MadSci Network: Chemistry Query:

This time, I was nearly stumped! As the apparent expert on "How many moles are in..." questions, I had some real research to do to simply find out how big a speck of dust is. After much searching through PubMed and reading several articles, I found that dust, or particulate matter (PM) comes in a variety of sizes mostly in the range of 1 to 10 �m (microns, thousands of millimeters). These are divided into groups by size, PM2.5 (around 2.5 �m), PM5 (around 5 �m), and PM10 (around 10 �m). Most housedust is in the PM5 to PM10 range, and yes, someone has gone to the trouble of determining the apporximate mass of these particles:

Edwards RD, Yurkow EJ, and Lioy PJ (1998), "Seasonal deposition of housedusts onto household surfaces," Science of the Total Environment 224(1-3):69-80.

Edwards et al. give an average PM5-10 particle mass of 6 �g (micrograms). Since 80% of household dust is organic debris, we can approximate a molar mass of 6.3 grams per mole, such that each particle would have a little under a micromole of atoms, or:

Around 5 hundred quadrillion atoms are in a speck of dust.

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