A technician is given the task of calibrating a calcium hydroxide solution with an approximate concentration of 0.01M using freshly calibrated 0.05M HCl. Propose an experiment that the technician can do in the laboratory to calibrate the sodium hydroxide solution. Using mock data and following the outline of the report sheets for the laboratory sessions, show how you could calculate the concentration of the calcium hydroxide based on the experiment described. Materials • Erlenmeyer flask or a beaker • Excess amount of 0.05M hydrochloric acid solution • Measured amount of sodium hydroxide solution • Indicator as Phenolphthalein • Freshly calibrated Burette • Burette Stand • Gloves and Lab coat Procedure 1. It is important to clean all the apparatus before performing the experiment. Clean thoroughly all the apparatus with distilled water. Set the required apparatus. Wear gloves, safety glasses and lab coat for protection. 2. For the satisfaction measure the initial pH of hydrochloric acid and sodium hydroxide solution. 3. Take a precisely measured amount of sodium hydroxide solution. In this case takeout 20ml of sodium hydroxide solution. 4. Put the precisely measured 20 ml NaOH solution of into the Erlenmeyer flask. But before transferring the solution rinse the Erlenmeyer flask with little portion of sodium hydroxide solution. 5. Takeout about four to five drops of the Phenolphthalein and put them in the Erlenmeyer flask that already contained sodium hydroxide. 6. Shake the Erlenmeyer flask in circular motion in order to mix the Phenolphthalein with the sodium hydroxide solution. The mixture will change to red at that time, as Phenolphthalein turns red in basic solutions. 7. Wash the burette with a little amount of hydrochloric acid solution. Clamp the burette to its stand and fill it with the hydrochloric acid solution to its upper limit. Takes care while clamping the burette, as it is much fragile. Moreover hold burette in such a manner that none of its surface touches any other surface. 8. Place the Erlenmeyer flask in such a manner that the tap of the burette lies above the mouth of the Erlenmeyer flask that is containing the aqueous solution of sodium hydroxide. 9. Note the initial reading on the burette that depicts the volume of the hydrochloric acid solution. All the reading from the burette should be recorded from the lower meniscus. 10. The tap on the burette will allow the hydrochloric acid to flow into the Erlenmeyer flask. Open the tap and allow only a small amount about 0.5ml of hydrochloric solution to enter the Erlenmeyer flask. There would be colour change in the solution in Erlenmeyer flask, as both the solution try to blend. Swirl the Erlenmeyer flask. The colour change will disappear that depicts that the amount of hydrochloric acid is consumed in the reaction. 11. Repeat the step described above again and again by allowing the similar amount of hydrochloric acid solution till the solution in the Erlenmeyer flask will have a permanent pink colour. This shows that there is no sodium hydroxide is remaining in the Erlenmeyer flask 12. After the endpoint is attained, end the process at once. Shaking the Erlenmeyer flask will have no impact on the colour of the solution in Erlenmeyer flask. Do not allow any amount of hydrochloric solution to enter the Erlenmeyer flask at this stage. Note, if the colour of the solution in Erlenmeyer flask is darker pink, you have allowed less acid to enter the Erlenmeyer flask. 13. After light pink colour is achieved, record the pH of the solution in the Erlenmeyer flask. 14. Note the reading on the burette by considering the lower meniscus. This will give the volume of the hydrochloric acid solution. Measure the concentration (morality) of the acid by referring to the formula given below. M acid . V acid=M base .V base 15. Repeat the experiment at least two more times, in order to evaluate any mistakes. Calculate the concentration by taking average of the volumes recorded. Readings Experiment no Volume of NaOH Volume of HCl pH of solution 1 20ml 43.5ml 8.1 2 20ml 43.2ml 7.6 3 20ml 43.3 7.7 Calculations Taking the average out of the three readings Volume of HCl = 43.3ml Applying formula for calculating concentration of sodium hydroxide. M acid . V acid=M base .V base 0.05 x 43.3 = M base x 20 M base = 0.108 M 2. Describe the preparation of 100mL phosphate buffer at pH 7.4, starting from H3PO4 solution (1M) and KH2PO4 (FW 136 g/mol). Show all calculations. Ka H3PO4 = 7.5×10-3. 