PAGs Flashcards
Describe the method for making an water soluble salt from an acid and an insoluble salt
Add base to the acid. As it reacts it will dissolve.
When fully reacted no more solid will dissolve.
Filter the excess base to leave only the salt solution.
Evaporate some of the water to make the solution more concentrated
And then allow to evaporate on its own for a longer period of time.
The salt will crystallise and pure and solid salt crystals will form
Describe the method for making a water-soluble salt from an acid an a soluble base
Carry out titrations to find amount of alkali needed to neutralise acid exactly using indicator. Repeat without indicator
Transfer neutralised solution to evaporating basin. Heat over bunsen flame to evaporate the water. Don’t heat too strongly as this will cause spitting. Once crystals start to appear, stop heating.
Let the mixture cool in basin, then filter. Wash the solid residue with cold distilled water.
Transfer the residue to a watch glass + heat in an oven to dry the solid. Make sure temp is lower than melting point of the salt
Remove the watch glass and cool in a dessicator, then weigh. Once the solid has dried to a constant mass, stop heating and cool in the dessicator.
Why is a dessicator useful when drying salts?
A dessicator allows allows substances to dry in a cool, dry atmosphere. This prevents moisture being reabsorbed.
What is the formula for calculating percentage error?
(Accuracy of equipment ÷ Measurement made) x 100
If a measuring cylinder has 1cm³ graduations, the error is ±0.5cm³. This value is the ‘accuracy of the equipment’
Need to multiply % error by however many times the equipment is used to make a measurement
Why might percentage yield be lower than expected?
(e.g. when making an insoluble salt)
Product may be lost from reaction vessels, especially if there are several stages to the reaction (lost when transfering product)
Side reactions occur, creating by-products
Impurities in the reactants
Changes in temperature + pressure
If the reaction is an equilibrium system
What is the formula for calculating percentage yield?
(Actual yield / Theoretical yield) x100
