VARIOUS USEFUL TECHNIQUES In this exercise we wi ll learn severa l things. Fi rst, how to weigh a solid. Place a piece of paper, abou t 15 em (6 in .) sq uare on the left pa n of a trip balan ce and cou nter balance this pape r either with ano ther pie ce of paper of approxima tely equal size or by ad just ing the '....ei gh ts. \ Veigh out 8 grams (g) of potassiu m chroma te as follows. From the bott le of K 2 Cr0 4 on the side shelf carefully po ur out, 011 a sepa rate sheet of p aper, wha t you estim ate to b e about 6 g of the ma te rial. (T his 'NiH be about 2 ta blespoons in vol ume.) Adj ust the balance

weights to weigh 8 g of substa nce . T ra nsfer the K 2 CrO" carefu lly and slowly to the pap er on the left pan of the balance, keeping one eye on the poi nter needle of the bal ance. If more is needed, estimate the amou nt and pou r it on to the separa te sheet of pa per, then add it to the pile on the left pan of t he bal ance u n til enoug h has been added to bri ng the balance to equ ili brium , wi th the poi nter needl e at the center of the scale .

A n)' excess rem aining sho uld no t, repeat: shou ld not , be ret urned to the original bottle. T oo many eco nom ically minded individuals have, in the past, ret urned m ate rial to the wrong bottle, or have somehow contam inated the excess material prior to ret urni ng it to the right bottle. T he results in both cases were uniortunate for the IJerson who next wed that contam inated bottle. I t is less wastef ul in the lon g run to adhere to this ru le: E xcess material is discarded. T o be economical, just Ire sur e you tak e as little as possible ill excess 0/ )'our estimated need . Also, note the use of the paper bet ween the ba lance pan and the potassiu m chroma te. You were asked to put the pa per there to p rot ect the balance pan as 'well as to facilitate your handling of the solid. Always, when weighing solids, tak e care to prot ect the balance pall from con tact with the solid itself. Use a shee t of JJaJJer, a watch glass, beaker, or oth er su itable dev ice. Liq ui d s, of course, must be weighed in containers, thus affording protect ion to the balance pa n without attention to th is d etaiL To re su me , clean u p an y spill ed ma teria l and take the 8 g of reagent back to you r bench . Be sure to clean up the area around the balance. Alway s leave the balance as clean, or cleaner, than it was before '}'ou used it. Pour the weighed sal t in to a 250-1111 beaker . Knowing that, when full , this beake r co n tains approxi mately 250 ml, esti mate how hi gh 100 ml o f water will fill the beaker . Ad d approximatel y 100 1lI1 of water to the beaker. Sti r unti l all of the salt has d issolved. If you wish , the d issolu t ion can be hasten ed by heating. If you do dec ide to heat th e water, use a wire gauze to pro tect the beaker from d irec t con tact with the flame of the burner . Now you nee d to use your grad uate (you r grad uated cylinder). Fill it abo u t half -full with water and note th at the surface o f the 'water is curved. T his curved su rface is called a m en iscus. The volume of the wa te r prese nt is read from the center, or lowest, po rtion of th e men iscus by com paring its height with the calibration mark s on the cylinder. Write d own the volume of wate r presen t and ask your instructor to read the volume. Yo u r reading:

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I nstr uctor 's read ing:

