Determine the concentration of a limewater solution.
Title
Determine the concentration of a limewater solution
Introduction
In this coursework, the objective is to plan and design an experiment and choose a range of appropriate equipment and apparatus, in the lines of normal laboratory apparatus and indicator solutions, which will allow me to find the concentration of a limewater solution. 100 cm3 of limewater is provided, which has been made such that it contains approximately 1g dm-3 of calcium hydroxide. Indicator solutions are also provided.
Hydrochloric acid of concentration 2.00 mol dm-3 is also available, however it is too concentrated to be used and so needs to be diluted. THE USE OF ANY OTHER CHEMICAL IS FORBIDDEN.
Aim
To propose an experiment that will allow me to establish the concentration, in g dm-3, of the limewater as precisely as possible.
The formation of limewater
Heating calcium carbonate, CaCO3, produces quicklime, CaO.
CaCO3(s)
›
heat
›
CaO(s)
+
CO2(g)
Calcium oxide reacts violently with water to produce calcium hydroxide (slaked lime).
Quicklime
+
Water
›
Slaked lime
CaO(s)
+
H2O(l)
›
Ca(OH)2(s)
Addition of further water to Calcium hydroxide produces the saturated aqueous solution known as Limewater. Limewater is a clear, saturated water solution of calcium hydroxide. Calcium hydroxide is only slightly soluble ("sparingly soluble") in water; about 0.2 grams per 100 cubic centimetres, because it has weak dipoles in its molecule and so the strong dipoles of a water molecule are limited in their interactions with them. Limewater can therefore only exist as a weak concentration. Therefore, its solutions are weakly basic. The saturated solution has a concentration of approximately 1.5 X 10-2 mol dm-3. As the concentration of calcium hydroxide in limewater is low, the solution has a low concentration of hydroxide ion, OH-. The limewater is likely to be a moderately strong alkaline (pH approximately 10-11).
Ca(OH)2(s)
+
aq
›
Ca2+(aq)
+
2OH-(aq)
In the experiment, it is imperative to react the Hydrochloric acid with the Limewater in order to establish the concentration of the limewater.
Acid
+
Metal Hydroxide
›
Salt
+
Water
The balanced equation that takes place is:
Limewater
+
Hydrochloric acid
›
Calcium chloride
+
Water
+
Hydrogen
Ca(OH)2(aq)
+
2HCl(aq)
›
CaCl2(s)
+
2H2O(aq)
+
H2(g)
Neutrality
The pH of an aqueous solution is a measure of its hydrogen ion concentration; it depends on temperature because the degree of dissociation of an acid does, i.e. Ka changes. Before you go on, write down the definition of pH.
It is: -1g {[H+] / mol dm-3}
Note that the concentration of the hydrogen ion has to be divided by its unit to get a number. You can only take logarithms of numbers, not of physical quantities.
A neutral solution can be at pH 8. How? By being cold. If you look up the values of Kw at 0oC, 25oC and 100oC you will be able to calculate the pH of water at these three temperatures; they are roughly 8, 7 and 6. All of the solutions are, however, neutral. The definition of this is pH-independent:
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It is: -1g {[H+] / mol dm-3}
Note that the concentration of the hydrogen ion has to be divided by its unit to get a number. You can only take logarithms of numbers, not of physical quantities.
A neutral solution can be at pH 8. How? By being cold. If you look up the values of Kw at 0oC, 25oC and 100oC you will be able to calculate the pH of water at these three temperatures; they are roughly 8, 7 and 6. All of the solutions are, however, neutral. The definition of this is pH-independent:
A neutral solution is one where [H+] = [OH-].
This is at pH 7 only at 25oC.
Therefore I have to keep the temperature controlled, preferably to 25°C, it really depends on the room temperature. As long as it stays constant, we can get an accurate reading.
Below is a table of variables:
Variables
Variable
Value or range
How measured
Dependent variable (continuous): Initial volume of acid
Volume of acid used in titration
Measured: in cm3 (ml)
Typical values: 20 cm3, 22.4 cm3, 23.6 cm3
With use of burette, whilst in titration.
