Chapter 13 Volumetric analysis (acid base titrations)

Transcription

1 Chpter 1 Volumetric lysis (cid se titrtios) Ope the tp d ru out some of the liquid util the tp coectio is full of cid d o ir remis (ir ules would led to iccurte result s they will proly dislodge durig the titrtio). Remove the fuel (stops drippig while you red the meiscus). Relese the liquid util the ottom of the meiscus is o the 0ml. Volumetric lysis is well estlished d verstile form of qutittive chemicl lysis. The purpose of this type of lysis is to use ccurtely kow volume d cocetrtio of oe solutio to fid the ccurte cocetrtio of secod. The experimetl procedure which llows us to do this is clled titrtio. This procedure is descried i detil elow. Preprig the pipette Titrtio procedure Fillig the urette Burette Cle the urette, rise it d dry the outside. Rise it with the solutio it is goig to coti (cid). Fill the urette to ove the 0ml mrk. Check for ir ules d ivert to remove y, if required. Wsh d rise well. Rise with the solutio it is to coti. Suck up solutio with pipette filler, ove the grd mrk. dry outside. Relese the solutio util the ottom of the meiscus is o the grd lie. Tip off y hgig drop (this should ot e couted). Allow to dri uder grvity (do ot low). Whe dried touch the tip off the side, y drops which should e ilcuded will dri i. Leve the rest.

2 Preprig the coicl flsk Coicl flsk Rise severl times with deioised wter. Dry outside. Add se solutio s descried ove, from the pipette. Rise dow wlls of the flsk with deioised wter (you kow exct volume dded of se) Titrtio procedure Add idictor to the flsk, or drops re eough ecuse ll idictors re wek cids or ses. Rise dow the sides with wter. Ru the solutio ito the flsk from the urette, slowly. Rise the sides of the flsk regulrly. Swirl the flsk costtly, to esure thorough mixig of regets. As the ed poit ers, dd the solutio drop y drop. Whe the ed-poit is reched the idictor will chge colour suddely. At this poit the cid will hve exctly eutrlised the se. Now red the meiscus of the urette, from the ottom, t eye level. Use filter pper, if ecessry, to mke the meiscus more redle. Record your result. Repet the titrtio severl times. Get the verge vlue. Oly iclude the vlues tht gree withi 0.ml of ech other. To prepre stdrd solutio of sodium crote. Weigh the smple Weigh 1.0 g of sodium crote o electroic lce, s ccurtely s you c. Use clock glss. Two plces of decimls would e est. Trsfer to eker Use sptul to trsfer the smple to eker of wrm wter (100ml). Rise the clock glss. Rise the remiig gris ito the eker with deioised wter. Rise the sptul ito the eker lso. All trces must e trsferred. Pour the wshigs ito the volumetric flsk. Pour the wshigs ito the volumetric flsk, usig fuel d glss rod. Wsh the rod s well. Rise the eker severl times with deioised wter. Pour these wshigs ito the volumetric flsk. Top up the volumetric flsk with deioised wter, util just elow the grdutio mrk.

3 Top up to the grdutio mrk with dropper. Red the ottom of the meiscus t eye level. Ivert d mix to esure proper mixig of the cotets. Clcultios Numer of moles of sodium crote moles 0.01 moles i50 cm moles i1 litre Molr Repet the titrtio severl times util two titrtio vlues gree to withi 0. ml of ech other. Equtio for the titrtio HCl N CO NCl HO CO Results Volume of the cid = 19. ml Fctor for the cid = (the umer i frot of HCl i the lced equtio) Molrity of the cid =? Volume of the se = 0 ml Fctor for the se = 1 (the umer i frot of sodium crote i the lced equtio) Molrity of the se = M To use this stdrd sodium crote solutio to fid the cocetrtio of (stdrdise) give hydrochloric cid solutio. Clcultios V M V M Plce 0 ml of Molr sodium crote ito coicl flsk usig pipette. Add two drops of methyl red idictor. This will give yellow colour to the solutio. Note; the umer of drops of idictor should e kept to miimum s most idictors re either wek cids or ses d will therefore tke prt i the eutrliztio process. Plce the hydrochloric cid i the urette d djust the level to zero, tkig ll of the usul precutios. Titrte i the usul mer. Record the volume of cid required to eutrlise the sodium crote. The poit of eutrlistio is reched whe the idictor just turs red (pik). 19. M M M moles per litre 0.1 Molr 0.1 M

