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4 Pages

Course Code
ENCH 213
Diane Beauchemin

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Chemical Concentrations and Preparing Solutions summary: • Molarity • Molality • Percent compositions • Other expressions of concentrations • How to prepare a solution • Correction for thermal expansion • How to calibrate volumetric glassware • Significant figures in arithmetic operations Chemical Concentration • Amount of analyte (solute), n = CV ◦ n = number of moles of analyte (mol) ◦ C = concentration of analyte (mol/L or M) ◦ V = volume of the solution (L) • In dilutions: ◦ n is constant ◦ C V1= 1 V 2 2 ▪ 1 denotes original solution ▪ 2 denotes diluted solution Expressions of Concentration molesof species • Equilibrium molarity (M): → eqm molarity of HCl in 2.0M sltn = 0M litresof solution used as molesof solute massof solute • Formal concentration (F): litresof solution = FM xvolume ◦ independent of analyte composition in solution ▪ (FM = formula mass = molecular mass) molesof species • Not to be confused with molality (m): kg of solvent → Molality is independent of temperature (based on weight) → Molarity changes with temperature Effect of Temperature • Water expands by 0.02%/degree nearo20°C ◦ Molarity changes by -0.02 %/ C ◦ concentration decreases as temperature increases • Pyrex and other borosilicate glasses expandby 0.0010%/degree near room temperature. ◦ for most work, this expansion in insignificant • Assume that the thermal expansion of a dilute solution is equal to that of pure water Correction for thermal expansion of solutions C '= C • Acorrection is possible because C is proportional to density: d ' d • C and d at original temperature • C' and d' at new temperature • See Table 2-7 for density of water at different temperatures Example: correcting concentration for a change in T A0.03146 M solution was prepared when the temperature was 17°C. What is the molarity of this solution when the temperature is 25°C? C ' C C ' at25 C 0.03146M d '= d → 0.99605g/ml =0.99878g/ml → C' = 0.03141M → The concentration would decrease by 0.16% on the warm day What is wrong with this demonstation of how to dilute a solution? Transfer pipets • Calibrated to deliver the most accurate fixed volume • Relatively difficult to use compared to displacement pipets (i.e., micropipets) • Have to be cleaned Displacement pipets and syringes • Easier to use than transfer pipets • Require more frequent calibration than transfer pipets ◦ piston and spring need to be replaced regularly • Less accurate than transfer pipets (1-2% uncertainty) Volumetric flask o • calibrated to contain a volume of water at 20 C when bottom of meniscus at centre of calibration mark Calibration of a pipet • Weigh an empty weighing bottle. • Pipet water with a pipet into the weighing bottle. • Note the lab temperature. • Use Table 2-7 to convert the mass into a true volume. Example: calibration of a pipet If the mass of the weighing bottle is 10.313 g and it weighs 35.225 g with the pipetted water when the temperature is 27°C, what volume was delivered by the pipet? mass of water = 35.225-10.313 = 24.912 g volume = mass * volume of 1 g of water at 27°C volume = 24.912g * 1.0046mo/g
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