Titrations

Question 1

What is a standard solution?

Answer 1

A solution whose conc is known accurately

Question 2

How to make a standard solution?

Answer 2

• weigh out accurate mass of solid into clean dry beaker
• add enough deionised water to dissolve solid
• transfer to volumetric flask using funnel
• rinse equipment with deionised water and transfer washings to volumetric flask
• add deionised water to flask to make up graduation mark (use dropping pipette) and ensure bottom of meniscus is on mark
• stopper flask and invert to mix thoroughly

Question 3

What is a titration?

Answer 3

a technique where a solution of known conc is used to determine conc of unknown solution

Question 4

Phenolphthalein in acid

Answer 4

colourless

Question 5

Phenolphthalein in alkali

Answer 5

pink

Question 6

What are the two stages of titrations?

Answer 6

1) rough titration
2) several more accurate titrations

Question 7

Why should you do a rough titration first?

Answer 7

To determine the approx amount of standard solution needed to neutralise the unknown solution

Question 8

What are the steps of a titration?

Answer 8

1) fill burette (including below tap) with standard solution + take initial reading to 2dp
2) Rinse pipette with deionised water + unknown conc solution and then transfer 25cm3 of solution from beaker into conical flask using volumetric pipette
3) add 3-4 drops of indicator to conical flask and place under burette
4) Add solution from burette into conical flask slowly and controlled, swirling the concical flask, until indicator just changes colour (pale pink)
5) write down final volume to 2dp (titre volume)

Question 9

Why should you always overfill your burette and let some out into waste beaker?

Answer 9

• ensures jet is filled
• titre volume will otherwise be too high

Question 10

Why is it important to rinse pipette and burette with deionised water and solution that will be in apparatus?

Answer 10

• Remove any soluble impurities
• Removes any drops of deionised water that would lower concentrations of the solution
• titre volume will otherwise be too high

Question 11

Why shouldn’t you add more than 3-4 drops of indicator to your conical flask?

Answer 11

• indicators are weak acids so will slightly neutralise solution
• titre volume will otherwise be too low

Question 12

Why is a conical flask used?

Answer 12

No solution spills when swirling

Question 13

What does concordant values mean?

Answer 13

2 titres within 0.1cm3 of each other - can be used to calculate average titre

Question 14

Why do burette readings have to be taken from the bottom of the meniscus?

Answer 14

• titre will otherwise be too low

Question 15

Why are the sides of the conical flask rinsed with deionised water?

Answer 15

• to ensure all reagents are in solution

Question 16

Why is the conical flask swirled during the titration?

Answer 16

• to ensure all reagents have reacted
• clear colour change

Question 17

What are the steps to determine unkown concentration?

Answer 17

1) write out balanced equation
2) work out moles of substance you know most about
3) determine number of moles of other substance using balanced equation
4) calculate unknown conc using moles/volume

Question 18

What would be the sample factor for a 250cm3 standard solution (assuming 25cm3 was used in the titration)?

Answer 18

x10
(can be used to calculate moles in original solution)

Question 19

How do you calculate the apparatus error for a piece of equipment?

Answer 19

(margin of error/quantity measured) x 100

• margin of error for burette x2 -

Question 20

What can you do to reduce % uncertainty in a practical?

Answer 20

Increase quantity measured