A new definition for the mole

The International Union of Pure and Applied Chemistry (Iupac) recommends revising the definition of the mole. Their new definition is based on a specified number of elementary entities. Until now, a mole has been defined with reference to a specified mass.

Iupac’s new definition describes the mole as containing exactly 6.02214076 x 10²³ (the Avogadro constant) elementary entities.

Textbooks published from 1971 onwards are likely to define a mole as the mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of ¹²C.

The new definition stresses that moles are to do with counting entities rather than measuring the mass of a sample of something else.

What does it mean for students?

Speaking to Chemistry World, Marcy Towns from Purdue University, US, pointed out ‘the new definition is well aligned with what students already use in practice.’ A chemistry education researcher and a member of Iupac’s task group, Marcy said, ‘it’s not going to have a big impact on what people do in the classroom. Although of course textbooks have to change.’

Textbooks won’t be rewritten immediately. The new definition needs to be approved by the General Conference on Weights and Measures – the only body that can make changes to SI units. If accepted, Iupac’s recommended definition will become the official one in May 2019. 

Major exam boards in the UK (including the Scottish Qualifications Authority, Edexcel, AQA, OCR, the State Examinations Commission for Ireland, the Council for the Curriculum, Examinations and Assessment in Northern Ireland and the Welsh Joint Education Committee) confirm both definitions for the mole would be accepted from student in exams this summer.

An OCR spokesperson says ‘once the proposed definition is confirmed, we will look at updating our GCSE and AS/A Level specifications’. The SEC Chief Examiner says the new and old definitions ‘will both be acceptable in examinations set by the State Examinations Commission in 2018 and 2019 and the situation will be reviewed again for examinations thereafter’.

Clearing up constants

Breaking the link between the definition of the mole and the definition of a kilogram has advantages. Apart from cutting to the chase with more direct logic, it means international bodies can establish better global conventions over the language of science.

The kilogram’s definition is also changing as part of this work to join up conventions, as are definitions of the ampere and kelvin.