Required Practical 1 TITRATION

Titrations involve neutralisation reactions to work out the concentration of an acidic or alkaline solution. 

To do this you react a known volume of a solution of an unknown concentration with a solution of known concentration (standard solution) and calculate the volume of standard solution required to neutralise the solution of unknown concentration. 

Equations needed:

A standard solution is any solution that you know the exact concentration of. 

Making 250cm³ of 0.1mol dm^-3 solution of NaOH

1. Calculate moles of NaOH needed          moles = conc. x vol (dm³) = 0.1 x 0.025 = 0.025mol

2. Calculate mass of NaOH needed          mass = moles x mr = 0.025 x 40 = 1.00g 

3. Weight out mass of NaOH into a weigh boat on a digital balance. Record the mass of the NaOH + weigh boat, then transfer NaOH to a beaker. Re-weigh the weigh boat (which may still contain traces of NaOH) and subtract from the mass of NaOH + weigh boat.

4. Add distilled water to the beaker and stir until NaOH dissolves. Using a funnel, tip the solution into a 250cm³ volumetric flask. Rinse the beaker and stirring rod with distilled water and add to the flask too. Top up the flask with distilled water until the bottom of the meniscus reaches the line (when in nears the line, add water drop by drop). Stopper the flask and invert several times to mix well. 

5. Calculate exact cencentration of standard solution          moles = mass / mr          conc. = moles / volume 

1. Using a pipette filler, fill a 25cm³ pipette with a solution of unknown concentration and transfer to conical flask. Add a few drops of of phenolphthalein indicator (colour change = pink to colourless).

2. Using a funnel, fill a burette with an acid of known concentration. Do this below eye level to avoid acid splashing in your face. 

3. Do a rough titration to get an idea wher the end point is. Record the initial volume of acid in the burette. Then gradually add the acid to the alkali, swirling the flask regularly. When the colour changes, take a final reading. 

4. Do an accurate titration. Take an initial reading, then run the acid in to within 2cm³ of the end point. When you get to this stage, add it drop by drop. Find the titre (amount of acid used to neutralise the alkali) by subtracting final reading from the initial reading. 

5. Repeat the titration until you have three concordant results (within 0.1cm³ of each other). Calculate the mean volume of acid used (using only the concordant results). 

1. Write a balanced equation 

e.g. H₂SO_{4 (aq)} + 2NaOH _(aq) --> Na₂SO_{4 (aq)} + 2H₂O _(l)

2. Calculate number of moles of solution of known concentration 

moles = conc. x vol (dm³)

3. Calculate number of moles of solution of unknown concentration (using molar ratio)

4. Calculate concentration of solution  

conc. = moles / vol (dm³)