CHMB16 Chapter 3

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Published on 27 Sep 2011
School
UTSC
Department
Chemistry
Course
CHMB16H3
Professor
Page:
of 4
Chapter 2: Tools of the Trade
Safe, Ethical Handling of Chemicals and Waste
Wear goggles or safety glasses with side shields at all times in the lab to
protect your eyes from liquids and glass, which fly around when least
expected.
Do not wear contact lenses because vapors can be trapped between the lens
and the eye.
Handle organic solvents, concentrated acids and concentrated ammonia
inside a fumehood.
Wear a respirator when handling fine powders.
Treat spills on your skin first by flooding with water.
Label all vessels to indicate what they contain.
Green Chemistry: a set of principles intended to change our behavior to help
sustain a habitable planet; seeks to design chemical products and processes
to reduce the use of resources and energy and the generation of hazardous
waste.
The Lab Notebook
The lab notebook must state what was done, what was observed and be
understandable to someone else.
Use complete sentences to prevent incomplete descriptions.
Paste hard copies of important data into your notebook.
Each page in your notebook should be signed and dated.
Analytical Balance
Electronic Balance: uses electromagnetic force compensation to balance the
load on the pan.
Readability: the smallest increment of mass that can be indicated.
Microbalance: weighs a milligram quantity with a readability of 1-microgram.
Tare: the mass of the empty vessel.
Chemicals should never be placed directly on the weighing pan.
Hygroscopic Reagents: reagents that rapidly absorb moisture from the air.
Weighing by Difference: first weigh a capped bottle containing dry reagent,
then quickly pour some reagent from the weighing bottle into a received, cap
the weighing bottle and weigh it again and the difference is the mass of
reagent delivered from the weighing bottle.
Mechanical Balance: it has two pans suspended on opposite ends of an equal-
arm lever balanced at its center on a knife-edge.
oIt should be in its arrested position when you load or unload the pan
and in the half-arrested position when you are dialing weights.
Samples must be at ambient temperature to prevent errors due to convective
air currents.
oWarm samples might appear lighter than their actual mass.
oCool samples might appear heavier than their actual mass.
oThe warmth of your hand and your fingerprints can affect the apparent
mass of an object so tweezers are recommended.
Analytical balances should be located on a heavy table to minimize vibrations.
Tolerances: allowable deviations from the standard.
Linearity Error: the maximum error that can occur as a result of nonlinear
response of the system to added mass after the balance has been calibrated.
Buoyancy: the upward force exerted on an object in a liquid or gaseous fluid.
oTrue mass is the mass measured in vacuum.
oAn object weighed in air is lighter than its actual mass.
oBuoyancy Equation: m = m’(1-da/dw) / (1-da/d), where m’ is the mass
read on a balance, m is the true mass, da is the density of air (0.0012
g/mL at 1 bar and 25°C), dw is the density of the calibration weights
(8.0 g/mL) and d is the density of the object being weighed.
Burettes
Burette: a precisely manufactured glass tube with graduations enabling you to
measure the volume of liquid delivered through the stopcock at the bottom
(0-mL mark is near the top).
oWhen reading the liquid level in a burette, your eye should be at the
same height as the top of the liquid.
oParallax: the error that occurs when your eye is not at the same height
as the liquid.
Operating a burette
oWash burette with new solution.
oEliminate air bubbles before use.
oDrain liquid slowly.
oDeliver fraction of a drop near end point.
oRead bottom of concave meniscus.
oEstimate reading to 1/10 of a division.
oAvoid parallax.
oAccount for graduation thickness in readings.
For best precision, measure the mass of reagent, instead of volume, delivered
from a burette or syringe.
Volumetric Flasks
Volumetric Flask: calibrated to contain a particular volume of solution at 20°C
when the bottom of the meniscus is adjusted to the center of the mark on the
neck of the flask.
oTo use a volumetric flask, dissolve the desired mass of reagent in the
flask by swirling with less than the final volume of liquid then add more
liquid and swirl the solution again.
oAdjust the final volume with as much well mixed liquid in the flask as
possible.
oHold the cap firmly in place and invert the flask several times to
complete mixing.
Adsorption: the process in which a substance sticks to a surface.
