# The mole

The mole (mol) defines the amount of substance (rahinga matū).

The definition of the mole was very slightly changed in 2019. It used to be the amount of substance that contains as many chemical units (atoms, molecules or other particles) as there are atoms in exactly 12 grams of carbon-12 (i.e. 6.023 X 1023), exploiting the fact that measuring mass is a useful way to count large numbers of very small things.

The value of the Avogadro number (NA) was fixed at 6.022 140 76 x 1023 mol–1 as part of the SI revision. One mole is now defined as the amount of substance of a system that contains 6.022 140 76 x 1023 specified elementary entities.

Discussion point: Mole Day is an unofficial holiday celebrated by scientists. The date is October 23 and the holiday begins at 6.02 am. Why has this date and time been chosen?

NOTE: This video was filmed prior to the change to SI definitions. As of 20 May 2019, the mole is now defined using the Avogadro number instead of 12 grams of carbon-12.

## Transcript

FARZANA MASOULEH

At the moment, one mole is the absolute – is the mass of a substance with the entities that are equal to atoms in 12 grams of carbon-12. But after 20th May [2019], we will use the Avogadro’s number, which is a familiar constant for us. So we will use this constant as one mole. And we will fix Avogadro’s number as we fixed Boltzmann’s constant, Planck’s constant and electron charge.

### Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology development.

**Published:**15 August 2019