1. Use the Henderson Hasselbalch equation to find the ratio of A- to HA. pH = pKa + log [A-] / [HA] 7.40 = -log (7.5×10-3) + log [A-] / [HA] 7.40 = 2.125 + log [A-] / [HA] 9.525 = log [A-] / [HA] Antilog (9.525) = [A-] / [HA] Antilog (9.525) = [A-] / [HA] * 3. What is the pH of a solution resulting from mixing 100 mL 0.5M NaOH with 500 mL 0.3M HCl? Show all calculations. For calculating the pH of the solution, the formula below can be applied Calculating the number of moles No of moles of acid HCl = 0.3 x 0.5 = 0.15 mol No of moles of base NaOH =0.5 x 0.1 = 0.05mol Moles of acid remaining after reaction Moles of HCL- moles of NaOH =0.15 – 0.05 =0.1 mol Calculating the morality of acid in whole solution No of moles of acid/ total volume of solution =0.1 /0.6 = 0.167 Hard Water 97 present of the earth is covered with water and the remaining 3 present has soil. Water has many useful properties. Among all these properties, it has a property to liquefy majority of minerals and salts in it. Sea water shows the same condition, where great quantity of salt is naturally liquefied in water. It is naturally a colourless, odourless and tasteless liquid. Hard water is mostly in below the layer of earth. Definition of hard water can be stated as “the condition of water in which majority of solid minerals liquefied in water and alters the main properties of water”. The liquefied minerals are mainly the carbonates of calcium and magnesium. These both minerals are in large amount on the earth surface. It is a well-known fact that the majority of rocks are consist of calcium carbonates and magnesium carbonates. Nevertheless, remaining minerals also stick together with them. Hard water has minerals but it has no harmful bacteria that cause diseases. Then it has no side effects on health. Everyone can take hard water. Still it leftovers a minimum product when using it in industries. (Hardwater.org, 2003). When rain falls, it absorbs carbon dioxide from air and produces a weak acid that is also known as carbonic acid. Then the mixture of water and carbonic acid falls on the rocks. This acidic water enters into the earth layer. When the rain falls on the rocks, it reacts with the rocks. Rocks are mostly reactive with acidic water or acids. Most rocks are consisting of calcium and magnesium carbonates. In the reaction water liquefies the calcium and magnesium carbonates in it. More mixture of calcium and magnesium carbonates creates harness in water. On the other hand, other compounds are also liquefied in water that depends on the certain kinds of minerals beneath earth’s outer layer. That type of reaction is written in below. CaCO3+ CO2+ H2O⇋Ca2++ 2HCO3− MgCO3+ CO2+ H2O⇋Mg2++ 2HCO3− When this type of water that contains lots of minerals in it, the scales of calcium carbonates can be easily observed. Temporary hardness The water that liquefied bicarbonate minerals that are calcium bicarbonate, magnesium bicarbonate etc has temporary hardness condition. These minerals detach in ions in water. Calcium (Ca2+) and magnesium (Mg2+) ions detach with the carbonates and bicarbonates (CO32-, HCO3 ). Metallic ions create hardness in water. This hardness is temporary. That is why it can be easily removed by heating it to 100 degree centigrade or combining it with calcium hydroxide. Boiling changes bicarbonates to carbonates that stays solid in water and can be precipitated out (Binnie, Kimber & Smethurst, 2002). Permanent hardness The water that liquefied sulphates of calcium and magnesium has permanent hardness condition. It is almost not possible to precipitate out the liquefied calcium sulphate and magnesium sulphates just by heating it to 100 degree centigrade. Hard water remains hard even with the rise of temperature. Special mechanisms permit water to be free from the sulphates. The Ion swap process is done mostly to make softer the permanent hardness of water ( Deevey Jr, Gross, Hutchinson & Kraybill, 1954). Effects of Hard water It is well known information that the soap is not useful with hard water. When hard water reacts with soap it produces precipitates in its place of lather. It is because the calcium ion in the hard water substitutes the sodium stearate, which is the most important component of soap that produces lather. The displacement reaction is given below (Boisen, A. M. Z., Amstrup, Novak & Grosell, 2003). 2 C17H35COO-+ Ca2+→ (C17H35COO)2 Ca Synthetic soaps and detergents have also no or less impact of hard water.
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