Titrations

Question 1

Describe the method for a general titration

Answer 1

• Firstly, you should use a pipette and pipette filler to transfer 25cm^3 of solution x to a 250cm^3 conical flask then you should add a few drops of methyl orange (when you’re adding a strong acid to a weak alkali) or phenolphthalein (when adding a strong alkali to a weak acid) to the flask
• You should then place a white tile under the conical flask and place the conical flask under the burette
• Now you need to fill your burette, that will be clamped using a clamp and retort stand, with HCl using a funnel and titrate solution x with the HCl until an endpoint is reached, this is your trial titration. You may wish to keep your solution from your first accurate titration in order to obtain concordant results by colour matching
• Repeat your titration accurately until you have two concordant results
• Make sure you record all your results accurately

Question 2

How do you calculate percentage uncertainty?

Answer 2

(Absolute uncertainty / measured value) x number of uncertainty occurs (basically how many times you weigh something) x100

Question 3

How do you make up a standard solution?

Answer 3

• Weigh a set mass of solid (Eg. 2.10g of sodium hydrogencarbonate), transfer the solid to a clean dry 100cm^3 beaker
• Add enough distilled water to dissolve the solid and stir with a glass rod
• Transfer the solution to a 250cm^3 volumetric flask and rinse the 100cm^3 beaker and glass rod and add the rinsings to the volumetric flask
• Add distilled water up to the mark, insert the stopper and invert the flask several times to mix thoroughly

Question 4

What is the meniscus?

Answer 4

The meniscus is the curve seen at the top of a liquid in response to its container

Question 5

What is a standard solution?

Answer 5

A solution of a known concentration, they’re often used in titrations to find out the concentration of another solution

Question 6

How do you find the percentage error of a value?

Answer 6

(True value - experimental value) / true value

Question 7

What colour does methyl orange turn in acid?

Answer 7

Red, yellow in base

Question 8

What colour does bromothymol blue turn in acid?

Answer 8

Yellow, blue in base

Question 9

What colour does phenolphthalein turn in acid?

Answer 9

Colourless, pink in base

Question 10

What colour is litmus in acid?

Answer 10

Red, blue in base

Question 11

Which indicator should you use when adding a weak acid to a strong alkali?

Answer 11

Phenolphthalein

Question 12

Which indicator should you use when adding a strong acid to weak alkali?

Answer 12

Methyl orange

Question 13

Which indicator should you use when adding a strong acid to a strong alkali

Answer 13

Bromothymol blue

Question 14

What are the main errors that can affect titrations?

Answer 14

• Systematic errors (a constant error with a piece of equipment eg. a balance used for weighing substances not being calibrated correctly)
• Random error (an error that may not always occur in an experiment eg. sometimes leaving an air bubble in the burette
• Margin of error of the piece of the equipment that you’re using eg. a pipette may have an error margin of +/- 0.06cm^3