4 To mke up pproximte solutio of sodium hydroxide d stdrdise it (fid its exct cocetrtio) y titrtio with the stdrd hydrochloric cid solutio ove. Plce 0 ml of the sodium hydroxide i the coicl flsk. Note; Alwys plce the se i the coicl flsk s they my rect with the groud glss i the tp of the urette. Add two drops of methyl red idictor d yellow colour is imprted to the solutio. Put the hydrochloric cid (previously stdrdized) ito the urette. Adjust to the zero level i the usul wy. Titrte i the usul mer. Whe the colour of the solutio i the coicl flsk chges to fit trce of permet pik the ed-poit hs ee reched. Record the volume of cid required to do this. Repet the titrtio severl times util two titrtio vlues gree to withi 0. ml of ech other. Equtio for the titrtio NOH HCl NCl HO Results Volume of se = 0 ml Fctor for the se = 1 Molrity of the se =? Volume of the cid = 19.8 ml Fctor for the cid = 1 Molrity of the cid = 0.1 M Clcultio V M M 1 1 M M M V M moles per litre To determie the percetge of ethoic cid i viegr. Viegr is solutio of ethoic cid dissolved i wter. The purpose of this titrtio is to fid the percetge of this cid i the viegr. Add 50 ml of viegr to volumetric flsk usig 5 ml pipette twice. Mke up the solutio to the 50 ml mrk with deioised wter. This is the solutio which will e used for the titrtio. Note; dilutig the solutio i this mer is ecessry for two resos (i) you will use less regets this wy d (ii) if error is mde mesurig dilute solutio it will ot hve gret implictios for the fil swer. Add 0 ml of 0.1 M sodium hydroxide solutio to the coicl flsk usig pipette. Add two or three drops of pheolphthlei idictor, just eough to imprt pik tige to the sodium hydroxide solutio. Put the dilute viegr solutio i the urette.titrte i the usul mer.

5 The ed-poit is reched whe the pik colour chges to colourless. Record the volume of cid used from the urette.repet the titrtio severl times util two titrtio vlues gree to withi 0. ml of ech other. Equtio for titrtio CHCOOH NOH CHCOON HO Results Volume of cid used = 1 ml Fctor for the cid = 1 Molrity of the cid =? Volume of se used = 0 ml Fctor for the se = 1 Molrity = 0.1 M Clcultios V M V M 1 M M moles / litre moles 0.77 moles/ litreof solutio g/l 46. g/l 4.6 g i100 cm 4.6% (w/v) / litrei the origil viegr. To determie the percetge of wter of crystlliztio i hydrted sodium crote (wshig sod). Wter of crystlliztio is the wter which is foud s prt of the structure of crystllie sustce. It hs othig to do with eig wet. The wter molecules referred to i the term occupy positios i the crystl lttice of the sustce. This wter of crystlliztio is geerlly represeted i the chemicl equtios of such compouds, t the ed of the formul e.g. N CO.xH O The x here is umer which represets the umer of molecules of wter i the crystl. The purpose of this experimet is to determie the percetge of wter of crystlliztio i sustce y titrtio. Weigh out ccurtely 5 g of hydrted sodium crote o clock glss. Mke up the solutio to 50 ml i volumetric flsk. Follow the sme procedure s for mkig stdrd solutio previously outlied ove. Pipette out 5 ml of this solutio ito cle coicl flsk. Add few drops of methyl red idictor, eough to imprt fit yellow colour to the solutio i the coicl flsk. Plce 0. M HCl i the urette d djust the level to zero tkig ll the usul precutios. Titrte i the usul mer util the yellow colour is replced y permet pik tige. This is the ed-poit of the titrtio. Record the volume of cid required to rech the ed-poit d repet severl times util two redigs (titres) gree to withi 0. ml of ech other.

6 Results Volume of cid used =.5 ml Fctor for the cid = Molrity of the cid = 0. M Volume of se = 5 ml Fctor for the se = 1 Molrity of se =? moles of NCO ; moles HO 0.05 : : 10 x 10 Clcultios V M A V M M M Molr sodium crote moles i50 cm g.491g 6.7g -.491g 4.9 g % wter of crystllistio % We c ow clculte the vlue of 'x'i the formul N CO We lredy kow tht there re 0.05 moles preset i the crystls. of sodium crote We lso kow tht there re 4.9 g of wter preset. This is 4.9 equivlet to moles moles.xh O