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Measurement and Experimental Techniques

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Measurement and Experimental Techniques 4/26/2010 Learning Objectives At the end of the lesson, you should be able to: Identify the various apparatus used in the laboratory for measurement State the accuracy of the apparatus used in the laboratory. Suggest suitable apparatus to be used for measurement for a variety of experiments In Chemistry experiments… Measuring Mass We measure 4 basic physical quantities: Apparatus: Electronic balance S.I. Unit: kilogram (kg) Mass Normally used to measure small masses of solid chemicals Temperature Smallest division: 0.1 g or 0.01 g Time Accuracy: 0.1 g or 0.01 g Volume Example of how to make recording: 2.4 g or 2.45 g Measuring Temperature Measuring Time Apparatus: Digital Stopwatch or Apparatus: Mercury in glass ( -10 °C to 110 °C) Analogue Stop clock thermometer or alcohol thermometer S.I. Unit: second (s) S.I. Unit: Kelvin (K) Smallest Division / Accuracy: However, degree Celsius (°C) is usually used. Digital – 0.1s / 0.01s Smallest division: 1 °C Accuracy: 0.5 °C Analogue – 0.1s Example on how to make recording: 35 °C or 35.5 °C and not 35.3 °C 1 4/26/2010 Measuring Volume Measuring Volume: Beaker For measuring volume, the apparatus chosen Used to measure rough volume of liquid to be used for an experiment will depend on: Unit: mili-litres (ml) or cm3 Example: 50 cm3 beaker 1. The volume that you are measuring Smallest division:10 cm3 2. How exact you need the volume to be cross-view Measuring Volume: Measuring Cylinder Measuring Volume: Burette Burette burette Setup Used to measure variable volume of liquid Used to measure a variable approximately. volume of liquids accurately. Often used in titration for It is more accurate than a beaker. formation of salts. Smallest division: 1 cm3 Smallest division: 0.1 cm3 It can measure volumes of liquids up to a maximum of 50.0 cm3 and accurately to 0.1cm3. Volume required = Initial Reading – Final reading Cross-view Measuring Volume: Pipette Comparison of Apparatus Apparatus Accuracy Reason Beaker approximate volume and e.g A 50cm3 beaker has markings in To measure a fixed, variable volume divisions of 10cm3. Readings are in 10cm3, specific set of volume (not accurate) 20…..50cm3 Measuring More accurate than e.g A 50cm3 measuring cylinder has of liquid accurately. cylinder beaker and can measure markings in division of 1cm3. Reading are variable volume in 11cm3, 12, 13, ….., 48, 49, 50 cm3 For example: Pipette More accurate than e.g A 50.0 cm3 pipette measures fixed 25.0cm3; 20.0cm3; beaker and measuring volume. It mean that every measurement cylinder. It has the same gives a fixed volume of 50.0cm3 . 3 50.0cm accuracy as burette but It is used during titration when making has a fixed volume. salts. Note: Cannot Burette More accurate than e.g A 50.0 cm3 burette can measure beaker and measuring volume such as 24.5, 24.6, 24.7….49.8, measure odd volume cylinder. It has the same 49.9, 50.0cm3 such as 26.0cm3 accuracy like pipette but cross-view measures variable volume. 2 4/26/2010 How to read the volume of a liquid? Measuring Volume of Gases When water or solution is placed in a glass container, it Apparatus: Gas syringe forms a curved surface called a meniscus. plunger Unit: cm3 A meniscus can be concave or convex. To read the volume of a liquid, align your eyes to the liquid level. barrel concave convex How to use a gas syringe? Collecting Gases Upward delivery - To collect gas that are soluble in water and less dense than air such as ammonia downward delivery - To collect gases that are soluble in water and denser than air such as chlorine and hydrogen chloride Displacement of water - To collect gases that are insoluble in water. E.g. Nitrogen gas, hydrogen gas, oxygen gas How to determine if a gas is light or heavy? Mini-Exercise As a general guide, a gas is less dense than What would a chemist use to measure air if its molecular mass is less than 30. exactly 25.5 cm3 of dilute hydrochloric acid? E.g. Ammonia (NH3) Molecular mass of ammonia A Beaker = 14 + 1 + 1 + 1 B Burette = 17 (less than 30) C Pipette Hence, ammonia is a light gas. D Measuring cylinder ANSWER: 3 4/26/2010 Mini-Exercise Mini-Exercise What is the reading of the volume of Mingde records a temperature of 48.2°C liquid shown in the measuring cylinder? from a mercury-in-glass thermometer. What is the mistake he made for his A 28 cm3 recording? B 30.6 cm3 Answer: C 36 cm3 For a mercury-in-glass thermometer, the D 40 cm3 accuracy of the apparatus is 0.5°C. Hence, it is not possible for Mingde to record a reading of 48.2°C. ANSWER: Mini-Exercise Determining Purity Name one suitable piece of laboratory apparatus A pure substance is made up of only one substance. for (a) measuring the volume of about 80 cm3 of a liquid, beaker Air is a mixture. It is made up of different gases. However, a white diamond is pure. (b) adding exactly 18.5 cm3 of liquid to a beaker, burette (c) collecting and measuring 80 cm3 of gas, and Gas syringe (d) adding exactly 25.0 cm3 of a solution to a flask. pipette Why is it important to determine How do we determine the purity of purity of a substance? a substance? Impurities in drugs must be detected as they may cause side effects. We can do so by doing one of the following: Chemicals are often added to food and beverages. It is important to ensure that only the pure chemicals are added. - Checking the melting point of a solid - Checking the boiling point of a liquid - Performing chromatography 4 4/26/2010 Determining Purity by Melting Effect of Impurities on Melting Points and Boiling Points Impurities affect the melting point of a substance in A pure solid has an exact and constant two ways: melting point. • They lower the melting point. A pure liquid has an exact and constant boiling point. The greater the amount of impurities, the lower the melting point of the substance. • They cause melting to take place over a range of temperatures. Effect of Impurities on Boiling Points If a liquid is impure, its boiling point will increase. The liquid will also boil over a range of temperatures. The greater the amount of impurities, the higher the boiling point of the liquid. 5.
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