Standardisation of NaOH Solution and BP Assay for Salicylic Acid

Question 1

HCl and NaOH react in a what ratio

Answer 1

1:1 ratio

Question 2

(1) Rinse a 25 cm3 pipette several times with small
volumes of the 0.1 M hydrochloric acid VS. why?

Answer 2

The volume delivered is of correct concentration,
There is no dilution or contamination,
And your titration results are accurate and reliable.

Question 3

(2) Rinse a 250 cm3 conical flask several times with
distilled water and then pipette 25.0 cm3 of the
0.1 M hydrochloric acid VS into it.
why

Answer 3

You rinse the conical flask with distilled water to:

Remove impurities or contaminants,
Avoid affecting the molarity of hydrochloric acid,
Ensure accuracy during the titration process..

Question 4

what indicator are we using

Answer 4

phenol red

Question 5

3) Rinse a burette several times with small volumes of the solution, removing the waste through the tap. Fill the burette to just above the zero mark and
then adjust the meniscus to near zero on the scale.
What solution

Answer 5

approximately 0.1 M sodium hydroxide VS

Question 6

All burette readings must be recorded to the nearest

Answer 6

± 0.01 cm3

Question 7

NOTE: Make sure there are no air bubbles trapped
below the tap - why?

Answer 7

your titre will be bigger if there’s an air bubble.

Question 8

(4) Titrate the hydrochloric acid VS with the sodium
hydroxide VS solution until what colour change

Answer 8

goes from yellow to pink / reddish violet
colour

Question 9

(5) Once you have worked out the concentration of the
Hydrochloric acid. what do we do

Answer 9

Place approximately 0.5 g salicylic
acid into a CLEAN and DRY weighing bottle and
determine the total mass to the nearest ± 0.1 mg using
an analytical balance. Transfer the contents
into a 250 cm3 conical flask and measure the mass of
the weighing bottle (including residue) again

Question 10

(6) Add approximately 30 cm3 of what solution to dissolve the salicylic acid sample and then add approximately 20 cm3 of water and a few drops of phenol red solution as indicator.

Answer 10

ethanol (96%)

Question 11

(7) Titrate with 0.1 M sodium hydroxide VS until a
what colour is obtained. Repeated until you have
three consistent 1 g readings.

Answer 11

pink ‘reddish’ indicates neutralisation

Question 12

The reaction between salicylic acid and DILUTE NaOH solution is 1:1. Why does only the carboxylic acid group react under these conditions?

Answer 12

Only the carboxylic acid dissociates under these conditions as it is much more acidic than the phenol

• the low concentration of NaOH used means that the pH is only higher than the pKa of the carboxyl group, not the phenol group

Question 13

What would you do if the three titrations were NOT consistent within 0.1 cm3 of one another?

Answer 13

Perform more titrations until three consistent volumes are obtained, then determine the mean volume from those three titrations

Question 14

Why should the three titrations all be consistent within 0.1 cm3 of one another?

Answer 14

Exactly the same volume of the same HCl solution and the same NaOH solution was used each time

Question 15

What is the 1g reading

Answer 15

the volume of sodium hydroxide that would be required to completely react with 1g of the sample (should be the same for all titrations ideally)

Question 16

how to work out the 1g reading

Answer 16

titre volume (cm3) / mass of naoh used (g)

Question 17

what do we calculate the % difference for

Answer 17

for the 1g reading e.g. between the biggest and smallest value

Question 18

% difference =

Answer 18

difference in values / mean value x 100

Question 19

what is the percentage purity (m/m)

Answer 19

mass found / mass taken x 100

this can be done with the 1g reading e.g.
the mass/1g x 100 (look at ur lab report if ur confused)