Acidic Solutions
INTRODUCTION:
The purpose of this assignment is to find the accurate concentration of a solution of sulphuric acid acid of which concentration is thought to be between 0.05 and 0.15moldm-3 . I am provided with solid anhydrous sodium carbonate and a range of indicators. I have to plan and carry out an experiment to determine the accurate concentration of the sulphuric acid in order to meet the requirement for the assessment of my experimental skills in unit 2852/02 courswork.
BASIC PLAN:
* I will make a standard solution of sodium carbonate and calculate its concentration.
* After making the sodium carbonate solution of known concentration, I will use it to determine the concentration of the sulphuric acid. This will be possible through an acid-alkali titration. Sodium hydroxide is a weak alkali and will neutralise the acid in the reaction to produce salt, water and carbon dioxide. The data obtained from this quantitaive analyses will then be used to calculate the concentration of the sulphuric acid.
MAKING A STANDARD SOLUTION OF SODIUM CARBONATE
APPARATUS:
* Safety spectacles
* Weighing bottle
* Spatula
* Solid anhydrous sodium carbonate
* A balance capable of weighing to within 0.01g
* A beaker, 250cm3
* Wash bottle of distilled water
* Stirring rod with rubber end
* Volumetric flas with stopper and labe, 250cm3
* Dropping pipette
* Mortar and pestle.
REACTING QUANITITES:
Na2CO3(s) + H2SO4(aq) Na2SO4 (s) + H2O(l) + CO2(g)
: 1
mole : 1mole
In the titration, I will be using a 25cm3 pipette therefore the volume of the Na2CO3 used will be 25cm3. From the chemical equation above, it can be seen that the stiochometry of the reaction is 1:1, therefore the volume of the H2SO4 will also be aimed at 25.00cm3 .
The concentration of the H2SO4 is thought to be between 0.05 and 0.15 moldm-3 . Assuming the concentration of the acid to be 0.05 moldm-3, the concentation of Na2CO3 can be calculated as follows:
Conc of H2SO4 (c) = 0.05 moldm-3
Volume of H2SO4 (v) = 25cm3 = 0.025dm3
Number of mole (n) = cv = 0.05 x 0.025 = 0.00125mols
0.025dm3 of Na2CO3 solution will contain 0.00125mols of H2SO4
concentration of Na2CO3 = n/v = 0.00125/0.025 = 0.05 moldm-3
n = mass/ relative molecular mass
number of moles (n) of Na2CO3 = 0.00125mols
Relative molecular mass of Na2CO3 = (23 x 2)+ 12 + (16 x 3) = 106g
therefore, mass of Na2CO3 = n x relative molecular mass
= 0.00125 x 106 = 0.1325g
If 25cm3 contain 0.1325g of Na2CO3,
250cm3 will contain 0.1325 x 250
25 = 1.325g ? 1.33g
Assuming the concentration of H2SO4 to be 0.15 moldm-3 , the concentation of Na2CO3 can be calculate as follows:
Conc of H2SO4 (c) = 0.15 moldm-3
Volume of H2SO4 (v) = 25cm3 = 0.025dm3
Number of mole (n) = cv = 0.15 x 0.025 = 0.00375mols
0.025dm3 of Na2CO3 solution will contain 0.00375mols of H2SO4
concentration of Na2CO3 = n/v = 0.00375/0.025 = 0.15 moldm-3
n = mass/ relative molecular mass
number of moles (n) of Na2CO3 = 0.00375mol
relative molecular mass of Na2CO3 = (23 x 2)+ 12 + (16 x 3) = 106g
therefore, mass of Na2CO3 = n x relative molecular mass
= 0.00375 x 106 = 0.3975g
if 25cm3 contain 0.3975g of Na2CO3,
250cm3 will contain 0.3975 x 250
25 = 3.975g ? 3.98g
From the calculations above, I must weigh accurately a sample of Na2CO3 beteeen 1.33 and 3.98g and use it to make a solution of Na2CO3 of concentration between 0.05 and 0.15moldm-3 .
PROCEDURE:
. I will tranfer some quantity of solid anhydrous Na2CO3 into a mortar and grind it using a pestle till fine powder is obtained.
