Solutions. Definitions pertaining to solutions

Transcription

1 Slutis Defiitis pertaiig t slutis Slute is the substace that is disslved. It is usually preset i the smaller amut. Slvet is the substace that des the disslvig. It is usually preset i the larger amut. Slubility is the limit t which smethig disslves. It is the maximum amut f slute fr a give amut f slvet. A saturated sluti is e that is at the slubility limit. A usaturated sluti is e that is less tha saturated. Ccetrati Uits Mlarity Mlarity (M) is defied as the rati f the umber f mles f slute t the ttal vlume f the sluti i liters. M mles f slute Liters f sluti Percet uits There are three percet uits that are cmmly used, percet by mass, percet by vlume ad percet mass-vlume. Percet by mass is defied as the rati f the mass f the slute t the mass f the sluti multiplied by 100. Percet by vlume is defied as the rati f the vlume f the slute t the vlume f the sluti multiplied by 100. Percet mass-vlume is defied as the rati f the grams f slute t milliliters f sluti multiplied by 100. mass f slute %( m / m) mass f sluti vlume f slute %( v / v) vlume f sluti grams f slute %( m / v) ml f sluti Related t the percet uits are the uits parts per milli, parts per billi ad parts per trilli. These are calculated the same way as the percet uits (part per hudred) except we multiply by a milli, billi r trilli. Mle fracti

2 Mle fracti (X) is defied as the rati f the mles f slute t the ttal umber f mles i the sluti. i X i T Mlality Mlality (m) is defied as the rati f the umber f mles f slute t the mass f the slvet i kilgrams. mles f slute m kg f slvet Yu shuld uderstad hw t cvert betwee these varius ccetrati uits. The sluti prcess Mlecular slutes A mlecular substace will disslve i a particular slvet depedig the types f itermlecular iteractis preset i the slute ad the slvet. If the slute ad slvet have similar iteractis they will ted t disslve i e ather. If they d t they will t disslve. Ethal 3 2 O H has a structure similar t that f water. It als has similar itermlecular frces. Ethal will disslve i water i ay prprti. Hwever, bezee HC H C HC ly has Ld frces. It des t disslve i water. The reas fr this that is fte give is that there are attractis betwee the bezee mlecules ad the water mlecules. This is t true. There are sigificat attractis betwee the tw mlecules. What makes bezee isluble i water is that the attractis betwee idividual water mlecules are far strger tha the attractis betwee bezee mlecules ad water mlecules.

3 Iic slutes All iic slids disslve i water t sme extet. The iic slids that are geerally csidered t be isluble are ly very slightly sluble (less tha 0.01 g f slute per 100 ml f sluti). What determies if a iic slid is very sluble r ly very slightly sluble? There are tw cmpetig iteractis that determie this. The first is the stregth f the iic bd hldig the is tgether. The secd is the stregth f the i-diple iteracti betwee the is ad the water. If the iic bd is strger the substace is ly slightly sluble i water. If the i-diple iteracti is strger the substace is mre sluble i water. Is with higher charges will ted t have strger iic bds. Smaller is will ted t have larger i-diple iteractis. Thigs that ca affect the slubility f a cmpud Temperature Fr slid slutes, i geeral, as we icrease the temperature the slubility f the cmpud icreases. There are a few exceptis t this (such as Ca 3 (SiO 4 ) 2 ). The reas fr this tred is that as the temperature is icrease the kietic eergy f the slute particles icreases which, i geeral, makes it easier fr them t eter it sluti. Fr gaseus slutes the ppsite is true. As the temperature is icreases, gases becme less sluble. This is because f the icrease i kietic eergy f the gas particles. With mre kietic eergy they are mre likely t leave the sluti. Pressure Pressure des t affect the slubility f slid slutes t ay great extet. Pressure des affect the slubility f gases thugh. If we icrease the pressure f a gas abve a liquid, the gas will becme mre sluble i that liquid. We must icrease the partial pressure f the gas. Icreasig the ttal pressure by addig ather gas will t icrease the slubility f the rigial gas. Clligative Prperties Vapr pressure f a sluti The vapr pressure f a sluti ctaiig a -vlatile -electrlyte slute is prprtial t the mle fracti f the slvet ad the vapr pressure f the pure slvet: P X sl' slv P slv Because the mle fracti f the slvet i a sluti is always less tha e, we see that the vapr pressure f a sluti is always less tha the vapr pressure f the pure slvet.

4 Ay sluti that beys this relatiship is called a ideal sluti. Mst slutis d t exactly bey this relatiship, but all dilute slutis (X slv >0.90) ca be csidered ideal. A sluti may als ctai a vlatile slute, fr example ethal i water. I this case the vapr pressure f the sluti is a cmbiati f the vapr pressure f the slute ad the slvet. We must calculate the ctributi f each with the abve equati ad add them tgether t get the vapr pressure f the sluti. Fr a ideal sluti with tw vlatile cmpets (A ad B), the vapr pressure is give by: P X P + X sl' A A B P B This lwerig f the vapr pressure f a sluti has implicatis als fr the sluti s meltig ad bilig pits. Bilig pit elevati ad freezig pit depressi Whe a slute is added t a slvet the vapr pressure f the sluti decreases. This is the same as lwerig the bilig pit curve the phase diagram. Whe the bilig pit curve lwers the meltig pit curve shifts t the left. This implies that the bilig pit has icreased ad the meltig pit has decreased (see phase diagram). The amut f the icrease r decrease ca be calculated if we kw the mlality f the sluti. This calculati usig mlality is a apprximati that wrks fr dilute

5 slutis. A mre accurate value ca be btaied by usig the mle fracti f the sluti. The amut by which a sluti s bilig pit icreases is give by the frmula: T b K b mi T b is the chage i the bilig pit, K b is the bilig pit elevati cstat ad depeds ly the slvet used, m is the mlality f the sluti, ad i is a value called the va t Hff factr. The va t Hff factr is a value that shws hw may is cme frm each frmula uit. Fr mlecular slutes i equals1. A similar frmula gives the depressi f the freezig pit: T f K f mi Here K f is the freezig pit depressi cstat ad, agai, depeds ly the slvet. Osmtic Pressure Osmsis is the prcess by which a slvet flws thrugh a semi-permeable membrae t equalize the ccetrati bth sides f the membrae. A semi-permeable membrae is a membrae that will allw sme mlecules, such as slvet mlecules, t pass thrugh, but t thers. We ca stp the smtic flw thrugh the membrae by applyig pressure t cuteract the flw. The amut f pressure we eed t stp the smsis is called the smtic pressure ad is give by: Π MRT This equati is related t the Ideal Gas Law. Π is the smtic pressure, M is the mlarity f the sluti, R is the ideal gas cstat i pressure-vlume uits (L atm ml -1 K -1 fr example), ad T is the Kelvi temperature. Because mlarity is the same as mles ver vlume the Osmtic pressure equati ca be rewritte as: Π V RT RT V This is the same as the Ideal Gas Law with Π beig pressure. Applicatis f Clligative Prperties The clligative prperties f a sluti ca be used t calculate the mlar mass f a slute. They ca als be used t calculate the percet dissciati f a weak electrlyte. Examples f these prblems are i yur textbk.