You have been provided with a sample of 2.0M hydrochloric acid, chemical formula HCl. Hydrochloric acid is described as a strong acid, which means the H+ ions and Cl- ions are fully dissociated into the solution. At any one time, virtually 100% H+ ions (protons) will have reacted with water to form hydroxonium ions, leaving Cl- ions. This particular sample is also quite concentrated.
You have also been provided with a sample of limewater. Limewater is a solution of calcium hydroxide and has the chemical formulas Ca(OH) 2. It is formed when calcium oxide, chemical formula CaO (quicklime) is mixed or “slaked” into water and has a pH of about 9-10. Ca(OH) 2 is described as a strong base.
When an acid and alkali are mixed, the reaction forms a neutral solution of metal salt and water, which has a pH of 7. This particular reaction we are interested in is shown in the balanced chemical equation below. One mole of calcium hydroxide reacts with two moles of hydrochloric acid, to form one mole of calcium chloride and two moles of water.
Ca(OH)2(aq) + 2HCl(aq) CaCl2(aq) + 2H20(l)
Base + Acid Metal Salt + Water
The sample of limewater we have been given contains approximately 1g/ dmï¿½ which is a concentration of approximately 0.135M. This is calculated by the equation below:
Mol = Mass = 1 = 0.0135M
Your objective is to determine the concentration as accurately as possible; the most appropriate method of completing this task is to carry out a titration.
An acid-base titration is a technique in chemistry by which you can determine the concentration of an unknown reagent using a standard solution of known concentration. It uses the neutralization reaction that occurs between acids and bases, and that we know how acids and bases will react providing we know their formula. The standard/known solution is referred to as the titrant. In this case the titrant will be the acid, which will be used to determine the exact concentration of the limewater, by delivering the acid to a known volume of limewater with a burette. The concentrations and volumes of the two reagents are related by the following equation, which is eventually how the unknown concentration will be calculated.
Volume of acid x Concentration of acid = Volume of base x Concentration of base
When the reaction is complete and the solution is neutral, it is referred to as the end point or equivalence point. The volume of titrant used to reach the end point is termed the titre. To determine this point precisely a chemical indicator needs to be used. Phenolphthalein is the most effective indicator because hydrochloric acid is a strong acid and calcium hydroxide is also a strong base. Phenolphthalein is colourless in acidic solutions and turns fuchsia in basic solutions. At the end point of this titration you are looking for a colour change from pink/fuchsia to colourless.
At the moment, the acid is 2.0M, which is too concentrated to see a satisfactory colour change of the solution. As mentioned above, one mole of calcium hydroxide reacts with two moles of hydrochloric acid so the ratio is 1:2. This means the concentration of the acid needs to be twice the concentration of the limewater, so 0.027M. To make a 250cmï¿½ solution of acid, you will need to start by working out the dilution factor:
Dilution factor = Original concentration = 2.0 = 74
New Concentration 0.027
Once the dilution factor has been calculated, the volume of acid required also needs to be worked out:
Volume of acid required = Dilute solution volume = 250 = 3.3784cmï¿½
Dilution factor 74
A volume of 3.3784cmï¿½ will be almost impossible to measure with the lab equipment which is available, so it will be rounded up to 3.4cmï¿½ which can be realistically measured in a normal classroom lab. Because this has been rounded up and is not precise, the exact new concentration needs to be calculated:
New concentration = Volume of acid x original concentration = 3.4 x 2.0 = 0.027M
Volume of dilute solution 250
The following equipment will be used; most of which is standard titration apparatus. The white tile will be used to judge the end point of the reaction better. The graduated pipette is the most suitable type of pipette to measure the acid, as it is very important to measure the volume as accurately as possible. The 25ml one mark pipette will be used for the same reason as the graduated pipette, the volume needs to be measured accurately and other pipettes for example a disposable pipette would not be accurate enough. By using the following equipment, the most accurate results will be obtained.
