chemistry prac

Ma x Va = Mb x Vb When heating solid, always test for co2 and look out for water droplets Procedure 1. Gentle heating: half air-hole open, move boiling tube; strong heating: full air-hole open, stay above flame Titration 1. Rinse burette and filter funnel a. Use tap water (fill up to ½) with a closed tap, then open tap and release some water to wash the tip, then flip it over and twirl to pour out the rest of the solution from top of the burette b. Repeat with deionised water and relevant solution c. Repeat procedure with filter funnel d. Then fill up the burette with the relevant solution with a filter funnel. give a strong jerk to release air bubbles. Record the reading on burette 2. Rinse conical flask a. Rinse with tap water and deionised water b. DO NOT use relevant solution 3. Rinse pipette a. Fill up a beaker with tap water. Use tap water (fill up to ⅓ with pipette filler) and swirl to wash the bulb, then pour out b. Repeat with deionised water and relevant solution c. Suck up relevant solution with pipette filler and empty into conical flask, tap twice gently to ensure all the solution has gone into conical flask - Burette readings Titration number 1 2 3 Final burette reading/cm3 2dp 2dp 2dp Initial burette reading/cm3 2dp 2dp 2dp Volume of X used/ cm3 2dp 2dp 2dp Best titration results ( ✓ )

Graph - Scale: 2, 5, 10 - Origin must label - Best fit line: must past through at least 1, don't have to be equal points out, sometimes must pass through origin - MUST EXTRAPOLATE Source of error and ways to improve - Water might have evaporated from the solution, increasing its concentration. This can be improved by covering the petri dish - Heat may be lost to the surroundings. This can be improved by adding lid, changing to styrofoam cup, wrapping test tube with towel, or any heat insulator - For mixing liquids: use a burette to measure the volume as burette is a precise apparatus for measuring volume of solution compared to measuring cylinder - For collecting hydrogen: test tube of hydrogen may have been contaminated with air that was present in the flask before the experiment started, hence test tube of gas collected is impure oxygen - For crystallisation: it does not give a 100% yield of crystals, some of the salt still remains dissolved in the water so the mass of crystals obtained is less than what it should have been - For evaporation to dryness: some of the salt formed may be lost due to “sputtering” - For paper chromatography: it was difficult to determine the centre of the red dye due to odd shape, hence distance travelled by red dye measured may be higher than expected, Rf value calculated may be higher - For heating in crucible to find empirical formula: some of the ash (MgO) may be lost to the surroundings, thus affecting the mass of the crucible and lid after weighing it. This is because the lid is often lifted to allow the introduction of air. The ash, being a fine and light particle, might easily be lost to the surroundings. This can be improved by opening the lid slightly and hovering above the crucible so that enough oxygen can enter, instead of lifting the lid completely. - Heat in solution may not be distributed evenly. Some parts may have higher temperatures than other parts. This can be improved by stirring mixture before taking temperature to ensure heat produced is distributed evenly around the solution - For titration with unknown concentration, the impurities inside sodium hydroxide might react with the hydrochloric acid and this will result in higher volume of hydrochloric acid recorded. The percentage purity would have been higher than the expected percentage purity. expt 3.6 (T) titration.pdf- It is difficult to compare colour of solution with universal indicator scale, resulting in inaccurate pH measurements - The reaction between metal and acid is exothermic, the heat produced might raise the temperature of the solution, leading to a faster rate of reaction. This can be improved by using a thermostatically controlled water bath

- Magnesium ribbon floats on the surface and is not completely submerged in the acid during the reaction, leading to a longer time recorded. This can be improved by using a glass rod to fully submerge the magnesium ribbon - Difficulty in reading the meniscus of potassium manganate due to it being a dark liquid, resulting in inaccurate reading of the titre volume. This can be improved by taking reading of the burette using the top of the meniscus instead - Last resort: human reaction time in starting and stopping the stopwatch. This can be improved by taking multiple readings and finding the average - Involving heat: use burette to measure volume of reactants, use a data logger with temperature probe to measure temperature Qns ● Why is magnesium ribbon cleaned before the experiment ○ Ans: to make sure there are no impurities on the surface of the magnesium ribbon that would tamper with the results ● Why is the lid necessary? (for heating in crucible) ○ Ans: it is to prevent some of the magnesium oxide powder from escaping into the surroundings, delivering inconsistent results ● Why should the lid be lifted occasionally ○ Ans: it is to allow the introduction of oxygen so the experiment can be carried out as combustion requires oxygen ● Why is conical flask used instead of beaker for titration ○ The sloping sides of a conical flask prevents loss of reactant due to splashing during swirling which could lead to inaccurate titration results ● Explain why a pipette and burette are used instead of a measuring cylinder ○ Pipette and burette are very precise apparatus used for measuring volume of solutions. This will reduce the percentage error as much as possible. ● Explain why during titration it is not necessary to dry the conical flask before adding the alkali ○ The water present will not change the number of moles of reactant added into the conical flask. Hence the titre value will not be affected ● Explain why the conical flask is not rinsed with the solution that it contains ○ Rinsing the conical flask with the solution will cause the volume of reactant to be higher than 25.0cm3. The volume of acid recorded will then be higher than expected ● Another student carries out this titration but average volume of B added was slightly higher than it should be, explain why this error is unlikely to change the calculated whole number value of x [2024 o levels qn1c