Practical 2 - Preparation of a soluble salt by titration Flashcards
What are we trying to achieve with this practical?
Preparing crystals of sodium chloride by using titration followed by evaporation
What type of crystals are we attempting to form via titration and evaporation here?
Sodium chloride
What does sodium chloride form in this experiment?
Crystals
The volume that could be held by the Burkett e
50cm^3
The volume that could be held by the pipette
25cm^3
The volume that cold be held by the conical flasks
100cm^3
Apparatus
50cm^3 burette
Funnel
25cm^3 pipette + filler
100cm^3 conical flasks
Evaporating basin
Molarities of the NaOH and HCl used
0.1moldm^3
What were the chemicals used?
NaOH solution
HCl solution
Phenolphthalein indicator
What type of indicator was used?
Phenolphthalein
Phenolphthalein purpose in the practical
Indicator
Risk of NaOH
Irritant
Risk of HCl
Irritant
Risk of phenolphthalein indicator
Flammable
Which two solutions share the same risk here and what is it?
NaOH and HCl - both irritants
What must we do before starting any titration and why?
Always rinse the burette and pipette with whatever we’re titrating with
(Water would increase the volume too much)
What do we rinse the burette and pipette with?
Whatever we’re titrating with
What do we rinse before beginning the titration?
The burette and pipette (with whatever we’re titrating with)
Why can’t we rinse the burette and pipette out with water?
It would increase the volume too much
How do we measure the NaOH solution?
With a pipette and filler
What’s the first step?
Using a pipette and filler, measure 25cm^3 of NaOH solution and pour it into the conical flask
Where does the NaOH solution go? Why?
The conical flask - could damage the burette
How much phenolphthalein do we add? What to?
2 drops to the NaOH solution in the conical flask
Where do we place the HCl solution and how?
In the burette using a funnel
Which chemical goes in the burette and which goes in the pipette then the conical flask?
Burette - HCl
Conical flak - NaOH
Where do we start our initial reading on the burette of HCl?
Anywhere that isn’t zero
What must we do before taking the initial burette reading of HCl in the burette and why?
Remove the funnel so that additional substance doesn’t drip in
How do we actually combine the NaOH and HCl?
Open the tap and add the HCl from the burette into the conical flask a little at a time while swirling the conical flask
How do we know if we’ve neutralised the NaOH?
The phenolphthalein turns from pink to colourless
When do we start and stop adding the HCl solution one drop at a time?
When the phenolphthalein starts to turn from pink to colourless
Stop when one drop is sufficient to turn the solution colourless
What colour does the phenolphthalein turn from and to when the NaOH has been neutralised?
From pink to colourless
What does it actually mean that the NaOH has been neutralised?
There’s the same number of moles of NaOH and HCl reacting
Why don’t we include the first titre in our final results?
It’s just a rough titre
What do we need two measurements to be between in order to move onto the next stage?
Within 0.2cm^3 of each other
What can we do once we’ve got two measurements within 0.2cm^3 of eachother?
Carry out the titration again using 25cm^3 of NaOH solution an exactly the same volume of HCl solutions and do not add indicator this time
What do we do differently in terms of the titration once we’ve got 2 measurements within 0.2cm^3 of eachother?
Carry it out using exactly the same volume of HCl solution needed
Do not add the indicator
Once we’ve titrated for the last time using the correct volume of HCl to react with the NaOH an no indicator, what do we do?
Gently heat the solution from the conical flask in an evaporating basin until its volume decreases by about half
How long do we heat the solution in the conical flask for?
Until its volume decreases by about half
Whats the final step of this practical?
Leave the evaporating basin to cool, allowing crystals to form
What happens when we leave the evaporating basin to cool?
Crystals form
How do we calculate the titre (cm^3)?
Final volume - initial volume
What decimal place to we write all of our results to?
2 d.p.
What must we ensure in terms of the burette before beginning the titration?
That there’s water in the jet
What must we ensure in terms of the pipette before beginning the titration?
That all bubbles have been flushed out
What do we do once crystals have formed?
Filter and dry them
Why don’t we use a universal indicator instead of pthenolphthalein?
The end point of the universal indicator is too narrow
