Aim: In this experiment I am finding out how much sulphuric acid is present in sodium carbonate solution.
How I will find the accurate concentration of sulphuric acid
I am to find the accurate concentration of sulphuric acid ranging between 0.05 and 0.15-mol dm-3, I am provided with solid, anhydrous sodium carbonate, and range of indicators. The indicator which I will be using in my experiment is methyl orange, because it is used in most titrations, and it changes colour as a mid strength acid.
It has sharper end point then universal indicators. The pH of methyl orange is 3.1 – 4.4.
This is the chemical balanced equation that will take place, during my experiment.
Equation: H2SO4 (aq) + Na2COaq) � Na2SO4 (aq) + H2O (l) + CO2 (g)
From looking at the equation we can see that one mole of aqueous sulphuric acid reacts with one mole of sodium carbonate. (These are the reactants). This forms the (products) which is one mole of aqueous sodium carbonate, one mole of water and one mole of carbon dioxide.
The balanced chemical equation is in a 1:1 ratio.
Choosing the concentration of sodium carbonate solution
The concentration of sodium carbonate which I will choose will be 0.1 mol dm-3, I have chosen this concentration because it is halfway between 0.05 and 0.15 moldm-3 of sulphuric acid concentration, I added, 0.05 and 0.15 and found out the average concentration which I could use was 0.1 mol dm-3, this has therefore given me a basic idea that my sodium carbonate solution should be ranging near that concentration.
I will be making a 250cm3 solution.
Working out the mass of sodium carbonate used
1000cm3 is equivalent to 1dm3. I need to convert 250cm to dm3. This is done by dividing 250cm by a 1000 (250 1000), which equals to 0.25dm3. Now I have worked out the volume of anhydrous sodium carbonate needed is 0.25dm3.
Working out the number of moles of sodium carbonate needed
We can use the formula n=cv to work out the number of moles of sodium carbonate needed.
N: Represents Number of moles (mol)
C: Represents concentration (mol dm-3)
V: Represents Volume (dm3)
I need to rearrange the equation to work out the number of moles of sodium carbonate needed.
This is the rearranged equation
Moles = concentration (mol-1 dm-3) x volume (dm3)
Therefore,
0.1 x 0.25 = 0.025 mol
0.025 mol of sodium carbonate is needed.
Working out Mass of sodium carbonate needed
I can work out the mass of sodium carbonate needed by using the equation:
Mass
________ = Amount in Moles
Molar Mass
I already know that 0.025 mols of sodium carbonate are needed. I need to work out the unknown, which is the Relative formula mass of sodium carbonate.
Firstly I know that the formula of sodium carbonate is: Na2CO3
Finding out the relative mass of each individual element, and then adding each of the elements relative mass together to give an overall molecular mass can work out the relative molecular mass.
The mass of Sodium (Na) is: 23
There are 2 sodium atoms; therefore the mass of sodium (which is 11) is multiplied by 2 to give 22 (23 x 2 = 46)
The mass of Carbon (C) is 12
The mass of Oxygen (O) is: 16, however there is 3 atoms of Oxygen therefore; we need to multiply the mass of Oxygen (which is 16) by 3 to give 48.
The masses of all the elements, are added together
Na2 C O3
46 + 12 + 48 = 106
Therefore the relative molecular mass of sodium carbonate is 106 RMM
Now that I know 0.025mols of sodium carbonate is needed, and the relative molecular mass of sodium carbonate is 106. I can apply this to the equation.
Mass
________ = Amount in Moles
Molar Mass
I will need to rearrange the equation
Therefore: Number of moles (mol) x Molar Mass = Mass
0.025 x 106 = 2.65g
2.65g of anhydrous sodium carbonate crystals are needed.
Apparatus used
1 Graduated Conical Flask
2 Volumetric Flask (250cm3) -Used to prepare standard solutions of solute
3 Burette
4 Pipette- to deliver accurate volume of sodium carbonate solution into the conical flask
5 Electronic Weighing scale- to weigh anhydrous sodium carbonate
6 Beaker (Optional)
7 White tile
8 Pipette filler to adjust the volume of sodium carbonate solution being entered in the conical flask and to make an accurate measurement of sodium carbonate solution.
