Question

CHEM 101 LAB 7 PRELAB QUESTIONS B C AND D

Experiment 7 Preparation of Alum PURPOSE oF EXPERIMENT: Prepare KAI(So,)2 12H20, an aluminum alum, from aluminum metal. (Note: ExPERIMENT 8 describes the gravimetric analysis of the sulfate content in alum.) Alums have the general formula MM(So) 12HO, where M* is commonly Na". K', Tl., NH,., or Ag, and Mt''is A广, Fe". Cr". Ti". or Co. . Samples of these compounds have a variety of uses in our everyday lives. Sodium aluminum alum, Nalso212H20, is used in baking powder. When water is added to baking powder, a mixture of Na,CO, and aluminum alum, the aluminum ion reacts with H,0 to form H', which in turn combines with HCO, to form H,CO,. Eventually, CO, is released which makes the cake "rise". Potassium aluminum alum, KAI(SO4 12H20, is used in water purification, sewage treatment, and fire extinguishers. Ammonium aluminunm sulfate, NH4Al(SO4)2 12H2O, is used in pickling cucumbers, whereas chrome alum, KCr(SO4)2.12H20, is used in tanning leather, that is, converting hides into leather for shoes, wallets, and other leather goods. In this experiment you will prepare potassium aluminum sulfate dodecahydrate, KAI(SO4)2.1 2H20, a double salt, starting with aluminum metal, Aluminum metal reacts rapidly with a hot aqueous solution of potassium hydroxide, KOH, to produce a soluble potassium aluminate salt. 2 AI(s)+2 K'(aq)+2 OH (aq)+10 H,O(0)- 2 K' (aq)+ 2 [Al(H2O), (ОН),]' (aq)+ 3 H, (g) When this salt is reacted with sulfuric acid, H,SO,, aluminum hydroxide, Al(H,0), (OH), , precipitates 2 K. (aq) + 2 [Al (H2O), (OHAT (aq)+ 2 H' (aq) + so,"(aq)- 2 K. (aq)-SO,'" (aq) +2 Al(H2O), (OH), (s) and then dissolves as additional acid is added. 133 EXPERIMENT 7 eressore/greater # of MOL 2 Al(H2O), (ОН), (s) +6 H' (aq) +3 S042-(aq)- 2 Al (a4)+3 SO, (aq)+12 H,O(U) (Stu Since the resulting solution contains K, Al, and SO,potassium aluminum sulfate precipitates as octahedrally shaped crystals when a nearly saturated solution is cooled to 0°C. pap 7.1 25 1、 What is the precipitate which forms and then redissolves upon adding H2S), to the mixture of K' , [Al (H2O), (OHAT and OH' ? when svl fvvic acid is ndded,itnevtralies simeofthe hydva xide tons (oH) causing the evalibrivm between AtloH) and Al(oH), (s) t shifttwards the precipitate a. Al (애), which form Aluminum hydroxide-precipita t formed is disselvesh f alvm' n v m b. Calculate the volume of 6.0 M sulfuric acid H,SO,, solution required to p tassim SvIfate neutralize the excess potassium hydroxide, KOH, solution from the first step of the synthesis. Indicate clearly your calculations and reasoning. Calculate the theoretical yield of the precipitate in a. Assume that you started with 1.00 g of Al(s) and could produce and collect the precipitate quantitatively. Indicate clearly your calculations and reasoning. c. d. Calculate the percent yield if you actually recovered 2.64 g of dried precipitate. 139 EXPERIMENT 7

Answer 1