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​0502 Avogadro constant: How many particles is that?


This post is closely related to 0502 Avogadro constant: How many particles is that?


KEY IDEAS

Please be patient while I learn how to make superscripts and subscripts on this site
​

1 mol of a substance is the amount of it that contains 6.023 × 10^23 specified particles. For example:
  • There are 6.023 × 10^23 iron atoms (Fe) in 55.85 g of iron, Fe(s)
  • There are 6.023 × 10^23 water molecules (H2O) in 18.02 g of water, H2O(l)
  • There are 6.023 × 10^23 sodium ions (Na+) in 58.45 g of sodium chloride, NaCl(s)
  • There are 6.023 × 10^23 nitrogen molecules (N2) in 28.01 g of nitrogen gas, N2(g)
 
This number (6.023 × 10^23) is called the Avogadro constant (symbol NA). In some older books, it is referred to as Avogadro’s number.
 
This number is so big that it is impossible to visualise. But there are stories that help us to realise that it is too big to visualise. Such as …
If we have 1 mol of a substance and we could remove one million particles every second, non-stop, day and night, it would take us 1.9 × 10^10 years to count out all of the particles. The duration since the “big bang” is estimated to be 1.5 × 10^10 years (15 billion) years. So, we would have counted out only 80% of the particles!
 
The important “takeaway message”: The number of particles in 1 mol of a substance is more than we can comprehend. To understand chemistry, we need to have good visualisation of the substances and reaction mixtures that we are considering. Try to form a mental picture of this many water molecules (6.023 × 10^23) moving and bobbing and weaving within an 18 g (= 18 mL) sample of liquid water, or of N2 molecules darting around in a sample of 28.01 g of nitrogen gas. It’s a crowded environment! This might be a useful image in future.
 
Recall: The ratio of the numbers of particles in samples of two substances is the same as the ratio of the amounts (in moles). We usually don’t need to know the absolute number of particles in a sample of stuff.
 


​SELF CHECK

1.         We usually write numbers out in full in groups of three digits (such as 10 600, or 1 342 000).Which of the following is the Avogadro constant?
            A:        60 230 000 000 000 000 000
            B:         602 300 000 000 000 000 000 000
            C:         0.000 000 000 000 000 000 006 023
            D:        60 231 023


​2.         How many years (approximately) would it take to count out, at 1 Fe atom per second, all of the atoms in 55.85 g of iron, Fe(s).
            A:        1.07 × 10^18 years
            B:         3.42 × 10^14 years
            C:         55.85 years
            D:        1.91 × 10^16 years



​3.         If we had a sample of iron with mass 55.85 g, and we could count out one atom per second continuously, what percentage of the mass would have been counted out by now if we began at the time of the Big Bang 15 000 000 000 years ago?
            A:        100%
            B:         7.85 × 10^-5 %
            C:         7.85 %
            D:        7.85 × 10^-5 %

4.         What mass of water contains the same number of water molecules as there are sodium ions in 5.845 g of sodium chloride?
            A:        5.845 g
            B:         0.100 g
            C:         1.802 g
            D:        6.023 × 10^22



Answers: 1(B); 2(D); 3(B); 4(C)
​

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  • Home
  • Modules
    • 02 The nature of matter >
      • 0201 Atoms: Building blocks
      • 0202 Classification of matter
    • 05 Chemical reaction, chemical equations >
      • 0500 Overview, preview
      • 0501 Amount of substance, mole
      • 0502 Avogadro constant: How many?
      • 0503 Avogadro constant: Why that number?
      • 0504 Chemical formulas: What do they mean?
      • 0505 What can equations tell us?
      • 0506 Limiting reactants
    • 09 Solutions >
      • 0901 What is a solution?
      • 0902 Miscibility of liquids
      • 0903 Like dissolves like?
      • 0905 Dissolution of ionic salts in water
      • 0906 Can we predict solubilities of salts?
      • 0907 Solution concentration
      • 0908 Chemical species, speciation
      • 0909 Solutes: Electrolytes or non-electrolytes?
      • 0910 Electrolytes - strong or weak?
      • 0911 Concentrated, dilute, strong, weak
      • 0912 Species concentration vs. solution concentration
      • 0913 Weak electrolytes: Getting quantitative
    • 11 Dynamic chemical equilibrium >
      • 1100 Equilibrium: An overview
      • 1101 Visualising dynamic equilibrium
      • 1102 The jargon of equilibrium
      • 1103 Equilibrium constants
    • 22 Evidence from spectroscopy >
      • 2200 Overview, preview
      • 2201 Spectroscopy: Quantization of energies
      • 2202 Light: Wave-particle "duality"
      • 2203 UV-Visible spectroscopy
      • 2204 Beer's law
    • 27 Communicating chemistry >
      • 2700 Overview, preview
      • 2703 The jargon we use
  • TOC
  • Index
  • Teachers' area
    • T01 Communicating chemistry
    • T02 Beer's law
    • T03 Professional amnesia of the chemistry teaching professio
    • T04 Law of equilibrium
    • T05 Visusalizing dynamic chemical equilibrium
  • Aha! Whatever
    • Playful dolphins
    • The University of Western Australia
    • Kings Park
  • Prof Bob?
    • Family
    • Travel
    • Perth
    • At work
  • Travelling
  • Contact
  • Blog
  • In four days for two days