R.P 1 (Acid-base titration)

Question 1

Outline the stages in making up a volumetric solution

Answer 1

• Weigh the sample bottle containing the required mass of solid on a 2 dp balance
• Transfer to beaker
• Reweigh empty sample bottle
• 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.

Question 2

Where do we read off from

Answer 2

Bottom of the meniscus

Question 3

How do we dilute an original volumetric solution

Answer 3

*Pipette 25cm3 of original solution into a 250cm3 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.

Question 4

Why is a volumetric pipette is more accurate than a measuring cylinder

Answer 4

Smaller uncertainty

Question 5

Outline the general method in an acid-base titration

Answer 5

*rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)

*pipette 25 cm3 of alkali into conical flask

*touch surface of alkali with pipette ( to ensure correct amount is added)

*adds acid solution from burette

*add a few drops of indicator and refer to colour change at end point

*phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [use if NaOH is used]

*methyl orange [yellow (alkali) to red (acid): end point orange] [use if HCl is used]

*use a white tile underneath the flask to help observe the colour change

*add acid to alkali whilst swirling the mixture and add acid drop wise at end point

*note burette reading before and after addition of acid

*repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other (then calculate mean)

Question 6

Why is a conical flask used over a beaker

Answer 6

easier to swirl the mixture in a conical flask without spilling the contents.

Question 7

Which substance is generally placed in the burette

Answer 7

One substance (generally the one we don’t know the concentration) is put in the conical flask. It is measured
using a volumetric pipette.
The other substance is placed in the burette

Question 8

What does the standard phrase: titrate solution A
with solution B mean

Answer 8

Means that A should be in the conical flask and B should be in the burette.