* Clamp stand
* White tile
* 10ml graduated Pipette
* 25ml one mark pipette
* 250ml volumetric flask
* Pipette filler
* Conical flask
* Distilled water
* Hydrochloric acid solution 2.0M (standard solution)
* Calcium Hydroxide (limewater) solution
* Lab coat
* Safety goggles
In this titration we are dealing with potentially harmful chemicals so there are several safety measures that need to be followed. Because hydrochloric acid is a strong acid, and fully ionized it can be extremely corrosive; the liquid itself can cause severe damage to skin and eyes, and the vapour is also very toxic if inhaled. Limewater can act as an irritant so make sure it does not make contact with skin or eyes.
Gloves, goggles and lab coats should be worn at all times throughout the experiment and long hair tied up. Be cautious with all chemicals used:
* Do not inhale any vapours
* If they come into contact with skin wash the area thoroughly with water
* Make sure not to rub face or eyes
* Clean up any spillages immediately
Before we start the titration we need to establish which factors have to remain constant.
* The amount of limewater used each time – 25cmï¿½
* The number of drops of Phenolphthalein
* Room temperature
Before the titration is started, the apparatus needs to be prepared and assembled correctly. The acid also needs to be diluted from 2.0M to 0.027M, with a dilution factor of 74. You are going to make up 250cmï¿½ of 0.027M HCl. Take the graduated pipette and fill it with exactly 3.4cmï¿½ of acid, you will notice the surface of the liquid is slightly curved; this is called the meniscus. Make sure the bottom of the meniscus is on the line, this is the most accurate way to measure. Empty the contents into the volumetric flask and fill it up to the mark with distilled water, so the meniscus is on the line.
The burette needs to be cleaned with the acid to avoid possible contamination from any chemicals previously used in it. Close the tap and rinse the inside with a small amount of acid, let the acid out into a sink to clean the tap. Place the burette upright in the clamp stand.
After the burette has been cleaned and clamped to the stand, fill it with the diluted acid; run the liquid out very slowly until the meniscus is exactly level with a marking, it is easiest to start at the zero mark, but if not then make sure you record the reading. This will also expel any air bubbles that might have been in the tip of the burette.
Fill the 25ml one mark pipette with the limewater using the pipette filler, again so the meniscus is on the line. Empty the contents into a clean conical flask, let the tip of the pipette touch the side of the flask to allow any liquid to flow from the tip. Then add a four of drops of phenolphthalein, the solution will turn pink/fuchsia. Place the flask under the burette.
Carrying out the titration
Firstly, a trial run should be carried out to determine an approximate volume of the titrant required to neutralize the solution.
Follow these steps:
* With the conical flask under the burette, let the acid out and mix the solutions with a swirling action.
* Continue adding the acid until the solution turns colourless
* Record the volume of acid required for the base solution to change colour
* Dispose of the neutral solution down the sink (this is safe as the solution is neutral)
* Rinse the conical flask with distilled water
* Refill the burette up to the zero mark
* Measure another 25 cmï¿½ of limewater and empty it into the conical flask
The trial run should be excluded from any calculations as it’s an approximate value. By having an approximate value, you can then prepare for the colour change in the main experiment. It saves time, as you can run the acid out quickly up to within a few cmï¿½ of the approximate value, and then add the acid very slowly to get an accurate result.
Repeat the process at least three more times, or until you have three titres within 1cmï¿½ of each other to determine an accurate average titre. Follow the steps above, but when volume of titrant delivered is within about 2cmï¿½ of the approximate value, start adding the acid more slowly. As the reaction gets nearer the end point add the acid drop by drop. Record the volume to the nearest 0.1ml.
Upon completion of the experiment, you will need to clean up.
* Dilute the excess reagents and dispose of them down the sink
* Dispose of the titrated solution down the sink
* Rinse the equipment with distilled water
* Return all equipment to its appropriate places
* Wipe your work area clean and dry
Once the experiment has been completed and an average titre obtained, the concentration of the limewater can be calculated.
Using the equation, simply substitute the concentrations and volumes into the equation
Volume of acid x Concentration of acid = Volume of base x Concentration of base
27.5 x 0.027 = 25 x Concentration of base
Concentration of base = 27.5 x 0.027
Because the ratio of base to acid is 1:2, the number needs to be divided by two so from this calculation the concentration of the limewater works out to be 0.015M. To work out the number of grams in one dm3 use the following equation:
Mass = Mr x Moles = 74 x 0.015 = 1.11g
Chembook, R.J.C. Brown, Chapter 14: Acid & Bases