# The Mole Will Change in 2019

The definition of a mole will change on 20 May 2019. However, the mammal won’t change, the measure will.

Most of us don’t need to measure the number of atoms, ions, or other tiny entities in a sample of a substance so we’re not aware that the mole (mol) is the unit that does it. Its definition used to be based on 0.012 kilogram of carbon-12 but physical properties can change, so its size could change minutely and throw off precise scientific measurements. This troubled chemical engineers who use the mole extensively.

To fix the problem, the General Conference on Weights and Measures (CGPM) voted at their 16 November 2018 meeting to unlink the mole from its physical definition, redefining it using a mathematical constant. The new definition will be:

One mole of a substance will have exactly 6.02214076×1023 specified “elementary entities” of that substance.

Wikipedia entry for mole (unit)

Three other measuring units had the same problem so the meeting redefined four:

• Mole: the unit for amount of substance.
• Ampere: a unit of electric current equal to a flow of one coulomb per second.
• Kelvin: a temperature scale similar to Celsius but 0 degrees K is absolute zero.
• Kilogram: the base unit of mass

Perhaps you’ve already heard that the kilogram will change on 20 May 2019. Originally defined as the mass of a litre of water, the kilogram (kg) was redefined in 1799 to be the mass of a block of platinum that’s stored in a vault in Paris. Even the platinum’s mass can change so now the kilogram will be defined by the Planck constant which was defined to be exactly 6.62607015×10-34 kg.m2.s-1. As Wikipedia points out, “This approach effectively defines the kilogram in terms of the second and the metre.”

Notice that the second is raised to minus one, s-1. Is your head spinning? There’s a simpler explanation here in Popular Mechanics.

Meanwhile, why is a hairy-tailed mole (Parascalops breweri) illustrating this article if the message has nothing to do with him? Well, there’s no illustration for the ‘measure’ mole so I’m showing you the ‘mammal’ mole.

(image from Wikimedia Commons; click on the caption to see the original)