RP1 - Making a Volumetric Solution and Acid-Base Titration

Question 1

How to accurately measure the mass of a solid

Answer 1

Weigh the sample bottle containing the required mass of solid on a 2dp balance
Transfer to a beaker, washing the bottle and adding to the beaker.
Reweigh the empty sample bottle
Record the difference in mass

Question 2

How to make a solution using an already measured mass of solid.

Answer 2

Add the solid to a beaker and add distilled water and use a glass rod to dissolve the solid.
Transfer the solution into a 250cm^3 graduated flask via a funnel.
Rinse beaker and funnel and add washings to the volumetric flask.
Make up to the 250cm^3 mark with distilled water
Shake the flask.

Question 3

Why do we have to shake the volumetric flask when making solution

Answer 3

To ensure a uniform concentration

Question 4

Why can we not heat or put hot solutions in the volumetric flask

Answer 4

The heat causes the flask to expand and the volume would then be incorrect

Question 5

Describe how we can dilute a solution

Answer 5

Pipette 25cm^3 of the original solution into a 250cm^3 volumetric flask.
Make up to the 250cm^3 mark with distilled water.
Shake the flask several time times to ensure uniform solution

Question 6

Concordant results

Answer 6

Readings within 0.1 of each other

Question 7

Why is a conical flask used instead of a beaker in titrations

Answer 7

Easier to swirl the mixture in a conical flask without spilling the contents

Question 8

What should you do to the burette before using

Answer 8

It should be rinsed out with substance that will be put in it.
If not rinsed out the acid/alkali added may be diluted by residual water or may react with previous substances.
This would decreases the concentration of the substance and so you would get a larger titre.

Question 9

Why do we have to remove the funnel from the burette

Answer 9

Small drops of liquid may fall from the funnel during titration leading to false burette reading and gives a lower titre volume

Question 10

What must we do to the jet space in the burette before using

Answer 10

Ensure it is filled with the solution and air bubbles are removed.

Question 11

How must you read the burette

Answer 11

From the bottom of the meniscus

Question 12

How much indicator should you add and why

Answer 12

A few drops as indicators are weak acids so if too much is added they will affect the titration results

Question 13

Which reactions are phenolphthalein used in

Answer 13

When strong alkalis are used

Question 14

Colour change when acid is added from burette to the alkali (phenolphthalein)

Answer 14

Pink in alkali to colourless

Question 15

Which titration reactions are methyl orange used in

Answer 15

When strong acids are used

Question 16

Colour change for methyl orange

Answer 16

Red in acid
Yellow in alkali
Orange at end point

Question 17

Why is a white tile used

Answer 17

To help observe the colour change

Question 18

How should you add solution from burette

Answer 18

Whilst swirling the mixture and adding drop wise at end point

Question 19

Why might distilled water be added to the conical flask during a titration

Answer 19

To wash the sides of the flask so all the acid on the side is wash into the reaction mixture to react with the alkali.

Question 20

Why does distilled water being added into the conical flaks not affect the titration readings

Answer 20

Water does not react with the reagents or change the moles of acid added.

Question 21

Why are titrations repeated

Answer 21

To spot anomalous results and so they can be discounted
To find concordant results
To calculate an average

Question 22

Colour of MnO4 -

Answer 22

Purple

Question 23

Colour change if manganate is in the burette

Answer 23

Colourless to purple

Question 24

% uncertainty =

Answer 24

Uncertainty / Measurement x 100

Question 25

Uncertainty of a burette in titrations

Answer 25

If the burette had an uncertainty of 0.05 then during a titration the uncertainty would be 3 x 0.05 = 0.15

Question 26

How to decrease apparatus uncertainty

Answer 26

Use apparatus with greater sensitivity or increase size of measurement