2. I will transfer between 1.33 and 3.98g of solid anhydrous Na2CO3 from the mortar into a weighing bottle and weigh it to the nearest 0.01g.
3. I will put about 50cm3 of distilled water into the 250cm3 beaker and then carefully transfer the bulk of Na2CO3 from the weighing bottle into the beaker of water.
4. I will reweigh the weighing bottle with any remaining Na2CO3 to the ...
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PROCEDURE:
. I will tranfer some quantity of solid anhydrous Na2CO3 into a mortar and grind it using a pestle till fine powder is obtained.
2. I will transfer between 1.33 and 3.98g of solid anhydrous Na2CO3 from the mortar into a weighing bottle and weigh it to the nearest 0.01g.
3. I will put about 50cm3 of distilled water into the 250cm3 beaker and then carefully transfer the bulk of Na2CO3 from the weighing bottle into the beaker of water.
4. I will reweigh the weighing bottle with any remaining Na2CO3 to the nearest 0.01g
5. using a stirring rod, I will stir the solid and water in the beaker to dissolve, adding more water is necessary.
6. I will transfer carefully the solution to the 250cm3 volumetric flask (pouring the solution down the glass rod to avoid spillage). I will rinse the beaker three times to make sure all the solution goes into the volumetric flask, each time pouring the solution down the stirring rod to rinse it.
7. I will carefully make up the solution to about 1cm of the mark on the neck of the flask using distilled water. I will insert the stopper and shake to mix the contents.
8. Using a dropping pipette, I will add enough distilled water to bring the bottom of the meniscus on the mark. I will then shake it thoroughly ten times to ensure complete mixing
9. I will label the flask as "Na2CO3 solution" leaving a space for the concentration to be filled in after I have calculated it. I will set the flask aside for titration.
RESULTS AND CALCULATIONS:
I will record the data from the experiment in the table below:
Molar mass of Na2CO3, M
06 gmol-1
Mass of bottle and contents before transfer, M1
g
Mass of bottle and contents after transfer, M2
g
Mass of Na2CO3, m = (M1 - M2)
g
Amount of Na2CO3 , n = m/M
mol
Volume of solution, V
0.25 dm3
Concentration of Na2CO3 , c = n/V
moldm-3
TITRATION OF A SOLUTION OF SODIUM CARBONATE WITH SULPHURIC ACID OF CONCENTRATION BETWEEN 0.05 AND 0.15moldm-3.
After making the solution of sodium carbonate of known concentration, I will titrate it with sulphuric acid.
Na2CO3(s) + H2SO4(aq) Na2SO4 (s) + H2O(l) + CO2(g)
Sodium carbonate will react with sulphuric acid to form sodium sulphate salt, water and carbon dioxide.
But firstly, I have to make a choice of indicator to use in helping me to know the stiochiometric point or equivalent point of the reaction, which will signify the end of the reaction.
Because sulphuric acid is a strong base and sodium carbonate is a weak alkali the most suitable indicator is methyl orange (OCR information sheet, 2000), which will turn from red to yellow (Chemistry, by Ann Fullick, 1994).
APPARATUS:
* Safety glasses
* Small filter funnel
* Burette, 50cm and stand
* 2 beakers, 100cm
* Sulphuric acid, between 0.05 and 0.15moldm
* Pipette, 25cm
* Pipette filler
* The standard solution of sodium carbonate already prepared
* 4 conical flasks, 250cm
* Methyl orange indicator
* White tile and a piece of white scrap paper
* Wash bottle of distilled water
PROCEDURE:
. Using the funnel, I will rinse the burette with sulphuric acid (rinsing and filling the tip) and fill it with same solution. I will record the initial burette reading as trial.
2. Using pipette filler, I will rinse the pipette with some solution of sodium carbonate and carefully transfer 25.0cm of the solution into a clean 250cm conical flask.
3. I will add 2-3 drops of methyl orange indicator solution
4. I will run the sulphuric acid solution from the burette into the flask, with swirling until the solution just turns yellow. This will be used as a trial run as I might propably overshoot the end-point. I will record the final burette reading.
5. I will then refill the burette with sulphuric acid and again record the initial burette reading to the nearest 0.05cm3 (one drop).