Absorption: the process in which a substance is taken inside another.
Acid wash glassware to replace low concentrations of cation on the surface
with H+ by soaking already cleaned glassware in 3-6M HCl or HNO3 for > 1hr
then rinse it well with distilled water and finally soak it in distilled water.
Pipettes and Syringes
Pipettes: deliver known volumes of liquid.
oTransfer pipette is calibrated to deliver one fixed volume (more
accurate).
oMeasuring pipette is calibrated like a burette and is used to deliver
variable volume.
oThe last drop does not drain out of the pipette and should not be blown
out.
To drain out the liquid from a pipette, touch the tip of the pipette to the side
of a beaker.
Micropipettes: deliver volumes of 1 to 1000-microliters.
oAerosol: a suspension of fine liquid droplets or solid particles in the gas
phase.
oUse tip recommended by manufacturer, other tips might make
inadequate seal.
oTake up and expel liquid three times before delivery to wet pipette tip
and equilibrate the inside with vapor.
oUnnecessary wiping of the tip can cause loss of sample.
oLiquid must be at same temperature as pipette; less than the indicated
volume of cold liquid is delivered and more than the indicated volume
of warm liquid is delivered (errors are greatest for smallest volumes).
oMicropipettes are calibrated at sea level pressure; they are out of
calibration at higher elevations so you can calibrate your pipette at
your elevation (errors are greatest for smallest volumes).
oReverse Mode: the plunger is depressed beyond the first stop and
excess liquid is taken in and depress the plunger to the first stop and
not beyond (good for foamy and viscous liquids).
Syringes: come in sizes from 1 to 500-microliters and have an accuracy and
precision near 1% (take up and discard several volumes of liquid to wash the
glass walls and to remove air bubbles from the barrel) more reliable than a
micropipette but requires more care in handling and cleaning.
Filtration
Gravimetric Analysis: the mass of product from a reaction is measured to
determine how much unknown was present and precipitates from the
gravimetric analyses are collected by filtration, washed and then dried.
Mother Liquor: liquid from which a substance precipitates or crystallizes.
Filtrate: liquid that passes through the filter.
Ignition: used to convert a precipitate into a known, constant composition.
Ashless Filter Paper: when a precipitate is to be ignited, this is used to collect
it because it leaves little residue when burned.
Slurry: suspension of solid in liquid (pour the slurry of precipitate down a glass
rod to prevent splattering).
Rubber Policeman: a flattened piece of rubber at the end of a glass rod.
Drying
Reagents, precipitates and glassware are conveniently dried in an oven at
110°C.
Anything that you put in the oven should be labeled and cover all vessels
whenever possible so that dust will not contaminate it.
Desiccator: a closed chamber containing a drying agent called a desiccant.
oThe lid is greased to make an airtight seal and desiccant is placed in
the bottom beneath the perforated disk.
oAfter placing a hot object in the desiccator, leave the lid cracked open
for a minute until the object has cooled slightly.
Good Drying Agents: magnesium perchlorate, barium oxide, calcium sulfate
and silica gel.
Calibration of Volumetric Glassware
Calibration: the process of measuring the actual quantity that corresponds to
an indicated quantity on the scale of an instrument.
To calibrate volume, measure the mass of water contained or delivered by the
vessel and using the density of water to convert mass into volume.

Document Summary

Treat spills on your skin first by flooding with water. The lab notebook must state what was done, what was observed and be understandable to someone else: use complete sentences to prevent incomplete descriptions. Paste hard copies of important data into your notebook. Each page in your notebook should be signed and dated. Electronic balance: uses electromagnetic force compensation to balance the load on the pan. It should be in its arrested position when you load or unload the pan and in the half-arrested position when you are dialing weights. For best precision, measure the mass of reagent, instead of volume, delivered from a burette or syringe. Ignition: used to convert a precipitate into a known, constant composition. it because it leaves little residue when burned. Slurry: suspension of solid in liquid (pour the slurry of precipitate down a glass rod to prevent splattering): rubber policeman: a flattened piece of rubber at the end of a glass rod.