6.5 CONCENTRATION OF AQUEOUS SOLUTIONAt the end of the lesson,pupils are able to: 6.5.1 State the meaning of concentration of aqueous solution 6.5.2 Solve numerical problems involving concentration of solution
Concentration???? The quantity of solute in each volumeof solutionwhich is usually 1 dm3 of solution.
1. The quantity of solute dissolved can be measured in gramor mole, hencetheconcentration of a solution can be measured in unit g dm–3 or mol dm–3.
2. Relationship between concentration and molarity:
3. Relationship between number of moles of solute dissolved inthesolution,itsmolarity and the volume of the solution:
6.6 STANDARD SOLUTIONAt the end of the lesson,pupils are able to: 6.6.1 State the meaning of standard solution 6.6.2 Describe the preparation of a standard solution through activity: • From a solid substance • Through dilution of an aqueous solution 6.6.3 Solve numerical problems involving preparation of standard solution and dilution
1. Standard solution??? Is a solution where the concentrationinexactly known. 2. Apparatus used to prepare the solution : Volumetricflask3. Parameters that have to be measured accurately: § mass of solute § volume of distilled water
Preparation Of Standard Solution From A Solid
1. To make sure all the residue of Na2CO3 on the wall of beaker and filter funnel havebneentransferred into a volumetric flask 2. To make sure that the standard solution produced is accurate as desired3. The eye level must aligned with the meniscus level observed 4. To avoid the solution from evaporate which may cause the concentrationof thethestandard solution change
1. To make sure all the residue of Na2CO3 on the wall of beaker and filter funnel havebneentransferred into a volumetric flask 2. To make sure that the standard solution produced is accurate as desired3. The eye level must aligned with the meniscus level observed 4. To avoid the solution from evaporate which may cause the concentrationof thethestandard solution change
**** Sodium hydroxide, NaOH is not suitable to be used for the preparationofastandard solution. Why???? Sodium hydroxide, NaOH is hygroscopic (absorbs water or moistureintheair).Sodium hydroxide, NaOH also absorbs carbon dioxide gas, CO2 intheairtoformsodium carbonate, Na2CO3 This causes difficulty to determine the exact mass of sodiumhydroxide, NaOH.
a) Weigh 8.0 g of sodiumhydroxideb) Add a little distilled water to dissolveNaOHinabeaker . Stir c) Transfer the solution into a 250cm3 volumetrikflask using a filter funnel d) Rinse the beaker and filter funnel withdistilledwater. e) Transfer into the volumetric flask. f) Remove the fiter funnel. g) Add distilled water into the volumetricflaskupto the graduation mark (250 cm3) h) Stopper the volumetric flask andshakethevolumetric flask. Calculation of NaOH needed:
Calculation of KOH needed:
a) What is the meaning of standard solution? [1 markah] b) After all the potassium hydroxide is poured into the volumetric flask, thebeakermustberinsed several times with distilled water. After each rinse, all the water istransferredintothe volumetric flask. Give one reason for doing this? [1 markah] c) Why is the volumetric flask stoppered after the standard solution is prepared?[1markah]d) Why is the distilled water is added drop by drop? [1 markah] e) Why do you need to shake the solution after all the water had been added?[1 markah]
a) What is the meaning of standard solution? [1 markah] A solution of which known its concentration b) After all the potassium hydroxide is poured into the volumetric flask, thebeakermustberinsed several times with distilled water. After each rinse, all the water istransferredintothe volumetric flask. Give one reason for doing this? [1 markah] To ensure that all sodium hydroxide is transferred into a volumetric flaskc) Why is the volumetric flask stoppered after the standard solution is prepared?[1markah]To prevent solution from spillage during shaking d) Why is the distilled water is added drop by drop? [1 markah] to get the actual volume of sodium hydroxide e) Why do you need to shake the solution after all the water had been added?[1 markah]To ensure that all sodium hydroxide is dissolved evenly
Preparation Of Standard Solution: DILUTION METHOD1. The process of diluting a concentrated standard solution (stocksolution)byadding water to form a less concentrated solution. 2. When a solution is diluted, ü volume of solvent increase but the number of moles of soluteremainsconstantü concentration of the solution decreases.
Procedure: 1. The needed volume of stock solution, V1 cm3 is determined by using theformula, M1V1 = M2V2 (1) (V1) = (0.2) (100) V1 = 20 cm3 2. Pour the stock solution from activity 6.1 into a beaker 3. Pipette 20cm3 of the stock solution from the beaker 4. Transfer solution in the pipette into 100 cm3 volumetric flask. 5. Add distilled water until the solution level approaches the calibrationmarkonthevolumetric flask. 6. Add distilled water slowly using a dropper until the meniscus level is alignedexactlyonthe calibration mark of the volumetric flask. 7. Close the flask using stopper. The volumetric flask is shaken and invertedseveral timesuntil the solution is homogenous.
Discussion: 1. 2. 3. because, beaker cannot measure the volume of standard solution that wewishtoprepare accurately. 4. The last drop of solution left in the pipette is not transferred out becausethepipetteiscalibrated precisely at which the volume of solution discharged fromthepipetteisaslabelled on the pipette
6.7NEUTRALISATIONAt the end of the lesson,pupils are able to: 6.7.1 State the meaning of neutralisation 6.7.2 Determine the concentration of an unknown solution through titration method 6.7.3 Solve numerical problems involving neutralisation
1. Meaning?? Reaction between an acid and a base/alkali to produceasaltandwater only.
Applications of Neutralisation in Daily Life