6. Using the pipette, I will transfer 25.0cm of the sodium carbonate into another clean conical flask, adding 2-3 drops of the methyl orange indicator solution.
7. Carefully, I will titrate this solution to the end point, adding the acid drop by drop when the colour is about to change.
8. I will repeat steps 5, and 7 at least twice more and keep flasks for comparison.
RESULT TABLE:
Pipette solution
Sodium carbonate
moldm-3
25 cm3
Burette solution
Sulphuric acid
moldm-3
Indicator
Methyl orange
Trial
2
3
4
Burette
readings
Final
Initial
Volume used
(titre)/cm3
Mean titre/cm3
CALCULATIONS:
I will use the data obtained from the titration to calculate the concentration of the sulphuric acid. Knowing the average titre, volume and concentration of the sodium carbonate, I will calculate the number of moles of sodium hydroxide and use it to find the concentration of the sulphuric acid.
Chemical/
Procedure
Hazard
Source of advice
Strategy to reduce risk
Disposal
Sodium carbonate
Irritant
Hazcards
Use lab coat and safety glasses
Rinse area of spillage with water and remove contaminated clothing
Sulphuric acid 0.05-0.15 moldm-3
Minimal hazard. It is of low concentration.
Hazcards
Use lab coat and safety glasses
Rinse area of spillage with water and remove contaminated clothing
Methyl orange
Minimal hazard.
Stains
Use lab coat and safety glasses. Use dilute solution. Do not swallow.
Rinse area of spillage with water and remove contaminated clothing. If swallowed, rinse mouth with plenty of water.
Running the burette tap
Overshooting
Tutor
Always turn the tap with the left hand and swirl with the right.
If overshoot, repeat titration.
Fixing a pipette into a pipette filler
Breaking/
stabbing
Tutor/
Common
sense
Always hold the pipette vertically close to the open end and not the tip.
If broken, do not touch, use parker and brush to pack into the broken glass container. If stabbed, seek medical advice.
Glass ware
Breaking /stabbing
Common sense
Use proper procedure
If broken, do not touch, use parker and brush to pack into the broken glass container. If stabbed, rinse under running water and plaster. Seek medical advice.
Weighing balance
Electric shock
Student Safety cleapss
Do not touch exposed conductor. Make sure it is in good working condition and has been inspected recently
If shocked, seek medical attention.
RISK ASSESSMENT:
EVALUATION:
The sources of information that helped in the devising of this plan were:
Preliminary work:
Preliminary work done during the course, preparation of a solution of copper sulphate containing 124.84gdm-3, using a calorimeter to find the concentration of an unknown solution of copper sulphate, preparing a standard solution of potassium hydrogenphthalate and Experiment 3 from the " Negus 6th Form Centre science safety handbook and induction booklet". Also, how much iron is in a sample of an iron compound? In activity EL2.1 of the activity booklet helped. The information sheet on the use of indicators in acid alkali titrations helped in the choice of indicator. The colour change of methyl orange was learnt from Chemistry.
Preparation of a solution of copper sulphate containing124.84gdm-3:
This experiment was making a standard solution and introduced me to the idea of standard solutions and what they mean. It helped me in the handling of scientific equipment and materials. It also helped me in calculating the mass of sodium carbonate to use.
Using a calorimeter to find the concentration of an unknown solution of copper sulphate:
This experiment introduced me to another way of knowing the concentration of an unknown. From a standard solution.
Preparing a standard solution of potassium hydrogenphthalate:
This served as a guideline in making the solution of sodium carbonate. It also gave practice on how to make solutions and calculate their concentrations. It helped me on how to record my results clearly and systematically.
Experiment 3:
This experiment was a titration of the potassium hydrogen phthalate I had already prepared with a solution of sodium hydroxide of unknown concentration, which I was supposed to find. It showed me how to use a standard solution of known concentration to calculate the concentration of another solution. This was also a guideline on how to carry out the acid-base titration and improve my practical technique. It also helped on how to record my result for easy understanding.
How much iron is in a sample of an iron compound? :
This work helped me in having the basic idea of titration and how it works as one method of quantitative analysis. It also allowed me to learn how to perform a titration and make use of accurately calibrated apparatus and using the data obtained to carry out calculations required.
