Finding out how much acid there is in a solution
Finding out how much acid there is in a solution
Aim: To determine the concentration of sulphuric acid by acid-base titration.
Introduction:
A titration is a procedure used in analytical chemistry to determine the amount or concentration of a substance.
Analytical chemistry involves the study of chemical substances usually by experiments. There are two branches of the analytical chemistry:
- Quantitative chemical analysis: concerned with the determination of the quantitative composition of pure compounds, mixtures and solutions.
2- Qualitative chemical analysis: involves deducing what substances are present.
Methods of quantitative analysis are:
- Volumetric (titrimetric) analysis.
2- Gravimetric analysis
3- Mass spectrometry
Volumetric analysis
It is used for to find the concentration of solutions by a technique know as titration. There are various types of titrations:
- A cid-base titration.
2- Conductimetric titration,
3- Potentiometric titration.
The principles of titration involve peppitting a portion (know as an aliquot) of the solution of unknown concentration (the titrate) into a conical flask. The solution is then titrated by adding a measured increment of a standard solution is then burette until the reaction is just complete.
The standard solution is the solution whose concentration is known. It is prepared by dissolving an accurately weighted amount of a substance in a known volume of liquid. The end-point of a titration is reached when the reaction between the titrant and the titrate is complete. Titre is the volume of the titrant added from the burette. In most titrations the end-point is visually recognized by a change in colour of an indicator, which has been added to the solution in the conical flask.
In this experiment I will prepare a solution of a know concentration of a sodium carbonate from solid anhydrous sodium carbonate. Using titration technique I will be able to find the concentration of sulphuric acid.
This is an acid-base titration therefore the chemical reaction I am expecting sulphuric acid to donate H, which will be accepted by the sodium carbonate. The end-point of this particular titration is when all the alkali has been neutralized and neither excess acid nor excess alkali is present in the solution. The solution consists of salt and water.
In order to able to recognize the end-point of reaction I need to add a suitable indicator. Different indicators are suitable for different combination of strong acid and weak acids and alkalis as follows:
Methyl orange: is used as the indicator for a titration between a strong acid and a weak alkali.
Phenolphthalein: is used as the indicator for a titration between a weak acid and strong alkali.
For a titration between a strong acid and a strong alkali, either methyl orange or phenolphthalein can be used as indicator.
For a titration between a weak acid and weak alkali, no indicator is really suitable therefore a ph meter, a conductivity meter or temperature probe has to be used. Hence sulphuric acid is a strong acid and sodium carbonate is a weak alkali, methyl orange is the most suitable indicator for this experiment.
Method
In this experiment the apparatus I will use are:
Apparatus:
* Sulphuric acid: the acid which is used for titration.
* Anhydrous sodium carbonate: the alkali which is used for titration.
* Methyl orange: the ...
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For a titration between a weak acid and weak alkali, no indicator is really suitable therefore a ph meter, a conductivity meter or temperature probe has to be used. Hence sulphuric acid is a strong acid and sodium carbonate is a weak alkali, methyl orange is the most suitable indicator for this experiment.
Method
In this experiment the apparatus I will use are:
Apparatus:
* Sulphuric acid: the acid which is used for titration.
* Anhydrous sodium carbonate: the alkali which is used for titration.
* Methyl orange: the best indicator for titration of a strong acid and weak base.
* Burette: is used to deliver precisely-measured Sulphuric acid to the flask, the burette also able you to read titres to 2 decimal places therefore it very accurate.
* Clam stand: used to hold the burette.
* 25cm3 pipettes: A pipette is used to measure small amounts of sodium carbonate very accurately.
* Pipette filler: is used to draw solution into the pipette.
* Small filter funnel: used to safety transfer all the sodium carbonate the volumetric flask.
* Glass rod: used to dissolve the anhydrous sodium carbonate in distilled water.
* 250 cm3conical flask:
* Distilled water: used to dissolve the anhydrous sodium carbonate.
* White tile: used to make colour changes clearer.
* Weighing bottle: to place the accurate amount anhydrous sodium carbonate.
* 250cm3 beakers (2): used for mixing anhydrous sodium carbonate with distilled water.
* Graduated volumetric flask: used to make up a solution of fixed volume very accurately
* Electronic balance: used to weight the accurate amount anhydrous sodium carbonate.
