Practical 4 - Back Titration

Question 1

What’s the aim of this back titration?

Answer 1

To determine the % of calcium carbonate in limestone

Question 2

Whats the volume of the volumetric pipette used?

Answer 2

25cm^3

Question 3

What’s the volume of the conical flask used?

Answer 3

250cm^3

Question 4

What’s the volume of the burette used?

Answer 4

50cm^3

Question 5

What type of tablets do we use?

Answer 5

Antacid tablets or limestone chips

Question 6

Concentration of the sodium carbonate

Answer 6

0.2moldm^-3

Question 7

Concentration of the hydrochloric acid

Answer 7

0.5moldm^-3

Question 8

Which indicator do we use?

Answer 8

Phenolphthalein

Question 9

What’s the risk involved with NaOH and HCl?

Answer 9

Irritants

Question 10

What are the irritants in this practical?

Answer 10

NaOH and HCl

Question 11

What’s the risk involved with phenolphthalein?

Answer 11

Flammable

Question 12

What is flammable in this practical?

Answer 12

Phenolphthalein indicator

Question 13

What’s the first step of this practical?

Answer 13

Grind the tablet into a fine powder using a pestle and mortar

Question 14

How much of the antacid powder do we weigh and what in?

Answer 14

0.5g on a 3 decimal place digital balance, in a conical flask

Question 15

What do we do after weighing the powder?

Answer 15

Add 25cm^3 HCl solution using a pipette

Question 16

How much HCl solution do we use?

Answer 16

25cm^3 (in excess)

Question 17

What do we do once we’ve got our HCl solution with our ground up tablet?

Answer 17

Stir the mixture vigorously with a stirring rod and heat gently with regular stirring

Question 18

How hot should we heat the HCl and tablet solution? Why?

Answer 18

Not too hot - will lose acid ions

Question 19

What happens if we heat the HCl and tablet solution too hot?

Answer 19

We will lose ions

Question 20

How could we lose acid ions in this practical?

Answer 20

Heating it too hot over the Bunsen burner

Question 21

What do we heat up during this practical and how?

Answer 21

HCl and powder solution, with a Bunsen burner

Question 22

How long do we heat the HCl and powder solution for?

Answer 22

Until the powder has dissolved

Question 23

What do we do after the antacid tablet has fully dissolved?

Answer 23

Cool it to room temperature

Question 24

What do we do after the tablet and HCl solution has cooled to room temperature?

Answer 24

Add a few drops of phenolphthalein indicator and titration against the standardised NaOH solution - will turn from clear to pink

Question 25

What do we titration the HCl and antacid tablet solution against?

Answer 25

Standardised NaOH solution

Question 26

What will the colour change of the indicator be when the reaction has ended?

Answer 26

From clear to pink

Question 27

What’s the final step of this practical?

Answer 27

Record the volume of NaOH solution needed to neutralise the excess HCl solution and use to calculate the % CaCO3 in the tablet (instructions are in the official deck)

Question 28

What value do we need from the titration in order to calculate the % of CaCO3 in the tablet?

Answer 28

The volume of NaOH solution needed to neutralise the excess HCl solution

Question 29

What would using more acid/less chalk do to our practical?

Answer 29

Give us larger titres for smaller % error

Question 30

How would we get larger titres and what would this lead to?

Answer 30

Use more acid/less chalk Smaller % error

Question 31

How would we get a smaller % error in this practical?

Answer 31

Use more acid/less chalk (Gives larger titres)

Question 32

Which methods used within this practical help speed up the reaction?

Answer 32

- crushing the tablet into a powder - heating the solution -stirring the solution

Question 33

What does crushing the tablet, heating the solution and stirring the solution all do to the reaction?

Answer 33

Speed it up

Question 34

Why is the flask heated in the practical?

Answer 34

To ensure all of the tablet has dissolved

Question 35

Why do we crush the tablet?

Answer 35

To increase the surface area for a faster reaction

Question 36

Why must we use a slightly large beaker?

Answer 36

To avoid loss of ions

Question 37

What could lead to loss of ions in this practical?

Answer 37

Using a beaker that’s too small

Question 38

How do we speed up the rate of reaction in this titration?

Answer 38

-Crush up the chalk (larger surface area) -Heat up the chalk in the acid