9 Wash bottle (Distilled water)- To make sodium carbonate solution, and wash of any impurities, in the apparatus being used.
10 Retort stand
11 Boss and clamp
12. Small filter
Making the sodium carbonate solution
1. Use a weighing bottle/boat weighs out accurately (using an electrical weighing scale), about 2.65 g of anhydrous sodium carbonate. Record the exact mass of anhydrous sodium carbonate transferred, in a suitable table; calculate the weight of the weighing boat.
2. Before using any apparatus make sure that all apparatus are clean, there is no impurities left over, make sure to wash apparatus with distilled water. This is done because impurities, or any precipitates left over can have an effect on the results of the experiment, or can alter the mass of solution.
3. The sodium carbonate solid should be transferred in to a 250-cm3 beaker. Distilled water should be added to the sodium carbonate and dissolved, make sure that the distilled water is made up to the meniscus.
.4 Transfer the Sodium carbonate solution into a 250cm3 volumetric flask; keep adding distilled until it is made up to the mark shown on the flask. Dilution is carried out by a factor of 10 exactly. A pipette is used to transfer 25.00ml of the solution to be diluted to a volumetric flask. The level of solution in the volumetric flask is brought nearly to the reference line by addition of water, the flask is stoppered and shaken, and then drops add water until the level corresponds exactly to the line. The flask is stoppered again, then shaken with inversion and swirling to give a homogeneous solution.
5. Additional distilled water should be added, rinsing the inside of the volumetric flask, funnel and beaker, to make sure that any sodium carbonate left over is added to the solution. If there is some sodium carbonate not dissolved this could affect the results, reducing the overall mass of sodium carbonate.
6. After making the solution make sure the lid on the volumetric flask is closed, as keeping the lid of the volumetric flask open, can lead to the solution being evaporated resulting in a loss of the solution. The volumetric flask should be shaken, by inversion and swirling for at least 1 minute to give a homogenous solution.
7 Before filling the burette with sulphuric acid, a small volume of sulphuric acid should be used in the burette making sure there are no impurities left over, and cleansing the burette. After adding the rinse solution the burette should be rolled between your finger and the rinsing solution should be poured out this allows the entire inside of the burette to be coated with rinse. A burette should be set up and filled with the acid solution; in this case it is Sulphuric acid (H2SO4). A small funnel should be used, to avoid sulphuric acid from spilling, and the sulphuric acid should be added to the burette slowly. Make sure to add the acid up to the desired starting point; in this case it is 0ml
.8.A small volume (2-3ml) of sulphuric acid should be released from the burette and transferred into a waste beaker, this is to make sure there is no air bubbles present in the jet and as this would give you a false, volume reading and also it rinses the burette tip. If this is not done then there are two risks: (a) contaminating the solution with any impurities present in the tip, and (b) having your solution volume going down inside the burette but no solution gets out of the tip and into the target reaction inside the flask.
9. The burette should now be placed in the burette holder, and properly adjusted. The entire assembly should be moved up or down as needed. It is best to use burette holder for a coarse height adjustment and then the burette should be moved up or down for a final adjustment. The burette should always be held in a vertical position when taking the readings and during the titration.
10. Attach the pipette filler to the pipette. A clean pipette should be used and it should be free from grease, so that the drops of the solution will not adhere to the walls, causing the pipette to deliver less than the rated volume. Any contaminant may affect the results. Using the pipette filler and keeping it in a vertical position place, the tip against the wall of the conical flask (approx 30 angle) just above the surface of the sodium carbonate solution, transfer 25 cm3 of the sodium carbonate solution from the volumetric flask into to a 250 cm3 conical flask. (The conical flask should be placed on a white tile). Remove the forefinger and allow draining at a vertical position until the continuous outflow ceases, then remove the pipette. A small amount of the solution will and remain at the tip of the pipette, this should not be blown out. Let the sodium carbonate solution flow out drop by drop.