* White paper: used to place behind the burette to obtain a clear reading.
* Goggles: used for eye protection through out the experiment.
Chemical hazard notes:
A hazard is anything that can cause harm if things go wrong. Hazards include:
- Chemicals (such as corrosive acids and alkalis, toxic gases and chemical which cause cancer) therefore when carrying out this experiment the chemical used MUST be used carefully and goggles MUST be worn for protection. Chemical such as sodium carbonate and sulphuric acid are harmful substances if a thing goes wrong these safety produces must be used:
Sodium carbonate is irritating to eyes and respiratory system.
If swallowed: give plenty of water. Seek medical attention
If substance gets in eyes: remove contaminated clothing. Wash off skin with plenty of water. Soak contaminated clothing and rinse repeatedly.
If spilt in laboratory: scoop up as much solid as possible. If a solution, add mineral absorbent and scoop up into a bucket. Rinse area of spill and the cloth or mop thoroughly.
Sulphuric acid
Dangerous with: water. Vigorous reaction when the concentrated acid is diluted.
Always add the concentrated acid slowly to cold water when
diluting, never the reverse. Stir frequently to ensure thorough
mixing.
If swallowed; wash out mouth and give glass or two of water. Do not induce vomiting. Seek medical attention as soon as possible.
If splashed in eye; flood the eye with gently running tap water for 10 minutes. Seek medical attention.
If spilt on skin or clothes; Remove contaminated clothing and quickly wipe as much liquid as possible off skin with a dry cloth before drenching the area with a large excess of water. If a large is affected pr blistering occur, seek medical attention.
If spilt in laboratory; wear eye protection and gloves. Cover with mineral absorbent and scoop it up into a bucket. Add anhydrous sodium carbonate over the mixture and leave to react. Add lots of cold water. Rinse the area of the spill several times with water.
Methyl Orange
Inhalation:
Remove to fresh air. Get medical attention for any breathing difficulty.
Ingestion:
Induce vomiting immediately as directed by medical personnel. Never give anything by mouth to an unconscious person. Get medical attention.
Skin Contact:
Immediately flush skin with plenty of soap and water for at least 15 minutes. Remove contaminated clothing and shoes. Get medical attention. Wash clothing before reuse. Thoroughly clean shoes before reuse.
Eye Contact:
Immediately flush eyes with plenty of water for at least 15 minutes, lifting lower and upper eyelids occasionally. Get medical attention immediately.
In order to carry this experiment accurately working area must be organised with all apparatus and goggles must worn and experiment set out like this.
A. Preparing the anhydrous sodium carbonate
) Place a weighing bottle on the electronic balance.
2) Add approximately 2.65grams of anhydrous sodium carbonate.
3) Record the mass of anhydrous sodium carbonate and the weighting bottle. Weight of weighting bottle - sample of Na2CO3+weight of bottle = weight of Na2CO3
4) Tip the solid into 250cm3 beaker tapping the bottom of the bottle to remove the contents
5) Carefully, rinse the weighing bottle two or three times (it vital that the entire solid is transferred into the beaker).
6) Pour further distilled water into the beaker until the beaker is filled half way(150cm3)
7) Used the glass rod to stir the solution until the solid has dissolved.
8) Transfer the solution into Graduated volumetric flask using a funnel so that all the solutions get into the flask.
9) Then rinse the remaining solution from the glass rod and funnel, remove the funnel carefully and add further distilled water using a dropping pipette until the bottom of the meniscus is exactly on the line checking at eye level.
0) Place stopper in the flask, mix the solution thoroughly, by inverting the flask and shaking.
1) Using the pipette filler, draw 25 cm3 of solution from the volumetric flask and transfer it into a conical flask.
2) There will be a small amount of liquid left on the pipette, carefully touch the tip of the pipette into the surface of the solution.
B. Preparing the Burette
) Use a clam stand to hold the burette.
2) Use a 250 cm3 beaker to fill a burette with sulphuric acid.
3) The burette can then be filled with the solution to a point above the top graduation.
4) Open the tap and let some solution run out so that the bottom of the meniscus is exactly on the line, be sure your eye is at the level of meniscus, not above or below.
5) Check to see that there aren't any air bubbles.
6) Record the volume of the acid before starting the experiment to the nearest 0.05cm3.
