I have chosen 0.1 mol dm as the as the concentration of sodium carbonate that I am going to use. This is because I am told that the sample of acid rain that I am given has “ a concentration between 0.05 mol dm and 0.15 mol dm .” since I know that the ratio of the solutions is 1:1, I need to make the concentration similar. I have taken the mid-point, which is 0.1 mol dm to be the concentration of sodium carbonate.
Method:
To start with you need to make a standard solution of sodium carbonate in a volumetric flask. You want to make 250cm ³ of a 0.1 mol dm of sodium carbonate solution. Therefore, to start with accurately weigh 2.65g of the solid sodium carbonate in a weighing bottle, by using a balance that reads to two decimal places. For accurate weights when using the balance, place the weighing bottle on the balance pan and allow the reading to stabilize. Press the tare bar to bring the reading to zero. Place the solid sodium carbonate in the weighing bottle using a spatula. Then allow the reading to stabilize. If you add to much sodium carbonate take great care removing it using the spatula.
If you accidentally drop any of tit around the balance remove it. Then put the 2.65g of sodium carbonate into a 100cm ³ beaker of distilled water and stir it using a glass rod. Avoid splashing when stirring. Make sure that all of the substance is used in making the solution by washing the weighing bottle repeatedly several times until none of the substance is remaining on it. Transfer the washings to the beaker each time. Keep stirring the solution with a glass rod until all of the sodium carbonate has dissolved in the water. Once the entire solid has dissolved, rinse the glass rod with distilled water, in order to remove any of the solution that is still on it. After that transfer the solution in the beaker into the 250cm ³ volumetric flask using a small filter funnel to pour it out without any spillage. Keep pouring the solution until it is a few centimetres away from the graduation mark, then fill slowly using a tit pipette until the bottom of the meniscus is level with the graduation mark. It is important that you read the volumetric flask at eye level with the bottom of the solution’s meniscus and take the reading from this point to avoid any inaccurate reading of the meniscus. When filling the volumetric flask allow time for the solution to run down the walls.
Now that the 250cm ³ solution of 0.1 mol dm is made place the top on the volumetric flask and turn it upside down several times so that the concentration is evenly spread throughout the volumetric flask. Ensure the burette tap is closed. Then pour a small amount of the sodium carbonate solution into the burette making sure that the jet is full and there are no air bubbles inside it. Use a small filter funnel to fill the burette. Wash out the burette with this solution and then open the tap to allow the liquid out. Fill the burette again this time allowing the liquid to fill the tubing below the tap before use. Mount the burette vertically on a clamp stand, being carefully that the clamp is not overtighten. Record the volume in the burette reading the burette at eye level. Take the meniscus reading, note down the value.
25cm³ of acid rain with the unknown concentration is put into a conical flask using a 25cm³ bulb pipette. At first, 25cm³ of acid rain is measured using a measuring cylinder and pour it into a beaker. Use a 25cm³ bulb pipette to draw up the acid rain. For safety reasons it is no longer permissible to use your mouth on the pipette. Therefore, use a pipette pump instead. The pump should be placed on the tip of the bulb pipette, which is in the solution; drawing up slowly the acid rain to the graduation mark on the scale. Check to make sure no air bubbles have been drawn up. Remove the pump slowly replacing it with your thumb. Keeping your thumb on the top of the bulb pipette move it to the conical flask. Release your thumb off the bulb pipette, placing the tip against the wall of the conical flask at a slight angle. Wait a few seconds for the liquid to leave the bulb pipette.
The conical flask is place on a white tile under the burette. The white tile makes it easier to see the end-point of colour change. Place the conical flask centrally on the white tile. Add 1-2 drops of methyl orange indicator. Turn on the burette tap, while swirling the flask at the same time. The end point of the titration should be when the two solutions have reacted without neither in excess. In order to achieve this, stop when the indicator changes colour to a red/orange colour. This red/orange tinge indicates an excess of sodium carbonate and therefore the end of the titration. A single drop of the alkali could being about this change, so add the solution from the burette to the acid rain drop by drop. Measure the new meniscus reading. The volume dispensed is the difference between the two readings.
Conclusion:
The plan I have devised is likely to provide precise and reliable results because the burette, pipettes and beakers are washed with the solution that is going in them before using them. This is because if water is in the vessels it would dilute the solution. If acid is present it would react with the alkali making its concentration lower. Therefore a bigger volume is used making the final concentration higher.
- Minimum drops of indicator is added.
- End-point colour = keep previous one to compare
Bibliography: student worksheet, chemistry in context, activity EL2.1 and practical skills in biology.
Analysis:
Results:
Mass of solid anhydrous Sodium Carbonate➔ 2.65g
Concentration (moldmˉ³) = Amount (mol) ÷ Volume (dm³)
C = n ÷ v
Amount (no. of mols) = Mass (g) ÷ Molar Mass (gmolˉ³)
n = M ÷ A
Sodium Carbonate (Na2C03)
(23x2) + (12) + (16x3) = 106gmolˉ³
Sulphuric Acid (H2SO4)
(2x1) + (32) + (16x4) = 98gmolˉ³
C (Na2CO3) = n ÷ v = 0.025mol ÷ 0.25dm³
= 0.1moldmˉ³
The concentration of the Sulphuric acid is therefore 0.10moldmˉ³.
Evaluation:
I feel that my practical went well, and was successful. I have achieved the correct concentration of Sulphuric acid, since the concentration of acid is 0.10moldm ˉ³, which is in between 0.05moldmˉ³ and 0.15moldmˉ³. Also, I had made sure that all my equipment had been washed with water, so that the experiment was fair. However there are some errors that may have come from measurements such as from burette, conical flask, and volumetric flask readings due to the percentage error, or due to the meniscus.
Overall, I think my titration was accurate, and so was my working out, since I have followed the correct procedure, and that my concentration of both acids were to a 1:1 ratio. Including that I have used the right type of indicator for the experiment.
