Core Practical 3: Make up a volumetric solution and : Find the concentration of a solution of hydrochloric acid

Question 1

Procedure for core practical 3

Answer 1

1. Wash out the 250 cm3 volumetric flask with distilled water, burette with acid and pipette with alkali
2. Use the pipette to transfer 25.0 cm3 of the hydrochloric acid solution into the volumetric flask. Make the solution up to the mark with deionised water.
3. Prepare your apparatus for the titration. The burette should contain the sodium hydroxide solution and the conical flask should contain the dilute hydrochloric acid solution.
4. Pour a 25.0 cm3 aliquot of the diluted hydrochloric acid into the conical flask. Add two drops of phenolphthalein indicator.
5. Titrate the contents of the flask against the sodium hydroxide solution (previously standardised). All burette readings should be recorded to the nearest 0.05 cm3
6. The end point of this titration is indicated by the contents of the flask becoming pale pink. Continued swirling will cause the pink colour to fade and disappear – if the pink colour persists for 5 seconds or more, then the end point has been reached.
7. Continue to conduct titrations until you have 2 concordant titres.

Question 2

what is the objective of core practical 3?

Answer 2

To find the concentration of a solution of hydrochloric acid

Question 3

equipment needed for core practical 3

Answer 3

● burette, clamp and stand
● sodium hydroxide solution (previously standardised)
● bench hydrochloric acid (approximately 1 mol dm–3)
● phenolphthalein
● 250 cm3 conical flask
● 25 cm3 volumetric pipette plus safety filler
● 100 cm3 beakers for transfer of solutions
● funnel for filling burette
● 250 cm3 beaker
● 250 cm3 volumetric flask

Question 4

What are the possible errors in core practical 3?

Answer 4

● Allow the titrant some time to drain down walls of burette before reading the burette.
● Swirl the conical flask so it mixes properly with the analyte.
● Use a white tile to make the colour change more noticeable.
● Diluting your solutions may produce a reading with smaller percentage error, (i.e. 10
cm3 titre has a smaller percentage error than a 30 cm3 titre).
● Phenolphthalein used in this titration may turn colourless at the end point if you
leave the solution to stand. This because NaOH reacts with CO2 from the air to form Na2CO3.

Question 5

Key points of core practical 3 to remember

Answer 5

● Use a pipette filler to draw a little solution of your analyte into the pipette to rinse it.
● Calibrate the pipette by ensuring there are no air bubbles in the tip.
● Add the suitable indicator to the conical flask.
● Rinse and fill the burette with the solution of your titrant using a funnel

Question 6

why should you touch the surface of the solution with pipette?

Answer 6

to ensure the correct amount is added. A small amount of solution will be left in the pipette at this stage. The calibration of the pipette will take into account this effect. It should not be forced out.

Question 7

why is a conical flask used in preference to a beaker?

Answer 7

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

Question 8

Why shouldn’t you leave the funnel in the burette?

Answer 8

small drops of liquid may fall from the funnel during the titration leading to a
false burette reading (would give a lower titre volume)

Question 9

Why should you make sure the jet space in the burette is filled with the
solution and air bubbles are removed?

Answer 9

If the jet space in the burette is not filled properly prior to commencing the
titration it will lead to errors if it then fills during the titration, leading to a
larger than expected titre reading.

Question 10

phenolphthalein colour in acid/alkali

Answer 10

acid: colourless
alkali: pink

Question 11

methyl orange colour in acid/alkali

Answer 11

acid: red
alkali: yellow

Question 12

why can distilled water be added to the conical flask during a titration to wash the
sides of the flask so that all the acid on the side is washed into the reaction
mixture to react with the alkali.

Answer 12

It does not affect the titration reading as water does not react with the reagents
or change the number of moles of acid added.

Question 13

Why should a titration be repeated?

Answer 13

A single titration could be flawed. Repeating allows for anomalous titres to be spotted and discounted.

Question 14

what are some common titration equations?

Answer 14

CH3CO2H + NaOH  CH3CO2 -Na+ + H2O
H2SO4 + 2NaOH  Na2SO4 +2H2O
HCl + NaOH  NaCl +H2O
NaHCO3 + HCl  NaCl + CO2 + H2O
Na2CO3 + 2HCl  2NaCl + CO2 + H2O

Question 15

how to reduce uncertainties in a burette?

Answer 15

you can either decrease the sensitivity
uncertainty by using apparatus with a
greater resolution (finer scale divisions ) or
you can increase the size of the
measurement made.

Question 16

how can you reduce uncertainties in measuring mass?

Answer 16

Using a more accurate balance or a larger mass will reduce the
uncertainty in weighing a solid.
Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added.