10. Add 1-2 drops of methyl orange solution to the sodium carbonate solution.
11. The sulphuric acid solution should be added slowly from the burette (2-3ml of sulphuric acid at a time). The stop clock should be turned slowly, the thumb and forefingers should wrap around the handle of the stopcock, to turn the cock, and apply inward pressure, to keep the plug seated. Use the left hand to open the stop clock. The conical flask should be swirled at the same time using your right hand, small volumes of sulphuric acid should be added, until the indicator changes colour, at the point where there is a spark of colour change the sulphuric acid should then be added drop by drop, this is because we want to record the exact moment of colour change. Use a white tile to see the colour change accurately.
12. The sulphuric acid drops should stop being added until there is a permanentnt colour change in this case the colour change from yellow to pink. Distilled water should be used to rinse the inside of the conical flask, since swirling may have unrelated sulphuric acid on the walls. If the colour change persists add one more drop of sulphuric acid (just to be sure) and then stop. Record the ending burette volume in the data table it should be recorded to at least 2 decimal places to maintain accuracy of the reading, at this point this will allow you to find an approximate value for the volume of sulphuric acid needed to react with the sodium carbonate solution.
13. Before repeating the titrations all apparatus should be cleansed with distilled, water, this includes the conical flask, the burette, and the pipette to avoid contamination and to get of any impurities.
14. The titration should be repeated a number of trials (possibly 4-5 trials) and. Repeat the titration until they are reliable and concordant results are obtained, (concordant means, results are very similar to each other, and there are no analomies present), that will allow you to find the accurate concentration of the acid solution.
15. For clean up the, contents should be dumped down the drain with
Plenty of running water. All apparatus should be rinsed and burette
Should be clamed upside down with the stopcocks open?
Weight of boat and sodium carbonate together is 3.77g. Therefore to find the mass of the boat it is: 3.77- 2.64 = 1.13g
Mass
Sodium Carbonate
2.64g
Weighing boat
1.13g
Risk Assessment
Chemical Procedure
Hazard
How to reduce risk
Disposal
Source of advice
Sodium carbonate
Irritant
Wear safety goggles, and wear lab coat. Avoid skin contact
In the case of spillages, skin contact, eye contact rinse immediately with water.
Methyl Orange
Methyl orange is toxic if swallowed, spilled or inhaled.
Avoid skin contact with the solution or the solid. Wear safety goggles and gloves, for protection of spillages on skin and eyes.
Small volume of Methyl orange solution can be flushed down a sink with a large quantity of water
Fixing Pipette into pipette filler
If pipette is made out of glass, it should be handled carefully as it can break.
Use the correct procedure, attach the pipette filler gently to the pipette
If pipette breaks, do not touch clear away with dust pan and brush
Glassware
Breaking
Handle glassware very carefully, such as conical flasks, beakers, burettes etc
If broken, do not touch immediately clear away with dustpan and brush.
Weighing balance
Electrical shock
Handle electronic weighing scales carefully
If shocked medical advice should be seeken.
Sulphuric Acid 0.05-0.15dm-3
Low concentration of H2SO4 can act as a minor irritant.
Wear goggles, for eye protection avoid contact with skin, in case H2SO4 is spilled.
. H2SO4 can be disposed down the sink as it is only very low concentration
Chemical Procedure
Hazard
How to reduce Risk
Disposal
Source of Advice
Spillages
Any chemical spillages can act as a irritant to the skin and eyes
Wear safety goggles, and a lab coat, handle all chemicals carefully
In the case of spillages immediately alert the teacher, and clear away with dust pan and brush
Long hair tied back
Analysis
Titration results
Starting point
Ending point
Titre (volume dm3)
Rough trial
0.0
23.63
23.7-0= 23.6
First trial
0.0
23.63
23.63-0.0= 23.63
Second trial
0.0
23.63
23.63-0.0=23.63
Third trial
0.3
23.82
23.82-0.0=23.82
Fourth trial
0.0
23.63
23.63-0.0=23.63
0.0
Average titre
23.575 = (23.6 )
To work out the concentration it is number of moles of divided by volume
Concentration (mol dm-3) = Number of Moles (mol)
________________
Volume (dm3)
To work out the concentration of sulphuric acid the number of moles need to be known. From analyzing my average titration results, I can see that the average volume of sulphuric acid is 23.6cm3. 23.6cm3 of sulphuric acid needs to react with sodium carbonate. I need to convert 23.6cm3 to dm3. I know that 1dm3= 1000cm3, therefore I need to divide 23.6cm3 by 1000 this equals to 0.0236dm3.
