Exam (elaborations)
CHEM 123 - Lab final - UBC (1)
CHEM 123 - Lab final - UBC (1)
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why do we perform iodometric titrations - ✅✅ -to find the concentration of anunknown by mixing it with a solution of known concentration
What are the 4 (general) steps of an iodometric titration? - ✅✅-combine solutions,
reach endpoint, calculate, determine the concentration of the unknown
What reaction takes place during an iodometric titration? - ✅✅-oxidation-reduction
reaction
what is used as the oxidizing agent in an iodometric titration? - ✅✅-potassium
iodate
top-loading balance - ✅✅ -- used for prepwork and obtains a mass to the nearest
+/-0.1g (uncertainty of +/- 0.05g)
- used prior to analytical balance
analytical balance -✅✅ -- used to determine a precise mass to the nearest +/-
0.0001 g
- masses are always reported to 4 decimal places
✅✅
does an analytical balance or top-loading balance need to be recorded to 4 decimal
places? - -analytical balance
when is a reading considered stable? - ✅✅-if it persists for 2 seconds
what temperature should objects be weighed? - ✅✅-at room temperature
should objects be dry or wet when weighed? - ✅✅-Dry
should you use the same balance for all measurements? - ✅✅-yes
Describe the steps for weighing by difference - ✅✅-- place clean empty weigh boat
on top loading balance
- tare top loading balance & weigh boat to 0.00g
- add approximate mass (+/- 20%) of chemical to weigh boat
- close all doors on analytical balance and tare it to 0.0000g
- weigh the sample & weigh boat and record the value (1)
- transfer weighed sample into beaker (from weigh boat)
- weigh the emptied weigh boat and record the value (2)
Subtract value 2 from value 1 to get the mass of the chemical
how do we know if something is a standard solution? - ✅✅-if it has a known
concentration to at least 4 significant figures)
, ✅✅
describe the steps to take when preparing a standard solution (in a volumetric flask)
- -- weigh & dissolve the solid compound (using weigh by difference)
- clean the volumetric flask, funnel, and stopper 3x with deionized water
- quantitatively transfer the dissolved sample into the volumetric flask (using and
funnel and rinsing beaker/stir rod with deionized water)
- fill volumetric flask to a few centimetres below the calibration mark (do not go
above this line)
- use a dropper to add water dropwise to volumetric flask until the bottom of the
meniscus is resting on the calibration line (if you go past calibration mark, you must
start over)
- stopper the flask and invert the flask 20x
what is a parallax error? - ✅✅-when your eye level is higher or lower than the
bottom of the meniscus where you must record a volume
how do you prepare a volumetric pipette? - ✅✅ -clean with deionized water and
then rinse with solution that will be pipetted (the sample)
how to clean a pipette - ✅✅ -- pour at least 25ml of deionized water into a clean
beaker
- use pipet bulb to half-fill the pipette with deionized water
- hold pipette horizontally and role around to coat the inside with water
- discharge the water through the tip into the sink or waste beaker
- repeat the wash 2 times with deionized water
- transfer 25 mL of the solution to be pipetted into a clean & dry beaker
- draw approximately 5 mL of solution into the pipet and hold in horizontal position
and coat inside with solution
- discharge liquid into the sink/waste beaker
How do you fill a volumetric pipette? - ✅✅ -- hold the pipette near the very top
(blunt end) with one hand and the bulb with the other) & make sure the tip is
submerged below the surface of the solution & bulb is not yet on the pipette
- completely squeeze all air out of pipette bulb & gently put tip of bulb into top of the
pipette (never pipette by mouth)
- slowly release pressure on the pipette bulb to allow the solution to rise into the
pipette (fill to at least 2 cm above calibration line, keep pipette tip in solution to avoid
bubbles from entering pipette)
- remove the bulb quickly and cover the top of pipette with index-finger of the hand
holding the pipette (not with thumb)
- drain solution down to calibration (slowly move index finger covering the top of the
pipette to drain solution to get meniscus resting on the calibration line)
- remove any suspended drops from the pipette by touching it to the side of the
beaker
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