Calculations - Titrations

Question 1

Titration

Answer 1

Used to determine the concentration of a solution e.g. acid or alkali

moles mol
conc conc =?
vol vol

Key idea - we take a fixed volume of a solution with a known concentration, it is then reacted with a solution with an unknown concentration

if we accurately measure the volume needed to react, then we can determine the concentration of our solution

we are going to react an acid with unknown conc. with an alkali with a known concentration

1. measure an accurate fixed volume of our alkali - using a pipette (usually with a volume of 25 cm^3)

rinse the pipette with distilled water to remove any unwanted chemicals

to remove any droplets of water in the pipette, rinse the pipette with our alkali and discard the alkali down the sink

pipette would contain
droplets of water would dilute any solution which we used the pipette to measure

Next place the conical flask right next to the beaker containing the alkali
The conical flask should have been rinsed carefully with distilled water to remove any traces of unwanted chemicals (water will not interfere with reaction)

Next place the tip of the pipette into the alkali
and draw the alkali into the pipette using a pipette filler

do this carefully slowly to avoid bubbles

fill the pipette past the 25 cm^3 mark
This is because the level of the liquid drops slightly when we lift the pipette out of the alkali

carefully lift the pipette out of the alkali - very slowly release drops of the alkali - so bottom of meniscus will exactly lie on 25cm^3 mark - also important to view meniscus at eye level

Now, move the pipette over the conical flask and release the alkali

there is now exactly 25cm^3 of the alkali in the conical flask

Add a few drops of indicator into the conical flask - phenolphthalein
only a few drops as indicators are weak acids - so if a lot of indicator was added, that could give inaccurate results

burette - is used to measure the volume of acid which reacts with our alkali

rinse the burette with distilled water
removes any unwanted chemicals from the burette
then rinse the burette with acid to remove any traces of water

next clamp the burette so that it is level

now use a funnel and slowly fill the burette with acid
the level of acid should be slightly above the zero line
next remove the funnel

this prevents acid from dripping from the funnel into the burette

now open the tap and allow acid to slowly leave the burette
close the tap when the bottom of the meniscus (for acid) is exactly on the 0cm3 mark

always read meniscus at eye level

place conical flask contianing the alkali onto a white tile
this will make the colour change of the indicator easier to see accurately

Open the tap on the burette and slowly release the acid into the conical flask

At the same time, swirl the conical flask - swirling ensures that the acid and alkali mix thoroughly so they can react

while we swirl/the acid is being added to the conical flask, watch the colour of the indicator

stop adding acid when the indicator changes colour to show the end point

at this stage, read the level of the acid on the burette

uncertainty of burette - 0,05

substracting the start volume from the final volume gives us the titre (the volume of acid which reacted with our alkali)
record start and final volume to 2d.p.

our first titration is considered to be a rough titration

Now rinse the conical flask out, and repeat the titration

This time we have an idea of the titre, so when we approach the end point, we add our acid drop by drop

keep repeating our titration until we have two concordant titres-
concordant tires are within 0.1 cm3

now calculate the mean of the concordant titres

Question 2

creating a standard solution - with known concentration

Answer 2

Calculate the mass of required substance needed to produce 250 cm3 of a 0.100 mol dm-3 solution.

mol of naoh = conc x vol
mass = mol x mr

place a weighing boat on a digital balance and weigh out this mass of solid.
Tip it into a beaker.
Now re-weigh the boat (which may still contain traces of the solid)
Subtract the mass of the boat from the combined mass of the boat and solid to find the precise mass of solid use

Add distilled water to the beaker and stir until all the sodium hydroxide has dissolved

Tip the solution into a volumetric flask. Use a funnel to ensure it all goes in

Rinse the beaker and stirring rod with distilled water and add that to the flask too - this makes sure there is no solute clinging to the beaker or rod

now top the flask up to the correct volume with more distilled water
make sure the bottom of the meniscu reaches the line/graduation mark

when you get close to the line add the water drop by drop

stopper the flask and turn it upside down a few times to make sure it is mixed

Question 3

percentage yeild formuula

Answer 3

percentage yield = yield/max theoretical yield x 100

Answer 4

not all of the reactant may react - especially the case for reversible reactions

side reactions take place, making other products

some of our product might be lost for example during filtration