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Topic 5b - Quantative Analysis - Coggle Diagram
Topic 5b - Quantative Analysis
Percentage yield
Theoretical yield
Where no atoms are gained or lost
total mass stays the same
Actual yield
Mass of the product that you get at the end of the reaction
Always less than the theoretical yiels
Percentage yield = actual yield / theoretical yield
Reacons for obtaining less than expected
Incomplete reactions
Not all reactants are convertd into products
Reaction mixture wasn't heated strongly enough
Product loss
You lose material when transfering it
Filtering also loses products
Impurities
One of the products is a gas
Unexpected reactions
Side reactions is when the reactants might react with the air rather than reacting to from the product you want
Can be caused by changes in conditions
100% yield means you got all the products you expected
0% percentage yield means that no reactants were converted into products
Atom economy
atom economy = Mr of desired products / Mr of all products
Measures the number of atoms wasted
100% atom economy
There is only one products
All of the products are used
Resources aren't used up very quickly
Low atom economies aren't very profitable
Industrial process
Higher atom economy, lower impact on environment
Produces less waste
Conserves limited resources
Titrations
Allows you to see what volume of a reactant is needed to react completely with a certain volume of another reactant
Method
1) Use a pipette with a pipette filler to measure out a set volume of alkali
2) Put the alkali in a flask with a few drops of indicator
3) Using a funnel, fill a burette with some acid of known concentration, make sure the jet is filled and place the alkali underneath on a white tile
4) Take a reading of the volume of the acid in the burette, read the bottom of the meniscus
5) Carry out a rough titration, let the acid quickly be added to the flask, make sure to swirl, and stop when there is a colour change indicating the acid has been neutralised
6) Record the volume of acid left in the burette
7) Calculate the amount of acid needed to neutralise the alkali by subtracting the initial readings of acid in the burette from the final reading of the burette, this is the titre volume
8) Now do an accurate titration, take the initial reading on the burette, and run the acid down to 2cm3 of the end point, then drop by drop add the acid until there is a colour change
9) Record the titre volume and repeat a few times until you have concordant results that are within 0.1cm3 of eachother
10) Repeat the titration without indicator once you know the volume of acid needed
Indicators
Universal indicator isn't suitable as it changes colours too gradually
Methyl orange, phenolphthalein and litmus give sharp colour changes so are ideal
The indicator you use can affect the accuracy of results
Concentration calculations
If you know the concentration of one of the solutions, you can use the volumes from the titrations experiment and the eqution to find the other concentration
Steps
Work out how many moles of the known substance you have
Moles = concentration X volume
Write down the balanced equation and work out how many moles of the unknown substance you have
Work out the concentration of the unknown, c = m/v
Concentration in grams/dm-3
concentration (g/dm3) = concentration (mol/dm3) X Mr
To convert between grams to moles, divide by the Mr
Calcultations with gases
Molar volume
Volume occupied by 1 mole of gas
Molar volume = gas volume / n of moles
Avogadros' law
under the same conitions, the same number of moles of different gases all occupy the same volume
One mole of gas always accupies 24dm3 (24000cm3) at room temperature and pressure (20degree and 1 atm)
volume = moles x 24