Class Notes (839,469)
Canada (511,354)
Chemistry (266)
ENCH 213 (25)
Lecture

3concentrations.pdf

4 Pages
62 Views

Department
Chemistry
Course Code
ENCH 213
Professor
Diane Beauchemin

This preview shows page 1. Sign up to view the full 4 pages of the document.
Description
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
More Less
Unlock Document

Only page 1 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit