REquired practical - titrations

Question 1

Describe a method for determining the concentration of an unknown solution of HCL using 0.1 mol/dm3 of NaOH by acid-base titration

Answer 1

• rinse burette with sodium hydroxide to remove any contamination
• overfill burette with sodium hydroxide so that jet space is filled ( jet space is area under tap on a burette)
• transfer 25 cm3 of HCL to conical flask
• add two drops of phenolphthalein or other suitable indicator
• slowly add the NaOH to the acid and swirl between additions
• rinse burettes and sides of the conical flask with wash bottle
• stop adding NaOH at the first colour change, record volume added
• rinse conical flask, prepare once more and repeat titration until concordant values are achieved. Use concordant data to calculate mean titre
• calculate number of moles of NaOH added using n = c x v. work out ratio between volume of NaOH and HCL
• then work out the concentration of HCl using number of moles / volume.

Question 2

What is the aim of a titration?

Answer 2

reacting an acid with a known volume & concentration with an alkali which we don’t know concentration of, to then find the concentration.

The solution with the known concentration is always placed in the burette and 25cm3 of the solution of unknown concentration is placed in the conical flask.

Question 3

Where is the 25cm3 mark on a pipette ?

Answer 3

very fine line towards the top of pipette highlighting the 25cm3 mark

Question 4

Why is the pipette rinsed with distilled water ? and then after this NaOH

Answer 4

rinsed with water to remove unwanted chemicals. pipette will contain water droplets, and this can dilute the solution placed in the pipette for measuring. so then also rinsed with alkali to remove water droplets.

Question 5

Should the conical flask and pippete filler be rinsed?

Answer 5

Yes

Question 6

What would be the best indicator to use in each situation ?

Answer 6

phenolphthalein or methyl orange - strong acid strong base( alkali) titration

phenolphthalein : weak acid strong base ( alkali)

methyl orange : strong acid weak base ( alkali)

Question 7

How many drops of indicator be added?

Answer 7

only a few drops ie 2-4. indicators are weak acids: adding too much will lead to inaccurate results

Question 8

What should the burette be rinsed with ?

Answer 8

water and then acid to rmeove water traces. acid good one to use because acid will be added to the burette anyways

Question 9

why should the funnel be removed from top of burette

Answer 9

to stop it dripping extra acid into the burette

Question 10

When reading volume in burette what should you look at?

Answer 10

bottom of the meniscus ( curve) should always be read at eye level as well.

Question 11

Why should acid and alkali mixture be swirled after one is added to other

Answer 11

mix thoroughly to ensure they react

Question 12

What are the calculations used to work out the mass of the standard solution ?

Answer 12

first of all find Mr of Na2Co3
then moles = mass/ Mr to find the number of moles.
we know volume of solution in volumetric flask is 250cm3, then use moles / volume (dm3) to find conc of the Na2Co3

Question 12

What are the steps for making a standard solution?

Answer 12

• weigh the mass of solute carefully. Can be done by weighing by difference. first weighing boat + sodium hydrogen carbonate and then empty weighing boat. finally subtracting the two.
• dissolve in the chosen solvent in a beaker ie distilled water from wash bottle
• pour solution into volumetric flask with a funnel on top. raise this so all drains into flask
• rinse beaker with wash bottle three times to make sure all sodium hydrogen carb transferred to vf
• add distilled water to get solution up to 250cm3 mark. meniscus should be on 250cm3 line
• stopper flask and invert several times