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
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
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
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)
