• Question: how many atoms is there in a blade of grass?

    Asked by anon-210342 to Tori, Titus, Stuart, Hannah, Gill on 13 Mar 2019.
    • Photo: Hannah Dalgleish

      Hannah Dalgleish answered on 13 Mar 2019:

      This is actually a very complicated question! I tried to figure it out but it involves a lot of Chemistry I don’t really remember. 😛 I think you need to know what the grass is made of (water and lignin apparently) and then know how many of each molecule there is in the blade of grass. When you know this, you can use Avogadro’s number to work out the number of atoms!
      Given that there are 50 000 000 000 000 000 000 atoms in a grain of sand, there are *a lot* of atoms in a blade of grass.

    • Photo: Stuart Higgins

      Stuart Higgins answered on 14 Mar 2019:

      As Hannah said, we can roughly work it out if we make some educated guesses. Googling around a bit:

      A blade of grass is about 0.1 grams, it’s made of roughly 70% water and 30% lignin.

      The chemical formula of water is H2O, and has a mass of 18 grams per mole
      The chemical formula of lignin is roughly C10H12O3, and has a mass of 180 grams per mole

      One mole is how much mass 602,214,076,000,000,000,000,000 atoms of a particular material is (that’s the Avogadro’s number Hannah mentioned). So a molecule of lignin has more mass than a molecule of water.

      Now we use the percentage of the weight of the grass that is water or lignin, and the numbers above to work out how many moles we have in total.

      So for water 70% of 0.1 grams is 0.07 grams. 0.07/18 = 0.004 moles
      And for lignin 30% of 0.1 grams is 0.03 grams. 0.03/180 = 0.002 moles

      0.004 + 0.002 moles = 0.006 moles

      So the number of atoms is 0.006 X 602,214,076,000,000,000,000,000
      which equals about 3,613,284,456,000,000,000,000 atoms in a blade of grass.

      (Which is about 72 times more atoms than in a grain of sand. Phew, the answer makes sense!).

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