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chemistry practicals work flow,procedures and calculations
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The document deal with workings on procedure,titrations, and calculations in chemistry practicals
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PRACTICAL CHEMISTRYIntroduction
Scientific subjects are, by their nature, experimental. It is accordingly important that an
assessment of a student’s knowledge and understanding of Chemistry should contain a
component relating to practical work and experimental skills. This booklet has been
produced to help students preparing for and taking practical Examinations. The material
contained in this booklet does not extend the curriculum specification content. Rather it
seeks to help the candidate succeed in practical examination by explaining in more depth
what is required of him or her in carrying out the exercises, making observations and
measurements with appropriate precision and recording these methodically. This booklet
advices candidate on how he or she should interpret, explain, evaluate and communicate the
results of the exercises clearly and logically using relevant chemical knowledge and
understanding and using appropriate specialist vocabulary.
1.1APPARATUS IN A CHEMISTRY LABORATORY
1.2 SPECIAL LABORATORY APPARATUS AND TECHNIQUES
(a) USING THE BURETTE
,A burette is used to deliver solution in precisely measured, variable volumes. Burettes are
used primarily for titration, to deliver one reactant until the precise end point of the
reaction is reached. To fill a burette, close the stopcock at the bottom and use a funnel. You
may need to lift up on the funnel slightly, to allow the solution to flow in freely.
You can also fill a burette using a disposable transfer pipette. This works better than a
funnel for the small, 10 mL burettes. Be sure the transfer pipette is dry or conditioned with
the titrant, so the concentration of solution will not be changed.
Before titrating, condition the burette with titrant solution and check that the burette is
flowing freely. To condition a piece of glassware, rinse it so that all surfaces are coated
with solution, then drain. Conditioning two or three times will insure that a stray drop of
water does not change the concentration of titrant.
Rinse the tip of the burette with water from a wash bottle and dry it carefully. After a
minute, check for solution on the tip to see if your burette is leaking. The tip should be
clean and dry before you take an initial volume reading.
When your burette is conditioned and filled, with no air bubbles or leaks, take an initial
volume reading. A burette-reading card with a black rectangle can help you to take a more
accurate reading. Read the bottom of the meniscus. Be sure your eye is at the level of
meniscus, not above or below. Reading from an angle, rather than straight on, results in a
parallax error.
Deliver solution to the titration flask by turning the stopcock. The solution should be
delivered quickly until a couple of mL from the endpoint. The endpoint should be approached
slowly, a drop at a time. Use a wash bottle to rinse the tip of the burette and the sides of
the flask. Your TA can show you how to deliver a partial drop of solution, when near the
endpoint.
(b) Volumetric (measuring) Flasks
,Erlenmeyer flasks and beakers are used for mixing, transporting, and reacting, but not for
accurate measurements. The volumes stamped on the sides are approximate and accurate to
within about 5%.
(c) Graduated Cylinders
Graduated cylinders are useful for measuring liquid volumes to within about 1%. They are
for general-purpose use, but not for quantitative analysis.
If greater accuracy is needed, use a pipette or volumetric flask.
(d) Pipette
A pipette is used to measure small amounts of solution very accurately. A pipette bulb is
used to draw solution into the pipette.
, Start by squeezing the bulb in your preferred hand. Then place the bulb on the flat end of
the pipette.
Place the tip of the pipette in the solution and release your grip on the bulb to pull solution
into the pipette. Draw solution in above the mark on the neck of the pipette. If the volume
of the pipette is larger than the volume of the pipette bulb, you may need to remove the
bulb from the pipette and squeeze it and replace it on the pipette a second time, to fill the
pipette volume completely.
Quickly, remove the pipette bulb and put your index finger on the end of the pipette. Gently
release the seal made by your finger until the level of the solution meniscus exactly lines up
with the mark on the pipette. Practice this with water until you are able to use the pipette
and bulb consistently and accurately.
(e) Volumetric Flask
A volumetric flask is used to make up a solution of fixed volume very accurately. This
volumetric flask measures 500 mL ± 0.2 mL. This is a relative uncertainty of 4 x 10 -4 or 400
parts per million.
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