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01

The expert's foundations: the mole, atomic weights and concentration

In simple words

A baker counts eggs by the dozen. Chemists count tiny particles by the mole, which is a much, much bigger "dozen". A great tutor knows exactly what a mole is, where the numbers for atom masses come from, and why some of those numbers are a small range instead of one number.

Before this course: Chemistry Lab Skills: safe lab technique (course 15.4.3). You should be able to write a risk assessment, read a hazard label and safety data sheet, measure with uncertainty, make and dilute solutions, run the kitchen titration and write a full lab report. Now you learn to teach all of that, and to check other people's chemistry the way a professional does.

Key facts

The mole (IUPAC): "The mole, symbol mol, is the SI unit of amount of substance." One mole contains exactly 6.022 140 76 x 10^23 elementary entities. This fixed number is the Avogadro constant, N_A = 6.022 140 76 x 10^23 mol^-1.
When it changed (NIST): the mole, kilogram, kelvin and ampere were redefined in terms of constants of nature in a change voted on 16 November 2018, which went into effect on 20 May 2019. Older books may still give the earlier definition; tutors should know that the current one fixes the number of entities exactly, and teach that.
Standard atomic weights (IUPAC CIAAW): some elements have interval values, because the isotope mix of normal materials varies naturally. Examples:

Element	Standard atomic weight (CIAAW)	Abridged value (5 figures)
Hydrogen	[1.007 84, 1.008 11]	1.0080 ± 0.0002
Carbon	[12.0096, 12.0116]	12.011 ± 0.002
Oxygen	[15.999 03, 15.999 77]	15.999 ± 0.001
Sodium	22.989 769 28 (one value)	22.990 ± 0.001
Chlorine	[35.446, 35.457]	35.45 ± 0.01

The latest table is Standard Atomic Weights 2024. CIAAW publishes the abridged values for everyday use, because the full table can give more detail than most people need.

Why it matters (checked with Python): using the intervals, the molar mass of water can be anywhere from 18.0147 to 18.0160 g/mol. With abridged values it is 2 x 1.0080 + 15.999 = 18.015 g/mol. So quoting water's molar mass to more than about five figures is false precision.
Counting particles: 1.00 g of water is 1.00 / 18.015 = 0.0555 mol, which is 0.0555 x 6.022 140 76 x 10^23 = 3.34 x 10^22 molecules.
Concentration words: NIST's checklist for reviewing writing lists "molarity" and the symbols M, N and m (molal) as obsolete terms that are not used. Many school books still use "molarity" and "M", so a tutor knows both, but writes the unit in full: 0.500 mol/L.

Step by step

Write the IUPAC definition of the mole on a card, with the exact Avogadro constant.
Open the CIAAW abridged table and copy the values for H, C, O, Na and Cl.
Calculate the molar mass of water, baking soda (NaHCO3) and carbon dioxide with the abridged values. (You should get 18.015, 84.006 and 44.009 g/mol.)
Calculate how many molecules are in 1.00 g of water.
Using the hydrogen and oxygen intervals, find the smallest and largest possible molar mass of water.
Rewrite two old worked solutions so that every concentration is written in mol/L.
Check: your molar masses match the ones above, and you can explain in one sentence why hydrogen has a range.

Watch on YouTube

What is the mole?Arizona State University · YouTube · youtube.com

Videos by other creators on YouTube, for reference. They are not made by or affiliated with The Future Market.

LESSON 01The expert's foundations: the mole, atomic weights and concentration

LESSON 01 / ANIMATED LESSON / AI VOICE00:00 / 01:15

Voice

Read the video script

[Host holding an egg carton] "Bakers count in dozens. Chemists count in moles."
[Text on screen: 1 mol = exactly 6.022 140 76 x 10^23 entities] "Since 2019, a mole is exactly this many particles. No weighing needed in the definition."
[Screen: CIAAW table with hydrogen shown as a range] "Look at hydrogen. Its atomic weight is a range, because the isotope mix in nature varies a little."
[Text on screen: water: 18.0147 to 18.0160 g/mol] "So water's molar mass is about 18.015. More digits than that would be pretending."
[Host] "Expert tutors know where every number comes from, and how many digits it deserves."

