Pure Chemistry

• Scientists use S.I. unit as a common standard for their measurements

  S.I. Units (System of Units)
• a stopwatch is used to measure time. S.I. Unit for time is second (s)
• A thermometer is used to measure temperature. the S.I. unit for temperature is the kelvin (K).

  Other commonly used units are degree Celsius
• Conversion:

  Temperature (K)=Temperature (degree celsius) + 273

  e.g. 0 degree celsius = 273K
• alcohol-in-glass thermometer or mercury thermometer is used for measuring temperature of reaction mixtures

  i) Alcohol-in-glass measures temperatures ranging from -10 degree celsius to 110 degree celsius

  ii) Accuracy +-0.5 degree celsius

  rule: 1/2 the smallest division (1 degree celsius)

  e.g. 18.0,18.5,19.0
• data logger connected to a temperature probe sensor can also be used to measure temperature and is more accurate than the mercury or alcohol-in-glass thermometer
• A beam balance or an electronic balance are used to measure mass. the S.I. unit for mass is the kilogram (kg). Other commonly used units are the gram (g) and tonne (T). 1kg=1000g 1 tonne = 1000kg=1000000g
• Beam balance and electronic balance.

  The electronic balance is used for precise and fast weighing of small amounts of substances with an accuracy up to +-0.01g (to 2dp)
• S.I. unit for volume is the cubic metre (m3). Other commonly used units are cubic centimetre (cm3) and *cubic decimetre (dm3) * 1dm3=1000cm3

  Conversion: 1000cm3=1dm3=1 litre
• Beaker:

  • Measures approximate volume e.g. 100cm3
  • LEAST ACCURATE apparatus to measure volume of liquids
• Measuring cylinder:

  • Measures volume to the nearest cm3, e.g. 54cm3
• Pipette:

  • Measures fixed volume very accurately, e.g. 5.0cm3, 10.0cm3, 15.0cm3, 20.0cm3 and 25.0cm3

  A) The mark is for accurate volume measurement

  B) use a pipette filler to fill it

  
• Burette:

  • Measures the volume of liquid to the nearest +-0.1cm3
  • The burette reading is accurate to +-0.05cm3, e.g. 26.70cm3 and 26.75cm3
  • Each marking is 0.1cm3. (accuracy=0.05cm3)
  • MOST ACCURATE apparatus to measure volume of liquid
• Gas syringe:

  • measures the volume of gas produced during a chemical reaction
• how to read the volume of liquid using a burette, align eyes to the liquid level

  Convex meniscus (n - shape) read the scale at the top of the meniscus

  Concave meniscus (u - shape) read the scale at the bottom of the meniscus
• During titration:

  Burette, pipette and conical flask are used
• Explain why it is good to practice to repeat a titration several times.

  It is to obtain a average volume of liquid added from a few accurate titration reading to eliminate any anomalous (not normal) results
• The gas syringe is commonly used to measure to volume of a gas.

  It measures a maximum volume of 100cm3 and used when needed to measure accurately
• When asked to draw and label an experiment and the reactant in the equation has a gas, draw a gas syringe (PG 9 of 1.1)
• Questions: The teacher selects apparatus in which to collect and measure the volume of oxygen gas. State the apparatus that could be used.

  Gas syringe.
• Question: In a further experiment, the teacher needs to measure 24.80cm3 of hydrogen peroxide. Explain why a measuring cylinder is NOT suitable for making this measurement.

  A measuring cylinder is only able to measure approximately to the nearest cm3.
• Collecting gases by

  • displacement of water
  • displacement of air
  • upward delivery
  • downward delivery
• Methods used for collecting a gas depends on TWO factors:

  • SOLUBILITY of the gas in water
  • DENSITY of the gas as compared to air
• Downward delivery (upward displacement of air):

  • Used when the gas is denser than air ( gas mass number is above 28)
  • Used when gas is SOLUBLE in water and DENSER than air
  • Air is being pushed out of the gas jar

  
• Upward delivery (downward displacement of air) :

  • used when the mass of gas is less than 28
  • gas Is SOLUBLE in water
  • LESS DENSE (lighter) than air
  • air is being pushed out of the gas jar

  
• Displacement of water:

  • Gas is INSOLUBLE or SLIGHTLY SOLUBLE in water
  • example of these gasses : hydrogen, oxygen, carbon dioxide

  
• Gas syringe:

  • used if volume measurement is needed to be accurate
  • any gas can be measured
• Chlorine is soluble in water

  hydrogen chloride and sulfur dioxide is very soluble in water

  ammonia is extremely soluble in water
• A student makes some crystals. what should the student check to test the purity of the crystals? A. melting point / B. shape of crystals/ C. size of crystals/ D. solubility

  A. Melting point
• Sublimation is used to separate a solid that sublimes on heating from another solid that does not sublime.

  sublimation method has very limited uses because very few solids will sublime on heating.

  examples of solids that sublime:

  • ammonium salts
  • iodine solution
  • dry ice
• How sublimation works:

  1. the mixture is placed in an evaporating dish
  2. an inverted filter funnel is placed over the mixture
  3. on gentle heating, ammonium chloride sublimes and is deposited on the cold filter funnel