate must have a molar mass that does notvary, so that stoichiometric calculations can be accuratelypreformedIf a sample absorbs substances from the atmosphere, theweighed mass will beand the calculatedpercentage by mass of precipitate will beIf a sample gives off substances to the atmosphere, theweighed mass will beand the claculatedpercentage by mass of precipitate will be 41. The precipitate must be stable when heated anddessicated The precipitate must be pure The precipitate must be easy to recover by filtration The molar mass should be high so that weighingerrors are minimised 42. Effects of incorrect techniques on calculationsERROR Mass of collected Mass of sample %precipitatecomponent compositionCo-precipitation ofionsHigh solubility ofprecipitateLoss of precipitateduring filtrationIncomplete dryingIncomplete washing 43. Question 1 A solution containing 10.0g of sodium chloride is mixed with a solution of silver nitrate. What mass of precipitate will be formed? 44. Question 2a A 1.595 g sample of silver alloy is dissolved in 50.00 ml(an excess) of nitric acid. A 10.00 ml sample was thentreated with excess sodium chloride solution toproduce a precipitate of silver chloride. Theprecipitate was filtered, dried and weighed. If the mass of silver chloride precipitated is 0.25g, findthe percentage of silver in the alloy. 45. Question 2b,c State the assumptions that were necessary to determine the percentage of silver in the alloy. What are the principal sources of experimental error in a gravimetric analysis? 46. Question 2d, e What conditions could be arranged to precipitate as much of the substance to be analysed as possible? Why was excess sodium chloride added to the reaction mixture? 47. Question 2 f Consider the following reaction:BaCl2(aq) + Na2SO4(aq) BaSO4(s) + NaCl(aq) Discuss why you would choose filtration techniques inpreference to evaporation when collecting theprecipitate of this reaction. 48. Question 3 a A sample of contaminated hydrated copper sulphate(CuSO4.5H2O) was tested for purity. A 15.0 g samplewas dissolved in water and filtered to removeinsoluble impurities. The sulphate ions wereprecipitated by the addition of excess barium chlorideand the resulting precipitate was collected, dries andweighed. If the final mass is 4.95 g: Find the percentage purity of the sample. 49. Question 3 b, c If the precipitate was not completely dry when weighed, what effect would this have on the calculated percentage purity of the sample? What would be the effect on the calculated percentage if barium nitrate was used instead of barium chloride? 50. Question 3 d, e What would be the effect on the calculated percentage if silver ions were present in the sample? Find the mass of contaminated hydrated copper sulphate that would be required to produce 100.00 ml of a 0.250 M copper sulphate solution. 51. Question 4 A 0.500 g sample of sodium sulphate and a 0.500 g of aluminium sulphate were dissolved in a volume of water, and excess barium chloride added to precipitate barium sulphate. What was the mass of barium sulphate produced? 52. Question 5 0.6238 g of copper(II) sulphate crystals with formula CuSO4.xH2O was dissolved in water, and the black copper (II) oxide was precipitated by treatment with boiling NaOH solution. The precipitate was collected by filtration, washed, dried and weighed. If the precipitate weighs 0.1988 g, calculate the value of x in the formula CuSO4.xH2O. 53. Question 6 In order to determine the molecular formula of acompound known to contain only carbon, hydrogenand oxygen, the following experiments were carriedout.1. A 0.60 g sample of the compound was burnt in excess oxygen. When the gases evolved were passed through anhydrous CaCl2, its mass increased by 0.36g due to the absorption of H2O. The remaining gas(es) when bubbled through a NaOH solution, increased its mass by 0.88 g. 54. Question 6 a, b2. A 1.21 g sample of the compound was vaporised. Thevapour occupied 0.403L at 150oC and 1.17 x 105 Pa Determine the mass of the gaseous products Determine the mass of carbon in the sample 55. Question 6 c, d Determine the mass of hydrogen in the sample Determine the mass of oxygen in the sample 56. Question 6 e, f Determine the empirical formula of CxHyOz Determine the relative molecular mass of CxHyOz 57. Question 6 g Determine the molecular formula of CxHyOz