Calculations:
The calculations of amount of substances and concentration were done through the help of this topics in the induction booklet: Relative Mass, Amount of substance-the Mole, Molar Mass, Amount calculations, Calculating the number of particles in a given amount, Stoichiometry, Amount in solution, Calculating concentrations from volume and amount and Calculating concentration from mass of solute and volume. Chapter 1 of Salters Advanced Chemistry Chemical Ideas "Measuring amounts of substances" also helped in the calculations.
JUSTIFYING MY STRATEGY:
The equipments I choose were such that accuracy, precision and reliability are ensured.
Volumetric pipette (25cm3):
This was used because of its high degree of accuracy in quantitative analysis. It has an error of 0.06cm3 if used correctly (i.e. if it is allowed to drain and retain the last drop).
The percentage error = error x 100 = 0.06 x 100
Reading 25 =0.24%
This is a relatively small percentage and ensures that the volume of solution measured is accurate to the highest degree.
Volumetric flask (250cm3):
This is used to make standard solution of a particular volume. If a 250cm3 volumetric flask is filled correctly i.e. the bottom of the meniscus rests on the calibration line, the error is 0.2cm3.
The percentage error = error x 100 = 0.2 x 100
Reading 250 =0.08%
The percentage error is low and therefore the flask measures the volume to a high degree of accuracy.
Burette:
One drop from a burette has a volume of approximately 0.05cm3. All the burette readings should include 2 decimal places in which the second figure is either 0 or 5. An error of one drop in a volume of 25.00cm3 gives a percentage error:
The percentage error = error x 100 = 0.05 x 100
Reading 25 =0.2%
This is also low, meaning that the equipment has high accuracy.
Beakers:
The beakers are useful for measuring broad volumes of a solution. This is not very accurate but can give an approximate volume. I used it to measure the volume of water because the volume of water needed was approximate.
Conical flask:
These are good for titrations as they have a narrow neck (enough to fit in the burette tip, without breaking it when swirling), but broad base. This is particularly useful as the narrow neck prevents spillage when swirling and the broad base makes the solution and end-point easy to see.
Weighing balance:
A two place weighing balance has an error to 0.01g. Using it to measure a mass of 5g will have percentage error:
The percentage error = error x 100 = 0.01 x 100
Reading 5 =0.2%
This is low and has high degree of accuracy.
Apart from the errors and uncertainties related to the precision of the equipments used, procedural error may arise from practical techniques. This would include:
* Not mixing the solution in the volumetric flask thoroughly would make the solution have uneven concentration through out. Some areas would be more concentrated than others. A lower concentration would use less volume from the burette and this would make my result lower. A higher concentration would use more burette solution and would make my result higher.
* Not washing the burette and pipette with the solutions they are to contain before titrating can be a source of error as other substances may be present in the equipment. This can affect the concentrations of the solution, hence the volume of the titre used. The substances that will increase the value of the titre will make my result higher and the substances that will reduce the value of the titre will make my result lower.
* The conical flask needs to be thoroughly rinsed with distilled water in between titrations. This will remove any solution from the previous titration and not affect the concentration of the solution in which it is to contain, as the concentration of the solution is already known.
* Using a white piece of paper makes it easy to read off the volumes from the burette. As the sulphuric acid is aqueous and transparent, so is the burette. This makes it difficult to read the volume and can affect the degree of accuracy in which the volume is read to. A lower volume gives a lower result.
The mass of sodium carbonate (between 1.33-3.98g) I used in making the standard solution of sodium carbonate was ideal because it was calculated theoretically taking into consideration the given concentration of sulphuric acid (between 0.05-0.15moldm3).
The volume of sodium carbonate used in the titration (25cm3) was appropriate because using a 25cm3 pipette gives more accurate and reliable result than using a pipette with a greater volume capacity. Using a pipette of volume less than 25cm3 would not make available enough solution for the titration and I would be working with very small values that can cause errors. The 25cm3 pipette I used is not too small or too big, it is just reliable. Moreover, this is used in most standard titrations.
Making the solution of sodium carbonate to a volume of 250cm3 in a volumetric flask made available enough solution in case I overshoot pass the end-point and need to repeat the titration. This also ensures that there is enough solution to repeat titrations so that different values of titre will be collected and the average of them taken. This ensures accuracy.
