102D FA18 wk02 doc1

102D week 2, doc 1 Section Day, Time: Team: Manager: Blogger: Scribe: Researcher: Units of Concentration ~ Changing units for concentration’s sake. ~ Recall from CHEM101 that molarity is a unit of concentration expressed in units of mole/L or M. The volume of a liquid is affected by temperature. For most liquids, as temperature increases, the volume increases. This increase in volume will also change the concentration (in units of M) of a solution (think about the units). We can use other units to express concentration that are not temperature dependent. Molality What is the key difference between molality and molarity? How would measuring concentration in units of m prevent the problems associated with changes in temperature? If you wanted to convert from molarity to molality, what information would you need? What is the molality of a solution prepared by dissolving 4.26 g of KBr in 348.0 g of water? (Please number and put your answer in the top righthand corner of the whiteboard so we may check it.) Besides molality, other units that are not temperature dependent include the following: Name Definition Units Percentage by mass (w/w%) % Parts per million ppm Parts per billion ppb DID YOU KNOW? Potassium bromide is an antiepileptic drug used in veterinary medicine. It is used to treat seizures in dogs. Unfortunately, it can take several months to be effective. APPLY YOUR KNOWLEDGE Which temperature independent unit of concentration would be most useful in stoichiometric calculations? Why? Suppose you have a 0.938 mM aqueous acetic acid (CH3COOH) solution. This is a dilute aqueous solution and its density is nearly equal to that of water. Express its concentration as Molality %(w/w) ppm ppb A solution of ethanol (CH3CH2OH) dissolved in water is marked 70.% (w/w). Convert this concentration to units of molarity when the solution is chilled to 15?C. (Helpful datum: density of 70% (w/w) ethanol-water mixture is 0.87184 g/cm3 at 15C.) Note: If you reach this point, go on to doc2, coming back later if there is time. You dissolve 6.38 g benzene (a liquid) in 345.0 mL toluene (a liquid). Express the concentration of benzene in units of: Molarity Molality %(w/w) (Helpful data: benzene, C6H6, density 0.8765 g/ml at 20C; toluene, C7H8, density 0.8669 g/ml at 20C) Can you think of another way to express the concentration of one liquid dissolved in another? A jug of concentrated sulfuric acid is labeled both 98% (w/w) H2SO4 and 18.0 M H2SO4. What is the density of the contents? What is the molality of the contents? What mass of this solution would you combine with 1.5 kg water to prepare a dilution that is 0.2 m aqueous H2SO4? Tiamulin (C28H47NO4S) is a water-soluble antibiotic used to treat swine dysentery. A veterinarian has prepared a 2.0% (w/w) solution. How much medicine should she give to Wilbur the pot-bellied pig for a dosage of 150 ppm, if Wilbur weighs 54 kg? What is the molality of the dosing solution? What is the molality of tiamulin inside Wilbur after he receives the dose? 4