23.6 1000 = 0.0236
I need to refer back to my planning and look at the chemical equation.
Equation: H2SO4 (aq) + Na2CO3 (aq) � Na2SO4 (aq) + H2O (l) + CO2 (g)
We can see that from the balanced chemical equation, the reactants are in a 1:1 ratio.
To find the concentration of sulphuric acid, you need to find the number of moles of sodium carbonate present in the 250cm3 volumetric flask. This can be done by dividing the mass by the relative molecular mass .
Mass (in grams)
_______________ = Number of moles of sodium carbonate
Relative Molecular Mass (mol-1g)
2.64 (g)
_______ = 0.0249 mol
106 (mol-1g)
0.0249 mol of sodium carbonate is present in 250cm3 solution.
Next I need to work out the concentration of sodium carbonate.
Rearranging the equation gives me:
Number of moles (mol)
_______________ = Concentration (mol dm-3)
Volume (dm3)
0.0249
_______ = 0.0996 mol dm-3
250
I have worked out the concentration of sodium carbonate solution, which is 0.0996 mol dm-3.
The number of moles present in the 25cm3 sulphuric acid solution used in the titration could be worked out by the following equation. But first I need to convert 25cm3 into dm3. I know that 1 dm3 is equal to 1000cm3. Therefore I need to divide 25cm3 by 1000 to get 0.025dm3 (25 1000 = 0.025).
This is the equation to work out the number of moles of sulphuric acid present in 25cm3 solution.
Concentration (mol dm-3) X Volume (dm3) = Number of Moles (mol)
0.0996 X 0.025 = 0.00242mol
The number of Moles present in the sulphuric acid solution is 0.00242. It is in a 1:1 ratio.
This leads me to working out the accurate concentration of sulphuric acid. I know the general equation:
Number of Moles (mol) = Concentration (mol dm-3) X Volume (dm3)
I can rearrange the equation to give me:
Number of Moles (mol)
________________ = Concentration (mol dm-3)
Volume (dm3)
0.00242
_______ = 0.097188mol dm-3 (0.0972 mol dm-3)
0.0249
The accurate concentration of sulphuric acid is therefore: 0.0972mol dm-3, which is therefore between the predicted range of sulphuric acid 0.05 and 0.15-mol dm-3
Evaluation
When conducting my titration experiment, I gained a few anomalous results. During the first few trials, my titres were not concordant and accurate. One of the mistakes I made was adding 3-4 drops of Methyl orange, I added too much indicator. Adding excess indicator to the solution or adding too much indicator can affect the results of the titration.
?
Equipment
Size (cm3) and class
Tolerance (cm3)
Percentage Error %
Vol Flask
250/A
+0.15
0.15
_____ x 100= 0.06
250
Pipette
25/A
+ 0.03
0.03
_____ X 100 = 0.12
25
Burette
50/A
+0.05
0.05
____ X 100 = 0.212
23.6
Weighing Scale
Electronic Weighing Scale
+ 0.005
0.005
______ X 100 = 0.18
2.64
?
Percentage Errors
Percentage errors are calculated: Uncertainty
___________
Result
The percentage errors, which I have calculated, are for volumetric apparatus mostly, and the percentage errors, involved are only very minimal, and negligible, therefore there will not be a huge variation, or error in my results. From analyzing my results, I can see that the largest percentage error was with the burette, which was 0.212%, and the smallest percentage error was the volumetric flask with 0.06 %. The difference between the largest and smallest percentage error is therefore 0.152%.
