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pH and Indicators
Derivation of pH, Uses of indicators
Question | Answer |
---|---|
What is the breaking up of water into H+ and OH- ions known as? | The self ionisation of water |
What is Kw the symbol for? | The Ionic Product of Water |
What Danish chemist discovered the pH scale? | Soren Peter Sorensen |
Definition of pH? | The pH of a solution is the negative log to the base 10 of the hydrogen ion concentration measured in moles per litre. |
What do the letters "pH" actually stand for? | It comes from the German word "potenz" meaning power |
What is Ka the symbol for? What does it tell us? | The Acid Dissociation Constant. It tells us how strong an acid is by telling us how much an acid dissociates. Strong acids have a large Ka |
What is Kb the symbol for? What does it tell us? | The Base Dissociation Constant. It tells us how strong a base is by telling us how much it dissociates. Strong bases have a large Kb |
What is the relationship between acids and conjugate bases? | Strong acids make weak conjugate bases, weak acids make strong conjugate bases. |
What formula is used to calculate the pH of a substance? | -log^10[H+} |
How do we calculate the pH of a base? | We use -log^10[H+} then take the answer away from 14 to get the number of OH- ions. |
How do we calculate the pH of weak acids and bases? | We use the formula (square root)Ka x Mol(acid) to get the concentration of H+ ions. We then sub this value into -log^10[H+} |
What range do most indicators need to show a distinct colour change? | A change of 2 units are required to show a colour change |
What is the range and colour change of Methyl Orange? | 3-5 Red to Yellow |
What is the range and colour change of Litmus? | 5-8 Red to Blue |
What is the range and colour change of Phenolphthalein? | 8-10 Colourless to Pink |
What is the correct indicator for a strong acid vs a weak base? | All 3 indicators, Methyl Orange, Litmus and Phenolphthalein will work for this titration. The range of the titration is from 3-10 |
What is the correct indicator for a strong acids and a weak base? | The range here is from 3-7 so Methyl Orange will be a suitable indicator |
What is the correct indicator for a weak acid and a strong base? | The range here is from 7-10 so Phenolphthalein will be a suitable indicator |
What is the correct indicator for a weak acid and a weak base? | The range for this titration is extremely small and cannot be detected by an indicator. Instead we would titrate the weak acid against a strong base and the weak base against a strong acid. |