Independent variable (continuous): Final volume of acid
Volume of HCl acid used
Range: 0 cm3 to 50 cm3
Number of results: 1
Repeats: 3 titrations
With use of burette, whilst in titration.
Control variables: Temperature
Room temperature
Measured: in °C
Using a thermometer
Apparatus
The apparatus, equipment and chemicals that I will use are as follows: -
. Pipette, 25cm3,
2. Pipette filler,
3. Volumetric flask 500 cm3,
4. Conical flask 250 cm3,
5. Burette 50 cm3,
6. White tile,
7. Clamp and stand,
8. Funnel,
9. Universal indicator solution*,
0. Limewater (100 cm3 with 1g dm-3 of calcium hydroxide),
1. Hydrochloric acid- standard solution (conc. 2.00 mol dm-3),
2. Wash bottle of distilled water,
3. Safety spectacles,
4. rubber lined gloves.
*Universal indicator is the most commonly used indicator because of the range in colours. It helps because other indicators do not give a colour when an acid-base solution turns neutral.
2
3
4
5
6
7
8
9
0
1
2
3
4
Very strong acid
Strong acid
Weak acid
Neutral
Weak alkali
Moderately strong alkali
Strong alkali
Very strong alkali
Strength of HCl acid
Acids are supplied as concentrated solutions in water. The table below gives the approximate molarity for HCl.
Chemical
Formula
Molecular weight
Molarity
Specific gravity
Weight %
Hydrochloric acid
HCl
36.5
7.5
.18
36-38
The reason why I have to dilute the acid is that, if I used HCl acid of concentration 2.00 mol dm-3, then each drop of acid in the burette will result in a large change of pH. This is because there are a larger number of HCl molecules in one drop of concentrated HCl, than in a molecule of diluted HCl, so it will be harder to indicate, when the titration is complete. Whereas if I use acid of a lower concentration then it will be easier to see the end point (pH 7) as each drop of acid would add a smaller change in colour.
Risk assessment
Diagram of apparatus
Safety
Safety is very important and it is necessary to follow safety procedures. It is important to wear eye protection when working with any acid. Also long hair should be tied back. In addition, as the chemicals used are corrosive it is important to take extra care while working around them.
Protective Equipment
. Ventilation - Work area should be fully ventilated to disperse HCl gas to atmosphere.
2. Clothing - Use appropriate clothing to prevent exposure of gas or solution to skin. Use rubber lined gloves inhandling the acid.
3. Eye protection - Safety glasses or goggles should be used to protect the eyes from HCl gas or HCl solution. Eye wash station should be available for emergency use.
4. Respiratory Protection - Respirator or gas mask is recommended.
Spills, Leaks and Disposal Procedures
. Spilled hydrochloric acid should be removed immediately by flushing the contaminated area with large volumes of water. Lime should be available for the immediate neutralization of spills.
2. Leaks in tanks, pumps, pipes and valves should be flushed with water and repaired immediately.
3. Hydrochloric acid for disposal should be neutralized with lime or powdered calcium carbonate before discharging to canals. The neutralized HCl should be flushed with plenty of water in order to dilute.
Emergency First Aid
. Contact with Skin - Wash with water. If skin irritation appears, get medical attention.
2. Contact with Eyes - Wash with water. Lifting eyelids occasionally. It eye irritation occurs even after washing, get medical attention.
3. If Taken Internally - Drink plenty of water to dilute solution.
Method
Titration technique
The purpose of this experiment is to determine the concentration of limewater by titration against a standard solution of HCl acid.
Bring to completion the following tasks:
HCl
In order to start with the experiment it is required of me to dilute the HCl. An accurate way of doing this is to use a volumetric flask and pipette. I want to dilute the acid to 0.1 moles.
. Transfer 25.0 cm3 of HCl with the use of the pipette filler.