C. Adding the Titrant
) Place the conical flask on a white tile, underneath the burette tap.
2) Make sure 4 drops of methyl indicator is added to the flask.
3) The solution should be delivered quickly until a couple of ml from the endpoint, swirling the flask after each point.
4) The endpoint should be approached slowly, a drop at a time.
5) The end of the reaction is when the first colour change is observed, which is yellow.
Recording the results
) Record the final burette reading.
2) The first experiment will be preliminary titration, which will give the general idea of where the end point is.
3) Titrations are repeated until 3 concordant tires are obtained which are within 0.1cm3 of each other.
4) Record the results in a suitable table.
Table of results
Mass of weighing bottle and solid 12.05g
Mass of weighing bottle 9.40g
Mass of solid 2.65g
Initial burette reading
Final burette reading
Titre
preliminary titration
0cm3
21.6cm3
21.6cm3
st titration
0cm3
21.6cm3
21.6cm3
2nd titration
0cm3
21.6cm3
21.6cm3
3rd titration
0cm3
21.7cm3
21.7cm3
Calculation (analysis)
This is the formula I am going use to calculate the concentration of the acid.
Average titre = 21.6+21.6+21.7 =21.633 cm3(3.d.p)
3
By looking at the equation above, we can predict exactly what is going to happen in the reaction. One mole of Na2CO3 will react with one mole of H2SO4. This will form Na2SO4 which is a neutral salt, water and carbon dioxide.
The two formulas that are needed to find out the concentration of a solution are;
) Moles = Mass/Relative Formula Mass
2) Concentration (mol dm-3) = Mass/Relative Formula Mass
The concentration will be measured in mol dm-3, therefore in order to use the formulas above; we will have to change the units of the results that we obtained which were in cm3. The volume has to be measured in dm3 not cm3 as the concentration is being measured in dm3.
Na2CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + H2O (l) + CO2 (g)
For the above reaction to take place properly, the molar mass for both Na2CO3 and H2SO4 should be equal. By looking at the equation we can see that this is true. We need to know this, as it will help us find out the concentration of H2SO4, which is unknown.
When working out the concentration of the Sulphuric acid, we instantly know that H2SO4 has the same number of moles as Na2CO3 as we have already calculated the molar mass ratio, which is 1: 1.
Calculating the relative molecular mass of Sulphuric acid and sodium carbonate
Sulphuric Acid (H2SO4)
(2 x 1) + (1 x 32) + (4 x 16) = 98
Sodium carbonate (Na2CO3)
(2 x 23) + 12 + (3 x 16) = 106
) Calculate the concentration of the sodium carbonate solution, which I made up.
Concentration (mol dm3) = Mass/Relative Formula Mass
2.65g/106= 0.025 mol dm-3
2) Concentration of NaCO3 = number of moles/volume
250cm³ water = 250/100 = 0.25mol dm-³
= 0.025/0.25
= 0.1 mol dm-3
(3) calculate the concentration of Sulphuric acid
I already know that the ratio is 1:1, as it takes 1 mole of sodium carbonate to react with 1 mole of Sulphuric acid. This is when neutralization occurs
=25x0.1
= 21.633.6×
= 2.5
21.633×
21.6× =2.5
=2.5
21.633
= 0.115
Concentration of sulphuric acid is 0.115 mol dm-3
Evaluating evidence and procedures
Looking at my results I can identify a slightly anomalous result 26.7 cm3. Although I had one anomalous result I can say that the rest of my results are up to a good degree of accuracy the reason been:
I used small number of drops of indicator, methyl orange, because having any more then 4 drop will affect the results.
) I made sure I used the same apparatus each time.
2) I made sure I used a white tile and paper through out the experiment.
3) When transferring sodium carbonate to the volumetric flask I made sure I wash the glass rod, beaker and filter funnel.
4) When I Opened the tap of the burette to let some solution run out I made sure that the bottom of the meniscus is exactly on the line.
5) I made all off my reading to 2 decimal places
6) After adding distilled water to the volumetric flask up to the graduation mark, I made sure that I invert it appropriately 20 times to make sure all the sodium carbonate has dissolved.
My first and second results are extremely accurate and precise; however my third result was anomalous although I followed my method accurately. I think the third reading was inaccurate because of contamination, the reason been is because I didn't clean the conical flask probably, therefore some alkali was left over.
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