Errors of Volumetric Glassware
Volumetric Vessels are mostly calibrated at 20 C .In order to gain precise and accurate measurements, all measurements should be taken at this temperature, and this was again beyond my control, as the room temperature varies. Instead the magnitude of the errors should be introduced into the volumetric procedures, by using standard solutions other then 20C (68 F). There is a change in capacity of glass volumetric apparatus with temperature is mostly one part, in 10,000 for each 5C change in temperature in the region of 20C. This source of percentage error, is mostly disregarded in most experiment, and one of which I have disregarded in my experiment.
Most apparatus are not clean and may be contaminated; it needs to be rinsed well with distilled water. Impurities, or precipitates left over can greatly affect the overall results, if a pipette had not been used for sodium carbonate solution, and it wasn’t rinsed, with distilled water, sodium carbonate would become diluted, and you would be adding less moles, of sodium carbonate then you thought. Also if a equipment had not been washed with distilled water there is the possibility of grease being present, grease prevents from glass walls being uniformly wetted, causing drainage to be uneven and imprecise.
Drainage Errors
Unless the titration to the end point has been slow and gradual. It is best to wait 30 seconds before taking the final titre reading from the meniscus, so the effect of further drainage will be negligible and to maintain complete accuracy. The meniscus reading should be read at eye level.
However, percentage errors, present in volumetric equipment, are beyond my control. There may have been some percentage errors in the chemicals that I was using, when conducting my titration experiment.
Meniscus Errors
When recording down my end point of the titration (i.e. the volume). I did not look properly at the point of the meniscus, therefore gaining a very minimal percentage error of about 0.10%. It is very important that the lowest point of the meniscus is observed very carefully and that the line of vision is in the same horizontal plane as the bottom of the meniscus. This is easily ascertained if the graduations on the glassware extend at least halfway around the tube. The eye is correctly positioned when both front and back portions of the graduation coincide). The meniscus may be seen more clearly if a small white card with a rectangular black patch is held behind the meniscus. Raise or lower the card until the bottom of the meniscus is clearly outlined. A wrong reading of the meniscus can lead to a inaccurate result, and the results will not be concordant.
.
Pipette errors
A pipette should be used with a perfect tip. A pipette, which has a broken or chipped tip, must be discarded since it will deliver a volume other then the rated volume when the tip is touched against the wall of the receiving vessel. Before I added, the sodium carbonate solution, I made sure that I wiped of any drops adhering to the outside of the pipette as this prevents droplets from the outside from draining into the receiving vessel (conical Flask), and affecting the results. I made sure that the pipettte was recleaned immediately before use, because if it stands more than an hour under ordinary conditions there is the possibility of air-contamination, which will affect my results.
Overall my results are accurate, and concordant. During my titration experiment, I had gained two anomalous results, which were not concordant; I rejected these anomalous results, when calculating my average, as taking the anomalous results when calculating the average titre, would give me inaccurate results, when working out the concentration of sulphuric acid. The anomalous results are shown in the table below.
Starting point
End point
Titre volume(dm3
Trial 5
0
20.35
20.3-0=20.3
Trial 6
0
21.45
21.4-0=21.4
In order to gain even more accurate concordant, results, each student could of had, a new batch of Sulphuric acid, although I made sure, that the solution was not contaminated, and used apparatus which were clean, and had been rinsed, I can not be sure, that other students, may have contaminated the sulphuric acid, solution, or the diluted water, this was beyond my control. Adding impurities, could affect my results, and lead to slight inaccuracy.
In most titration experiments, a number of trials are done; I had only done one rough trial and 3 trials, following this. In order to gain even more concordant results I would have to conduct more trials, as increasing the number of trials will gain a more accurate end point.
References
(Volumetric analysis titration)
http://216.239.59.104/search?q=cache:g-Wqe01MihoJ:firstyear.chem.usyd.edu.au/LabManual/A5.pdf+white+tile+in+titration&hl=en&ct=clnk&cd=2&ie=UTF-8
http://ptcl.chem.ox.ac.uk/~hmc/hsci/chemicals/methyl_orange.html
Chemical Safety- data methyl orange
Volumetric questions
http://www.wpbschoolhouse.btinternet.co.uk/page06/Mtestsnotes/ExtraVolCalcs1.htm.
Salter’s chemistry
Working out concentration (pages 10)
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