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Empty the gr ad ua te. Notice that some re sidual droplets of 'water ad here to the interior. If no t rem oved, th is residual wate r will d ilut e the solution of bariu m chloride whi ch is nex t to be put into t he gra d uate. These water droplets can be removed by dryin g the gr ad uate or they can be re placed wit h droplets of bariu m chloride solu tion. Ad d abou t 5 m l of bar iu m ch loride solution to the grad uate and twist and turn the graduate , thu s wash ing the in teri or surface ,.. . ith the barium chloride solu t ion. D iscard this liq u id. The adherent droplets now on the walls of the gradua te are approximatel y eq ua l in composition to the bar iu m chloride solut ion in th e stock bottle . (Keep th is trick in mind. I t is qu icker an d easier than d rying the grad uate ; and of course it will also be su itable wi th a beaker, or any ccnta iner.) Now fill the graduate to the 50-ml ma rk with bariu m ch loride solution . Back to the solut ion of po tassiu m ch ro mate in water : Pour about one-fou rth of this solut ion into another 250-m l beaker. Save the remain ing three-fo urths; it will be used later. Slowly, wit h st irring, pour abou t 25 m l of the ba riu m chloride solu tion from the gr ad uate in to th e one-fourth portion of the po tassiu m chromate solut ion. N ote th at the liq u id becomes cloudy. The precipitate which has formed is bar iu m chromate , BaCrO..; it is insoluble, or almost so, in wat er. Some of the particles of precipi tate arc so sma ll that they sett le \-cry slow ly. Let the mixtu re stand with ou t agita tion for abo ut five m in u tes an d note that this is so. Our immediate object is to separate the preCipita te from the liqui d. Set up th e ap para tus for filtering: Fol d a circle o f filter pap er exactly in half . Fold th is half so as (O for m an approxim at e q uart er circle, with the two folded edges form ing an angle of about 10° , as shown in Figure 6. Tear a small triangul ar piece off of the inside corner, ma king sure that the ou ter edge is torn back further than the in ner edge, as illustrated . Open the folde d filter paper in to a con e , mak ing su re tha t the flap with the torn co rn er is on th e o utsi de o f the cone . Place the cone loosely in a fu n nel and mo un t the funnel on a fu n nel su p port with the stem touch ing the in ner wall of a beaker, near the r im . Turn the funnel so that the d iagonally cut end of the fu nnel ste m slan ts downward toward the beaker wall, as illustrated. Fi ll the filter paper cone with di stilled water and pres s the u p per edge of the paper cone gen tly against t he co nica l sides o f the fu nnel. If these d ire ctions are correctly follow ed , a column of water, which promotes rapid filtration, will form in the stem of the fu nn el. Di scard any water which ha s collec ted in the beaker and ask your ins tructor to check you r setu p. His init ials go here :

Pour a few mi llil iters of the clou dy liqu id into th e filter pa per cone. No te that no t all of the pan icles of precipitate ar c re tained on the filt er paper. Som e are small en ough to pass th rough the pores of th e paper. Show your instructor that th is is so. His init ials:

So, to separate the prenpItate from the liq uid it is obvious that we must first remove t hese \'ery small part icles of precipitate before filtering. This is easy to do . Just le t the mixtu re stand for about on e yea r. By then the very sma ll particle s will have slowly di ssolved and rc-prccipitar ed on the larger part icles. T he n we can filter witho u t d ifficult y. 17

HaC I z solu t ion to the su pe rn atant liq u id. If no preCipitate forms, precipi tat ion is in deed com plete. If a precip itate d oes for m, more BaC h m ust be ad ded, follow ed by an other d igestion and test for complete pre cipi tation . \ Vhen you believe pr eci pitat ion to be complete, show your instructo r that the su pern atant liq ui d is colorless and that no addit iona l precipi tate forms when a few d ro ps of BaC h solu tion are added. H is in iti als:

Di scard the or iginal filter pap er , prepare another fi lter pa per cone, and separ ate the pre ci pitate from the liqu id as follows: First, let the m ix ture stand wit hout agitat ion, so that the preci pi tate settles to the bottom of the beaker. Mo st of the su pernatant liqu id can be de canted from, that is, poured off, the to p of the precipi tate and passed th ro ugh the filter paper into the beaker below. To d irect the flow of the liq ui d, touch a st irring ro d to th e beaker spo ut, hol di ng the st irring rod so that its lower end is within the conical space of the fun nel. If the st irring rod is held in this positio n, all the liq ui d will flow as desired ; even the last drop, which otherwi se would ru n down the outside of the beaker, will go dow n the stirring rod into the funnel. During the decantation, try to m inimize any transfer of prec ip ita te to the filter pape r. If a large q uanti ty of precipitate iuad vcrtcn tlv gets on to the filt er papcr, no har m will resu lt, but the t ime req u ired for filt ra tion will be un necessar ily lengthened. \ Vhen mo st of the su pernatant liq ui d has been decanted, t ilt the beaker until it is almost inverted , and most of the precipitate will slide out on to the filter paper. At all tim es, exercise care to in sure that the level of the liq uid in the fun nel remains below the top edge of the filter paper cone and tha t no droplets of liq uid r un down the outside of th e beaker. \Vh en almost all of the su pernatant liq u id has pa ssed th rough t he filter paper and m ost of the precipitate has been transferred to the filter paper, rem ove the collectin g beaker below the funnel and replace it with a test tu be . T he liq ui d in the collec ti ng beaker, an d the liq u id which will ru n in to the test t u be, is called the " filtrate." Make sure that a litt le filtra te has collected in the test t u be and th en rem ove the test tube, re placi ng it with the origin al beaker. Put approximately 5 m l of silver nitrate solution in another test t u be, est ima ting the volume for practice. Using your medic in e dropper , add two drops of th is silver nitrate solut ion to th e filtrate in th e test tube . Do not immerse the dropper in the filtra te. T hi s wou ld contam inate the d ropper. Hold it well abov e the liq u id , j ust insi de the mo uth of the test tube, and force o ut the two d rops. A pre ci pitat e of silver chloride will form. T he filtrate contains potassium chlor ide, KC I, wh ich reacts wi th the AgN 0 3 to form a white, inso luble su bstance, AgC l. Notice tha t you were no t directed to take the silver nitrate solutio n di rectly fro m th e stock bou le wit h yo ur medi ci ne d ro p per . H ad you done so, the stock solu tion would have been contam inated if your dropp er was not thoroughly clean. H ence a r ule :

