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Investigation to determine the concentration of a sample of limewater

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Introduction

Investigation to determine the concentration of a sample of limewater Introduction 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 Objective The sample of limewater we have been given contains approximately 1g/ dm� which is a concentration of approximately 0.135M. ...read more.

Middle

By using the following equipment, the most accurate results will be obtained. * Burette * Clamp stand * White tile * 10ml graduated Pipette * 25ml one mark pipette * 250ml volumetric flask * Pipette filler * Phenolphthalein * Conical flask * Distilled water * Hydrochloric acid solution 2.0M (standard solution) * Calcium Hydroxide (limewater) solution * Lab coat * Safety goggles * Gloves Safety 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 Method Preparation Before we start the titration we need to establish which factors have to remain constant. * The amount of limewater used each time - 25cm� * The number of drops of Phenolphthalein * Room temperature Before the titration is started, the apparatus needs to be prepared and assembled correctly. ...read more.

Conclusion

Follow the steps above, but when volume of titrant delivered is within about 2cm� 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. Trial Run First run Second run Third run Average Titre(cm�) 28.0 27.5 27.4 27.6 27.5 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 25 = 0.0297M 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 Sources Chembook, R.J.C. Brown, Chapter 14: Acid & Bases http://en.wikipedia.org/wiki/Strong_base#Strong_bases http://en.wikipedia.org/wiki/Acid-base_titration http://ptcl.chem.ox.ac.uk/~hmc/hsci/chemicals/calcium_hydroxide.html ...read more.

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