Part 2 - Salt Preparations

When are titrations used

when both reagents (+product) are soluble*

What is a titration used for

To make a soluble salt by mixing:
• an acid (like HCl or H₂SO₄)
• with a soluble base (like NaOH or KOH)

What is the goal for this acid-base neutralisation

Goal: To only get salt + water in the end
(no extra acid or base left over)

2. Why is titration required if both reagents are soluble?

Because both the acid and the base are:
• soluble → they dissolve in water
• colourless → you can’t see when the reaction is “done”

So you need something to tell you when the acid and base have fully reacted. That’s why an pH indicator is needed

3. Why use an indicator like phenolphthalein?

Because it shows you when the acid and base are balanced perfectly (this is called the end point of the reaction): Refer to indicators table

In a base it’s pink, while in an acid it turns colourless. So when you’re slowly adding drops of acid to the base it will turn from pink to colourless/very faint pink indicating the amount of acid needed to neutralise the base.

 

Note: The end result of a titration should not be colourless, it should be a very faint pink -> if it goes colourless it means too much acid was added and it is overshot

5. The problem if you DON’T use titration:

If you just randomly mix the acid and base:
• You might have extra acid left over → the solution is acidic
• Or extra base left over → the solution is basic
Either way, your salt is contaminated.

Titration lets you find the exact amount of acid needed to react with the base perfectly.

6. Steps and Procedure

1. Place a known volume of alkali/carbonate solution to a conical flask by means of a volumetric pipette

2. A few drops of phenolphthalein is then added to the alkali in the conical flask (pink)

3. Acid is then added to the alkali by means of a burette until the indicator changes colour indicating the end point has been reached (note: flask should be placed on a white tile to observe the colour change better)

• Note: The acid is added a little at a time and the flask is swirled in a controlled way, allowing the acid and alkali to mix

4. The amount of acid used is the volume of acid needed to completely neutralise the alkali/carbonate solution

5. This experiment is repeated multiple times to get an accurate reading (average is taken)

6. The experiment is then repeated without the indicator (it is only used to indicate how much acid is needed). To get a pure dry sample of the salt, evaporation or crystallisation is carried out.

Label the diagram

When is the precipitation method used

to produce insoluble salts when both reagents are soluble

What kind of a reaction is this

This is a double-displacement reaction (not an acid-base one: contains no acid or base)

Explain how this method works

Solutions of the two soluble salts are mixed and, in that mixture, an insoluble salt forms (precipitate). To isolate that precipitate: it is filtered, washed with distilled water and dried. The filtrate would be a soluble salt.

Label the diagram

What is the rule for titrations and precipitation methods

Rule for titrations: soluble base + soluble acid → salt + water (+CO₂ if the base is a carbonate)

Rule for precipitation: soluble salt + soluble salt → insoluble salt (precipitate) + soluble salt