Try it now

Make a one-page "constants and masses" reference sheet for your learners: the mole definition, N_A, the abridged atomic weights of ten common elements from the CIAAW page, and molar masses of five kitchen substances (water, salt, sugar, baking soda, acetic acid), each calculated by you and checked twice.

ClaudeStuck? Copy a ready-made prompt for Claude

Copy this, open Claude, paste it, and Claude will walk you through the lesson step by step.

For a grown-up: Claude is for adults (18+). Copy this, paste it into Claude, and use the answer to explain the lesson to your child.

I'm learning "Chemistry Tutor: teach" on The Future Market. Lesson 1: "The expert's foundations: the mole, atomic weights and concentration".

What the lesson teaches:
- The mole (IUPAC): "The mole, symbol mol, is the SI unit of amount of substance." One mole contains exactly 6.022 140 76 x 10^23 elementary entities. This fixed number is the Avogadro constant, N_A = 6.022 140 76 x 10^23 mol^-1.
- When it changed (NIST): the mole, kilogram, kelvin and ampere were redefined in terms of constants of nature in a change voted on 16 November 2018, which went into effect on 20 May 2019. Older books may still give the earlier definition; tutors should know that the current one fixes the number of entities exactly, and teach that.
- Standard atomic weights (IUPAC CIAAW): some elements have interval values, because the isotope mix of normal materials varies naturally. Examples:
- Element - Standard atomic weight (CIAAW) - Abridged value (5 figures)
- Hydrogen - [1.007 84, 1.008 11] - 1.0080 ± 0.0002
- Carbon - [12.0096, 12.0116] - 12.011 ± 0.002
- Oxygen - [15.999 03, 15.999 77] - 15.999 ± 0.001

The exercise I'm working on:
Make a one-page "constants and masses" reference sheet for your learners: the mole definition, N_A, the abridged atomic weights of ten common elements from the CIAAW page, and molar masses of five kitchen substances (water, salt, sugar, baking soda, acetic acid), each calculated by you and checked twice.

I'm stuck. Please:
1. Explain the main idea in plain, simple words with one everyday example.
2. Walk me through the exercise one small step at a time. Ask me what I've done so far before giving the next step.
3. Don't just give me the final answer. Help me do it myself, and check my work when I share it.

I'm a parent helping my child (about 8 years old) with a lesson called "The expert's foundations: the mole, atomic weights and concentration" from the course "Chemistry Tutor: teach".

Here's what the lesson says:
A baker counts eggs by the dozen. Chemists count tiny particles by the mole, which is a much, much bigger "dozen". A great tutor knows exactly what a mole is, where the numbers for atom masses come from, and why some of those numbers are a small range instead of one number.

The activity:
Make a one-page "constants and masses" reference sheet for your learners: the mole definition, N_A, the abridged atomic weights of ten common elements from the CIAAW page, and molar masses of five kitchen substances (water, salt, sugar, baking soda, acetic acid), each calculated by you and checked twice.

Please explain this so an 8-year-old understands: short sentences, a fun everyday example, and no hard words.
Then give me 3 simple questions I can ask my child to check they understood, and a way to do the activity together, one tiny step at a time.

Open Claude

Sources

IUPAC, The new definition of the mole has arrived: iupac.org/new-definition-mole-arrived/
NIST, SI redefinition (vote and effective date): www.nist.gov/si-redefinition
NIST, Meet the constants (Avogadro constant): www.nist.gov/si-redefinition/meet-constants
IUPAC CIAAW, Standard atomic weights: www.ciaaw.org/atomic-weights.htm
IUPAC CIAAW, Abridged standard atomic weights: www.ciaaw.org/abridged-atomic-weights.htm
NIST, Checklist for reviewing manuscripts (item 22, obsolete terms): physics.nist.gov/cuu/Units/checklist.html

My notes

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