Prac 1 Flashcards
Required Practical 1a
Preparing a Standard Solution
- Weigh the sample bottle containing the required mass of solid on a 2 dp balance.
Transfer to beaker
Reweigh sample bottle and record the difference in mass.
Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
- Pour solution into a 250cm3 graduated flask via a funnel.
Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask. - Make up to the mark with distilled water using a dropping pipette for last few drops.
Invert flask several times to ensure uniform solution.
Required Practical 1a
Preparing a Standard Solution: Why reweigh sample bottle?
Some of the solid could still be in the weighing bottle.Doing this ensures you only record the mass of solid
that was transferred into the flask.
Required Practical 1a
Preparing a Standard Solution: Why rinse the beaker and add washings?
Some of the solution may still be in the beaker, the funnel or on the glass rod. If you don’t wash this content through, the solution in the flask would be less concentrated than you have calculated.
Required Practical 1a
Preparing a Standard Solution: Why invert the flask?
The shape of the flask means that mixing the content is very difficult to achieve by shaking it. Inverting the flask is the best way to ensure a uniform mixture.
Required Practical 1b
Acid-Base Titration: Why ensure the jet is filled with acid?
Air bubbles in the jet cause errors. Your titre
reading would be higher than the true value
Required Practical 1b
Acid-Base Titration
Rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water).
Pipette 25 cm³ of alkali into conical flask. Add a few drops of indicator.
Fill burette with acid using a funnel. Make sure the jet is filled. Remove the funnel.
Record initial burette reading to 2 d.p. (read meniscus properly).
Add the acid from the burette whilst continually swirling the flask. Slow down when you approach the endpoint, add just a drop at a time.
Stop when the endpoint (expected colour change) is reached.
Record the final volume in the burette to 2 d.p.
Repeat until you get concordant results (within 0.1 cm³ of each other).
Take an average (2 d.p.) of the two concordant results.
Why Remove the funnel.
Some acid could remain in the funnel and drip into the burette during the titration. This would mean your titre value is lower than the true value.
Why Add the acid from the burette whilst continually swirling the flask.
This ensures that the reacting acid and alkali particles collide in the flask and react.
The burette and volumetric pipette are chosen because
They are more precise than measuring cylinders or beakers.
When is the end point:
Not when the indicitar just changes colour
Due to possible incomplete reaction.
Permanent colour change is seen.
Why is too much indicator a problem?
It may react and affect endpoint reading, so use a few drops only.
2 Safety precautions for this experiment:
Fill burette and eye level
Wear gloves