NEVER insert any object into a bottle of liquid or solid unless ,'our instructor first makes sure that the object to be inserted is truly clean and free from contamination. 19

N o w a few part icles of barium chromate still remain in the beak er. Also, not all of

the supernatant liqu id has yet passed th ro ugh t he filter pap er. A small q uantity, wh ich conta ins KCl in solu tion, is held u p by the HaCrO l in the fil ter papcr cone . Since we wish to obtai n pure barium chromate, thi s potassium chloride m ust be removed, an d at the same time. we can tra nsfer the residual preci pita te from the beaker to the filter paper. Using you r wash bottle , gently squ irt a little d istill ed water in to the bea ker , wh ile ho lding the beaker tilted so that th e wash water. and residu al precip itate, run out int o th e filter pa per cone. Using a m in imum amoun t of wa ter, wash the en tire interior surface of the beaker with t he stream from your wash bott le, thus washi ng all of t he prec ip itate out. If a few particles stick to the beake r walls. set th e beaker dow n and loosen them wit h a pol ice. man affixed to the e nd of a stirring rod. \ Vash any part icles, as necessary , off the policeman. Make su re not to let the water in th e filter pap er cone overflow in your zeal to get that last littl e ch unk of stu bborn barium chromate. \\' hen most of the wash water has passed throu gh the filter pa per. collect the last few drops in a d ean test tube and test th is liq u id wit h silver ni tra te for the presence of ch loride. Additional AgCl will probab ly precipi tate. Repeat the washin g of th e BaCr04 (since the beaker is cleaned out. direct the wash water direc tly upon the precipitate) test ing for chloride as before. Repeat as necessary until the precipitate is washed free of chlor ide, as determ ined by the results of th e test on the filtered wash wate r. Show you r inst ructor that the precipi ta te is washed free of chlor ide . His ini tials:

It is reasonab le to assume that if th e barium chromate is free of potassiu m ch lor ide, it is also free of all other solu ble contaminant s wh ich migh t have been present or igina lly. Since there ar e no likely insolu ble contam inants, it may be assum ed th at the bar ium chromate is reason abl y pure, except for the wate r that is still present.

Let us now turn our atten tion to the remai n ing three-fourths of the origin al potassium chromate solution. If the water is evaporated from th is solution , the origi nal salt will re ma in. In genera l, th ere are two ways to evaporate a solven t (water in this case) from a solut ion. \Ve can evap ora te ra pidly or slowly. If we evapora te rapidly, some of the K 2 Cr0 4 will be lost: slow evaporation does not result in loss of the solu te (K 2Cr04 in th is case). T o demonstrate that rapid evaporat ion causes loss of solute, pour about half of th e remai ni ng K2 Cr0 4 solution into an evapora ting dish and heat it. You will note that even though care is taken to hea t it gently. as the q uan tity of water remain ing becomes less and less, the solution spu tters and bubbles, Slow evapo ration. which you should no w attemp t, is more satisfactory. Pour all of the rem aining potassium chro mate solution in to an evapo rating dish and place this d ish on a bea ker which is about half filled with water . The beaker sho uld be small enough so tha t the evaporat ing d ish is supported on the r im of the beaker : the evapora ti ng dish shou ld 20

no t be inside t he beaker, Gently bo il the water in th e beaker . reple nishing it from t ime to t ime , u nt il all th e water in the K 2 C rO ~ solut ion has. by slow evaporat ion , been driven off. Note that there is no loss of solu te in th is slower eva pora t ion process . If you wish. some or all of the filtrate (\v·h ich contains KC I) may be evaporated by the slow or fast p rocess. or a com binat ion of the two-fast evaporation to remove most of the solvent. t hen slow evaporation wh en it becom es likely that loss of material will occur if the fast evaporat ion is co ntin ued .

T h is has bee n a long experiment : what have you learned ? I.

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Your instructor's init ials here' 22

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