2. Pour the HCl from the pipette filler into the 500 cm3 volumetric flask.
3. Add 475 cm3 of distilled water to the same volumetric flask, I measured the amount of water by looking at the graduated mark on the flask and filled it until the 500 cm3 mark.
Burette
. Rinse with HCl acid, then pour washings down the sink.
2. Fill the burette, including the jet.
3. Check there are no air bubbles - hold jet up, open clip until all air bubbles are expelled.
4. Place in a stand.
5. Read the burette to + or - 0.05 cm3.
Pipette
. Wash by filling with limewater, empty down the sink.
2. Fill to above the graduation mark.
3. Hold the pipette above the beaker with the graduation mark at eye-level, and allow limewater to drain out drop by drop until the meniscus just rests on the mark.
4. Allow the measured quantity of limewater to drain into a conical flask previously rinsed with distilled water. Touch tip of jet against the side of the flask.
Procedure
. Place the white tile under the conical flask; it helps to see the colour changes.
2. Using the pipette filler, carefully transfer 25.0 cm3 of the limewater to a clean 250 cm3 conical flask.
3. Add two or three drops of universal indicator solution.
4. Run HCl solution from the burette into the flask, with swirling, until the end-point, adding the alkali drop by drop when you think the colour is about to change.
5. Record the final reading.
6. Empty the solution in the conical flask and then rinse.
7. If there is not enough HCl in the burette then fill up remembering to take a reading and recording the initial volume of HCl before titration.
8. Repeat steps 1, 2, 3 and 4 at least twice more.
9. Empty the burette and wash it carefully immediately after the titration.
Results table
The readings should be placed in the correct places in the following table of results:
Reading No.
2
3
Burette readings
Of HCl
Initial (cm3)
Final (cm3)
Average titre (cm3)
Volume used (Titre)/cm3
Accuracy
In order for the experiment to be accurate and reliable, it will be necessary to dilute the Hydrochloric acid significantly. It is in my presumption that a concentration of 0.1mol dm-3 would enable me to achieve reliable results. This would require a dilution factor of twenty. I will be making 500 cm3 of dilute hydrochloric acid. I will also be using 25 cm3 of Limewater.
You should record burette readings to the nearest 0.05 cm3 (approximately one drop). Consecutive titrations should agree to within 0.10 cm3 and, strictly, you should repeat the titrations until this is achieved. With practice, your technique should improve so that it is not necessary to do more than three titrations. Calculate the mean of the two or three closest consecutive readings and quote this to the nearest 0.05 cm3.
In order to reduce errors it is necessary to choose accurate and reliable equipment. That is why I have chosen the pipette, volumetric flask and burette. All of these instruments have an accuracy of ±0.05 cm3, which is suitable for my experiment and should produce accurate and reliable results. In making my choice of indicator, I also had to be very careful. The Limewater used will be a relatively weak base so it will be appropriate to use universal indicator as it has an end-point on the pH scale and this is seen at each change of colour, it is easier to see a change in colour. Whereas phenolphthalein indicator has an end point much higher up in the pH scale. Therefore, if I were to use the phenolphthalein indicator then the end-point (colour change) would be reached before the equivalence point (when the chemicals, acid-base break up).
Bibliography
* Letts AS revision notes - found very useful for equations.
* Cambridge AS Chemistry 1 - useful for terminology and chalk and lime chemistry.
* Dictionary of Science by E. B. Uvarov and Alan Isaacs - useful for terminology.
* The Osborne Science Dictionary by C. Stockley, C. Oxlade and J. Wertheim -useful for facts and in depth information on elements.
* IFinger look-up system encyclopaedia by Oxford University Press - useful for the information on acids and bases.
* www.chemfiesta.com - useful for information on concentrations.
* Http://library.caltech.edu - examples of titrations and readings made.
* http://www.infoplease.com - information concerning pH and indicators.
* http://www.webref.org/chemistry - brief notes on acids.
* http://www.inchem.com.ph/hcl.htm - information about the chemical properties i.e. safety and hazards.
AS Chemistry- Assessed Practical
By Priyesh Patel 12O
