Practical 4 - Back Titration Flashcards
What’s the aim of this back titration?
To determine the % of calcium carbonate in limestone
Whats the volume of the volumetric pipette used?
25cm^3
What’s the volume of the conical flask used?
250cm^3
What’s the volume of the burette used?
50cm^3
What type of tablets do we use?
Antacid tablets or limestone chips
Concentration of the sodium carbonate
0.2moldm^-3
Concentration of the hydrochloric acid
0.5moldm^-3
Which indicator do we use?
Phenolphthalein
What’s the risk involved with NaOH and HCl?
Irritants
What are the irritants in this practical?
NaOH and HCl
What’s the risk involved with phenolphthalein?
Flammable
What is flammable in this practical?
Phenolphthalein indicator
What’s the first step of this practical?
Grind the tablet into a fine powder using a pestle and mortar
How much of the antacid powder do we weigh and what in?
0.5g on a 3 decimal place digital balance, in a conical flask
What do we do after weighing the powder?
Add 25cm^3 HCl solution using a pipette
How much HCl solution do we use?
25cm^3 (in excess)
What do we do once we’ve got our HCl solution with our ground up tablet?
Stir the mixture vigorously with a stirring rod and heat gently with regular stirring
How hot should we heat the HCl and tablet solution? Why?
Not too hot - will lose acid ions
What happens if we heat the HCl and tablet solution too hot?
We will lose ions
How could we lose acid ions in this practical?
Heating it too hot over the Bunsen burner
What do we heat up during this practical and how?
HCl and powder solution, with a Bunsen burner
How long do we heat the HCl and powder solution for?
Until the powder has dissolved
What do we do after the antacid tablet has fully dissolved?
Cool it to room temperature
What do we do after the tablet and HCl solution has cooled to room temperature?
Add a few drops of phenolphthalein indicator and titration against the standardised NaOH solution - will turn from clear to pink
What do we titration the HCl and antacid tablet solution against?
Standardised NaOH solution
What will the colour change of the indicator be when the reaction has ended?
From clear to pink
What’s the final step of this practical?
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)
What value do we need from the titration in order to calculate the % of CaCO3 in the tablet?
The volume of NaOH solution needed to neutralise the excess HCl solution
What would using more acid/less chalk do to our practical?
Give us larger titres for smaller % error
How would we get larger titres and what would this lead to?
Use more acid/less chalk
Smaller % error
How would we get a smaller % error in this practical?
Use more acid/less chalk
(Gives larger titres)
Which methods used within this practical help speed up the reaction?
- crushing the tablet into a powder
- heating the solution
-stirring the solution
What does crushing the tablet, heating the solution and stirring the solution all do to the reaction?
Speed it up
Why is the flask heated in the practical?
To ensure all of the tablet has dissolved
Why do we crush the tablet?
To increase the surface area for a faster reaction
Why must we use a slightly large beaker?
To avoid loss of ions
What could lead to loss of ions in this practical?
Using a beaker that’s too small
How do we speed up the rate of reaction in this titration?
-Crush up the chalk (larger surface area)
-Heat up the chalk in the acid
