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Solutions

Solutions. What is a solution?. Solution is a mixture of solute and solvent. Solute (normally solid) dissolves in solvent (liquid) to form the solution . In salt solution, salt is the solute and water is the solvent.

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Solutions

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  1. Solutions

  2. What is a solution? • Solution is a mixture of solute and solvent. • Solute (normally solid) dissolves in solvent(liquid) to form thesolution. • In salt solution, salt is the solute and water is the solvent. • If solute amount is high, the solution is called a concentrated solution. • If the solute amount is low, the solution is a dilute solution.

  3. Concentration of a solution • Concentration of a solution indicates how much solute is present in the solution. • Concentration is expressed as mole/dm3 (mole/L) • 1 dm3 (L) = 1000 cm3 (mL) • So concentration is also moles/1000cm3 (moles/1000 mL). • How many moles of a solute present in 1000 cm3 of a solution is called Molarity of the solution (M). • So a solution of 2moles/dm3 is called 2 Molar solution OR 2M

  4. Find the concentration (Molarity) of a solution if 15g of sodium carbonate Na2CO3 is dissolved in 300 cm3 of the solution. • Answer • Change mass in to moles 15g/106g = 0.14moles • It means the solution is 0.14 moles in 300 cm3. But Molarity is moles in 1000 cm3 • So concentration (Molarity) of the solution = 0.14X 1000 300 = 0.47 moles/dm3

  5. 225cm3 of a 3M solution of sodium hydroxide is given. • (a) How many moles of sodium hydroxide are present in 1dm3 of the solution? • 3moles • (b) How many moles of NaOH are present in 225 cm3 of the solution? • 3 X 225 1000 = 0.675 moles • (c) What is the mass of NaOH present in 225 cm3? • 0.675 X 40g = 27g

  6. Titration • Titration is a method to find the concentration of a solution by using another solution whose concentration is known. • The solution with known concentration is called a standard solution. • Titration is useful in neutralisation reactions (acid – base reaction) • A chemical indicator is used to know the end point of a titration. • (a) Phenolphthalein (colourless in acid and pink in alkali) • (b) Methyl Orange (Red in acid and yellow/orange in alkali)

  7. Set up for a titration using hydrochloric acid with sodium hydroxide solution

  8. Titration calculations • 25 cm3 of a sodium hydroxide solution was neutralised by 30 cm3 of hydrochloric acid of concentration 1.5 mole/dm3. • Equation: • NaOH + HCl NaCl + H2O • (a) How many moles of HCl are present in 30cm3? • 1.5 X 30 = 0.045 moles 1000 • (b) How many moles of NaOH used to react with it? • 0.045 moles (Mole ratio is 1:1) • (c) What is the concentration (Molarity) of NaOH? • 0.045 X 1000 25 = 1.8 mole/dm3

  9. In a titration, 20 cm3 of a 0.5 M sodium carbonate solution was completely neutralised by 16.5 cm3 of Hydrochloric acid. • Equation: Na2CO3 + 2HCl  2NaCl + CO2 +H2O • (a) Find the number of moles of Na2CO3 present in 20cm3 • 0.5 X 20 1000 = 0.01 moles • (b) How many moles of HCl present in 16.5cm3? • 0.01 x 2 = 0.02 moles (mole ration is 1:2) • (c) Calculate the molarity of the acid. • 0.02 X 1000 16.5 = 1.21 mole/dm3

  10. Complete the following passage. Use the words in bold underneath. • The burette is filled with ________ up to the 0 mark. We know the _______________ (strength) of the acid but not of the ____________. A known volume of the alkali is put in a ___________ ________ under the burette. It also contains an _________. The acid is carefully let into the flask. When the alkali is ________________, the indicator changes colour so we stop the flow of acid. This is called the ______________. indicator acid concentration conical flask alkali endpoint neutralised

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