Measurements in Chemistry
17
Experimental Chemistry Topics covered: Measuring volumes Methods of Gas Collection 7 th February 2015 Page 1
Transcript of Measurements in Chemistry
Experimental Chemistry
Topics covered:
Measuring volumes
Methods of Gas Collection
7th February 2015
Page 1
Measurements in ChemistryThe following physical quantities are usually
measured when performing experimental
chemistry.
1. Volume
2. Temperature
3. Time
4. Mass
What are the SI unites of the above mentioned physical
quantities?
What apparatuses would you use to measure then?
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Measuring Volume of Liquids
• The apparatus used depends on:
– the volume being measured
– how accurate the measurement needs to be
• Common apparatus:
– Measuring cylinder
– Burette
– Pipette
– Beaker*
– Conical flask**Beaker and Conical flask are for containing liquids,
they do not measure liquids accurately Page 3
Measuring or Graduated cylinder
– Only used for measuring approximate volumes. Cannot be used for precise volume measurements.
– Accuracy of 1 cm3
–Not used for mixing, containing or storing chemicals
What is the volume measured in the
oval above?
Measuring Volume of Liquids: Measuring Cylinder
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Measuring Volume of Liquids: Bulb Pipette
– Measures out precise
volumes.
– EXACTLY(precisely)
20.0 cm3, 25.0 cm3 or
50.0 cm3
– Used with a pipette filler
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Measuring Volume of Liquids: Burette
- Long scale: 0 cm3 to 50 cm3
– Each graduation or marking has an accuracy of 0.1 cm3
– Reading error of 0.05 cm3
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Apparatus used for Experimental Chemistry
• Heating a flammable liquid
THINK TANK: Why can’t
we heat the test tube
directly over the flame?
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Apparatus used for Experimental Chemistry
• Measuring change in mass during a chemical
reaction.
THINK TANK: Why
is cotton wool placed
at the mouth of the
conical flask?
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• Collecting and measuring a volume of gas
Apparatus used for Experimental Chemistry
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Types of Gas Collection
Methods depends on the physical properties of the gas:
– the solubility of the gas in water
– the density of the gas compared to air
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Solubility & Density of Some Common Gases
Activity: Decide their method of collection
Gas Solubility in waterDensity compared to
air
ammonia extremely soluble less dense
carbon dioxide slightly soluble denser
chlorine soluble denser
hydrogen not soluble less dense
oxygen very slightly soluble slightly denser
sulfur dioxide very soluble denser
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Checklist for deciding method of gas collection
Solubility in waterDensity compared
to air
Type of Gas
Collection
Soluble less dense Upward Delivery
Soluble denserDownward
Delivery
Insoluble/slightly
solubleless dense
Displacement of
Water
Insoluble/slightly
solubledenser
Displacement of
Water
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Collection of Gases: Dry Samples
Dry samples of gas can be obtained by passing the gas through a
drying agent. Drying agents have a great liking for water
molecules. Water must be removed from gases to obtain gas
samples of high purity and some chemical reactions tend to be
dangerous in presence of water or water vapor.
Drying agents must not react with the gas being dried.
Some common drying agents:
• concentrated sulfuric acid
• quicklime (calcium oxide)
• Fused calcium chloridePage 13
Concentrated Sulphuric acid – Liquid Drying
Agent as Sulphuric acid is in liquid form
Concentrated sulfuric acid ( used for acidic gas
i.e. HCl gas, Hydrogen sulfide gas, H2S)
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Solid Drying Agents:
Quicklime & Fused Calcium Chloride
•Quick lime aka Calcium oxide ( used for
alkaline gas i.e ammonia)
•Fused calcium chloride ( used for general
gases like hydrogen and oxygen as it is neutral
in nature)
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Important Points to Take Note
• If a gas is required DRY it must NOT be
collected over water.
• A soluble gas must NOT be collected over
water.
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Not sure of density of Gas?
Density of a gas is approximately equal to its relative
molecular mass, Mr;
• Density of gases as compared to density of air can be
obtained by comparing the Mr of gases to that of air. The ‘Mr ’ of
air [a mixture] can be taken to be approximately equal to ‘29’
[since air is about 20% oxygen and 80% nitrogen; Mr of air is
closer to that of nitrogen]
• Example: find the density of carbon dioxide as compared to
air. Mr of CO2 = 12 + 2(16) = 44 , therefore CO2 is denser than
air. 17
