9. Titrations

Question 1

define strength of an acid/alkali

Answer 1

the extent of its dissociation into H+ or OH- ions

Question 2

define strong acid

Answer 2

an acid that fully dissociates in solution

Question 3

define weak acid

Answer 3

an acid that partially dissociates in solution

Question 4

name some strong acids

Answer 4

hydrochloric
nitric
sulphuric

Question 5

name some weak acids

Answer 5

citric
formic (methanoic)
vinegar (ethanoic)

Question 6

define standard solution

Answer 6

a solution for which the concentration is known

Question 7

define molarity

Answer 7

concentration in mol/dm^3 expressed using M

Question 8

define concentration

Answer 8

the number of moles or mass present in a stated volume

Question 9

what is the dilution factor of a 25cm^3 sample made up to 250cm^3?

Answer 9

10

Question 10

give examples of indicators

Answer 10

phenolphthalein
methyl orange
litmus

Question 11

What us the colour change for phenolphthalein?

Answer 11

Acidic = colourless
Neutral = colourless
Alkaline = pink

Question 12

What is the colour change for methyl orange?

Answer 12

Acidic = red
Neutral = orange
Alkaline = yellow

Question 13

What is the colour change for litmus?

Answer 13

Acidic = red
Alkaline = blue

Question 14

What pH does phenolphthalein change colour at?

Answer 14

around pH 9

Question 15

What pH does methyl orange change colour at?

Answer 15

around pH 3-4

Question 16

What pH does litmus change colour at?

Answer 16

7

Question 17

What types of titrations is phenolphthalein suitable for?

Answer 17

Strong acid-strong base

Weak acid-strong base

Question 18

What type of titrations is methyl orange suitable for?

Answer 18

Strong acid-weak base

Strong acid-strong base

Question 19

How do you percentage purity of a substance?

Answer 19

(Actual mass/expected mass) x 100

Question 20

How can you calculate % uncertainty?

Answer 20

(Uncertainty/measurement) x 100

Question 21

How do you calculate moles?

Answer 21

Mass/rfm

Question 22

How do you calculate concentration?

Answer 22

(Mass or moles)/volume

Question 23

Describe how to make a standard solution

Answer 23

• calculate mass required to make the solution
• weigh solid/liquid on top pan balance/cylinder/burette
• dissolve solid in deionised water into volumetric flask
• rinse beaker/glass rod etc with deionised water into flask
• make up to mark and invert to mix

Question 24

How can you ensure accuracy when making up a standard solution?

Answer 24

• ensure bottom of meniscus lies on the line
• ensure every drop of solution is washed into volumetric flask
• wash inside of volumetric flask with deionised water

Question 25

What are the practical rules for titrations?

Answer 25

- in one volumetric analysis, 3 titrations should be carried out - first titration should be rough but no more than 1cm3 greater or less than accurate - 2nd and 3rd titration should be accurate and concordant, with dropwise addition until the end point is reached. - all values should be recorded to 2 dp - calculate the average titre with the two accurate concordant results - rinse burette and pipette with deionised water and the acid or base

Question 26

How can you ensure the reliability of a titre result?

Answer 26

- average the accurate titres | - repeat the titration to obtain more accurate titres