Raoult's Law: A General Chemistry Experiment Margaret J. Steffel The Ohio State University Marion Campus, Marion, OH 43302 General chemistry textbooks that present Raoult's Law and discuss deviations from it do so to varying extents. Some mention deviations without citing examples; some show graphs of vapor pressure versus solution composition for hypothetical materials; some show graphs for specific comnounds and eive the comnounds'names but not their formulas; %,mearequite thorough ( 1 ) . Tu m ~ k this e material more meanincful to s~udents.and therefore more easily applied . u ~ d appreciated later in organic and physical chemistry courses, we have them carry out a series of exercises on paper, in the laboratory, and with molecular models. The paper exercise gives them practice using Raoult's Law and convinces them of the validity of graphical addition. The laboratory work shows them the hehavior of real solutions. Finally, the work with mm~lecularmodels helps them t u rwogni7c the relationshin hetween un,uerties oi indi\,idual nioleculcs and of matter in bulk. ~ h L l a t t e ris an important purpose of the exercises-to reinforce this most fundamental principle of chemistry.
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Paper Exercise The paper exercise is like problems given in many textbooks. Students are told that a solution of benzene and toluene behaves ideally, i.e., as described by Raoult's Law: P,, = XAp*O (1 - X A ) P ~They ~ . are told the vapor pressures of benzene and toluene a t room temperature, and they use Raoult's Law to calculate partial pressure of benzene, partial pressure of toluene, and total vapor pressure over solutions of benzene and toluene a t mole fractions of benzene equal to 0.25,0.50, and 0.75; they present these data graphically (including points for Xbenzene= 0.00 and 1.00).
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Choice ol Liquids The pain of liquids most commonly cited in textbooks' discussions of Raoult's Law are benzene and toluene (ideal behavior), acetone and carbon disulfide (positive deviation), and acetone and chloroform (negative deviation) (2). These examples are g o d for classroom discussion; thvn, for studenh to have H senw ofexperimtmtation, other comhinations s h o ~ ~ l d he inwatigated in the lahtmu~ory.i.41~0,we do not use benzene, carhon diaulfide, ur chlordwm in o.lr grneral chemistry (ahoratories because of their potential toxicity/can.inogenicity.) A restriction was that we wanted to use liquids for which deviations result from oh\.~ical effects rather than frtm chemical " changes (3).We chose acetone, hutanone, dichloromethane, ethvlacetate. heotane. h e m e , and methanol. These seven 19 ifwhich are pairs of miscible liq;ids compkse'21 liauids: methanol is not miscible with heptane or hexane and thkse pairs are not used. The behaviors of these pairs were found to he as follows:
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No deviation: acetnne-butanone,acetone-ethyl acetate, butanoneethyl acetate, and heptane-hexane. Positive deviation: acetone-heptane, acetone-hexane, acetonemethanol, butanone-heptane, butanane-hexane, butanonemethanol, dichloromethane-heptane, dichlorometbane-helrane, dichloromethane-methanol.ethvl acetate-hevtane,ethyl acetateher in*. and cil,,I acerarr -nwthanol. Segs~ivedeviamm: acetonc-
