Solubility & Salt Preperation

When would you use Titration?

To make a soluble salt from an acid and an alkali

When would you use the Precipitation method?

To make an insoluble salt

When would you use the Excess Base method?

To make a soluble salt from an acid and an insoluble base

Solubility of Acids

All acids are soluble

Solubility of Bases

Most bases are insoluble.

Bases that are soluble include: Sodium Hydroxide, Pottassium Hydroxide, Ammonia gas, Calcium Hydroxide, Sodium Carbonate, Pottassium Carbonate & Ammounium Carbonate

Solubility of Salts

Most salts are soluble.

Insoluble salts include: Silver Halides, Lead (II) Halides, Barium Sulphate, Lead (II) Sulphate & Calcium Suplhate

Compounds of what are always soluble?

Sodium, Pottassium and Ammonium Bases, and all Nitrates

Describe the steps of a Titration

• Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (which can’t be universal indicator - we used phenolphalien)

• Add the acid into the burette and note the starting volume

• Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour (purple)

• Note and record the final volume of acid in burette and calculate the volume of acid added

  • Final volume of acid - initial volume of acid when using a burette

  • Repeat this until you get concordant results

• Add this same volume of acid into the same volume of alkali without the indicator

• Heat to partially evaporate, leaving a saturated solution

• Leave to crystallise decant excess solution and allow crystals to dry

What are ‘concordant results’?

Results of a Titration within 0.2cm³ of eachother

How many decimal places are titres measured to?

2 Decimal Places, with the second always being 0 or 5

Describe the Precipitation method

• The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water

• The preparation of an insoluble salt follows this pattern:

soluble salt 1 + soluble salt 2 ⟶  insoluble salt + soluble salt 3

AB + CD ⟶ AD + CB

• The method involves measuring out a fixed volume of one salt solution and then adding the second salt solution until it is in a slight excess

  • This ensures the maximum amount of precipitate will be obtained

• The precipitate is recovered by filtration

  • Then it must be washed with distilled water remove reactants that are contaminating the residue (recovered solid)

  • It is then left to dry

Describe the Excess Base method

• Add powdered insoluble reactant to acid in a beaker, one spatula at a time, stirring to mix. Continue adding powder until it is in excess (some unreacted powder is left over). All the acid has now reacted.

• Filter the mixture in the beaker to remove the excess solid. The filtrate now contains only the salt and water.

• Heat the solution in an evaporating dish over a water bath. Stop heating when small crystals start to appear around the edge of the evaporating basin. The solution is now saturated.

• Leave the saturated solution at room temperature for a day or two. This gives time for large crystals to form.

• If necessary, dry the crystals by dabbing gently with filter paper.

Why use a pipette to measure volume?

It only measures one volume, but does so exteremely accurately.