= 10.6g
OR 0.25 moles = 2.56g
This therefore tells me that 2.56g dissolved in 250cm3 water will produce a solution with a concentration of 0.1moldm-3
The sodium carbonate should be weighed using a top pan balance to 2 decimal places using the tere facility. The beaker in which the sodium carbonate will be placed should be weighed first, or zeroed before adding the solid by spatula so that the weight of only the sodium carbonate is weighed.
Distilled water should added to the beaker and stired using a stirring rod utill all of the sodium carbonate is dissolved in the water.This can now be added to the volumentric flask using the funnel.
Once it has been added it is important that the stirring rod, beaker and funnel are washed down with distilled water so that all traces of the sodium carbonate enters the flask.
Distilled water can be added until the liquid level is 1-2cm below the mark.
When the graduated mark was nearly reached, a 1cm3 pipette should be used to add water into the volumetric flask to make an exact 250cm3 solution when the bottom of the meniscus is just touching the graduation mark.
The solution should be mixed to ensure that the solution is homogenous.
Titration
Once the solution has been made, a small amount of this should be transferred to a conical flask for the titration. Using a pipette 25cm3 of the sodium carbonate solution will be transferred to the conical flask and a few drops of methyl orange indicator will be added, this is a measure of the alkalinity of the sample, changing colour as it changes from alkaline to acidic and is usually used in titrations such as this mixing a strong acid (sulphuric acid) and a weak alkali (sodium carbonate) This is therefore approprite for the experiment I am doing.
Before the titration can take place the apparatus must be set up.
Before the burette can be clamped in place it is important that it it rinsed through with sulphuric acid to ensure that any chemicals remaining in the burette are disposedof and so there is no contamination which may affect the results. This is best done by placing a small amount of sulphuric acid and tilting and rotating the buret as to rinse out the entire inside wall and allowing some of the acid to run through the buret tip.
Clamp the burette to the clamp and fill it up with sulphuric acid past the zero mark, and slowly open the burette tap until the bottom of the sulphuric acid meniscus is on the zero mark allowing the acid to flow through the tap and place the conical flask containing the sodium carbonate underneath on top of a white tile, this tile will help colour changes to be seen more clearly.
The acid can now start to be added to the conical flask in small amounts at a time, swirling the flask after each addition.
Once it reaches the end mark the solution will turn from a red colour to a yellow one.
Once it had reached this point I will stop adding the acid to the flask. A reading will be taken of the amount of acid was added to the nearest 0.05cm3.
The burette can be refilled and the titration done several times until several concordant results are achieved, this means that my results will be more reliable.
Safety
There are some safety precautions which should be taken to ensure that no-one is harmed during the experiment. The first is that googles should be worn as all times. This is because the acid is an irritant an so eye protection should be worn as well as gloves and a lab coat to protect the skin. Handle with care. Any acid spilled on the skin or splashed into your eyes should be rinsed with water and cleaned up immediately.
Reliability
There are some things which can be done to ensure results are as reliable as possible. The first is to ensure that no quantity of any chemical is lost after measuring by rinsing out containers after the solution is transferred. The burette should be read to the nearest 0.0 5cm3 where the bottom of the meniscus is touching. The titrations should be repeated until three concordant readings are achieved to 1.d.p.
Average titre of sulphuric acid = 23.45
The first titration was a trial to get the rough idea of the amount of acid which should be added therefore this reading is discarded the average titre is calculated using readings 2, 3 and 4.
Calculating percentage uncertainties for each measurement
Uncertainty using pipette = (0.04/25) x 100 = 0.16%
Uncertainty using balance = (0.008/2.56) x 100 = 0.31%
Uncertainty using volumetric flask = (0.2/250) x 100 = 0.08%
Uncertainty using burette = (0.08/23.45) x 100 = 0.34%
Finding the percentage of uncertainty of each stage of the experiment allows me to calculate the total percentage uncertainty of the experiment
Total percentage uncertainty = 0.16 + 0.31 + 0.08 + 0.34 = 0.89%
Calculating the concentration of the acid
H2SO4 (aq) + Na2CO3 (aq) = Na2SO4 (aq) + H2O (l) + CO2 (g)
Since sodium carbonate and sulphuric acid react in a 1:1 mole ratio –
Moles of H2SO4 = moles of Na2CO3
Conc. H2SO4 x Vol. H2SO4 = Conc. Na2CO3 x Vol. Na2CO3
Therefore
Conc. H2SO4 = Conc. Na2CO3 x Vol. Na2CO3 / Vol. H2SO4
= 1 x 25 / 23.45 = 1.06 moldmcm-3
As the overall uncertanty of the experiment was 0.89 I can say that the absolute uncertanty is
1.06 x 0.0089 = 0.009
therefore - concentration of H2SO4 = 1.06 +/- 0.009 mol dm-3
Sources
- ‘Experimental error and error analysis’ – Chemical Review Magazine - Nov’01 - by Alasdair Thorpe
