Making a mountain out of a Mole Day

Today is Mole Day, chemists’ #1 holiday! Mole Day occurs every year on October 23 from 6:02am to 6:02pm to commemorate Avogadro’s Number and the basic measuring unit of chemistry, the mole.

What is Avogadro’s Number?

Avogadro’s Number is currently defined as the number of atoms in 12 grams of carbon-12, which comes to 6.02 x 10²³.

Amadeo Avogadro was a 19th century Italian scientist who first proposed in 1811 that equal volumes of all gases will contain equal numbers of molecules to each other (known as Avogadro’s Law).  Nearly one hundred years later, in 1909, chemists decided to adopt the mole as a unit of measure for chemistry. At the time, the scientists decided to define the mole based on the number of atoms in 12 grams of carbon-12. Jean Baptiste Perrin suggested this number should be named after Avogadro, due to his contributions to molecular theory.

Molecules and atoms are very tiny and numerous, which makes counting them particularly difficult. To put it into perspective, an atom is one millions time smaller than the width of the thickest human hair. It’s useful to know the precise amount of certain substances in a chemical reaction, but calculating the number of molecules would get very messy if every time we had to use numbers like 602,214,129,270,000,000,000,000.

Enter Avogadro’s number! Using the mole simplifies complex calculations. Before the mole was adopted, other units were inadequate for measuring such miniscule amounts. After all, one millilitre of water still has 33,456,340,515,000,000,000,000 H₂O molecules!

This doesn’t mean that one mole of different substances equal each other in mass or size; it simply refers to the number of something, while size and mass vary by object. For example, a mole of water molecules would be about 18 millilitres, while a mole of aluminium molecules would weigh about 26 grams. However, a mole of pennies would cover the Earth at a depth of over 400 metres.  And a mole of moles would weigh over half the size of the moon!

Why Mole Day?

Schools around the U.S. and other places use the day as a chance to cultivate an interest in chemistry among students. Mole Day goes back to the 1980s, when an article in The Science Teacher magazine proposed celebrating the day. This inspired other teachers to get involved and  a high school chemistry teacher in Wisconsin founded the National Mole Day Foundation in 1991. The American Chemical Society then planned National Chemistry Week so this it falls on the same week as Mole Day every year.

Every year, chemistry teachers use this as an opportunity to perform fun experiments, bake mole-shaped desserts, and teach random facts about Avogadro’s number to students, with the aim of increasing science engagement

revised-SI-logoWhat about the revised SI?

In a previous blog post, we outlined how several of the units of the International Standards of units are undergoing a change. For example, the kilogram will no longer be based on a physical artefact, but on a constant. In the case of the mole, the current definition defines one mole as containing as many molecules as “atoms in 12 grams of carbon-12”. The new definition, which will likely come into effect next May, simply defines the mole as containing exactly 6.02214076 x 10²³ elementary entities. This eliminates any reference to mass and lays out the exact number of molecules as Avogadro’s constant, so the mole will not be dependent on any substance’s mass.

More Mole Facts

A mole of doughnuts would cover the earth in a layer five miles deep!

All of the living cells in a human body make up just over half a mole.

A mole of rice grains would cover all of the land area on Earth at a depth of 75 metres.

A mole of turkeys could form sixteen earths.

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