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How long will it take to count the molecules in a spoonful of water ? Presentation designed by: Dr. Ravi Divakaran, Dept. of Chemistry, St. Albert’s College, Ernakulam

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Confused ? Let me put it in another way … One tablespoon full of water is about 18 mL That is, about 18 grams … Which is one mole of water. So … How many molecules?

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That’s it! Avogadro number! 6.02 x 10 23 How long will it take you to count that many molecules? (a) 10 minutes (b) 1 hour (c) 6 hours (d) 24 hours [Think, and write down your answer before you proceed]

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Imagine that we are going to count them At the rate of one every second … No eating, no sleeping … Just counting non-stop, day and night …

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Oh yes, now you’ve got it. It will take you only 6.02 x 10 23 seconds. Just how long is that?

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60 seconds in a minute, 60 minutes per hour … 3600 seconds per hour. 24 hours in a day … 3600 x 24 = 86400 seconds per day. 365 days in a year … 86400 x 365 = 31536000 seconds a year. Or, roughly, 3.1 x 10 7 molecules in a year.

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That is, you count roughly 3.1 x 10 7 molecules in a year, or, multiply by two, 6.2 x 10 7 molecules in two years. So let us say, we will count upto 10 7 in two years.

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How long will it take to count upto 10 23 ? (a) 2 years and 2 months (b) 3 years and 4 months (c) 8 years (d) 10 years (e) 23 years [Think, and write down your answer before you proceed]

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10 8 = 10 7 x 10 So to count upto 10 8, 2 years x 10 = 20 years. Then … Upto 10 9 in 200 years And 10 10 in 2000 years. If Jesus Christ started counting the molecules, he will still be counting …

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Now a bit faster … Upto 10 13 in 2 million years. If the dinosaurs had started counting, they would be somewhere here now. And upto 10 16 in 2 billion years … That is if you started counting when the earth was formed … (Scientists estimate the age of the earth to be 4.5 billion years)

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Upto 10 17 in 20 billion years … That is, if we started counting long long before the universe itself was formed, we will now reach 10 17. (Scientists estimate the age of the universe to be 14 billion years. The age of the universe is less than 10 17 seconds!) We still have to multiply by 10 6 to finish. (That is, by a million!)

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That means, a million universes will be born and dead by the time we finish counting the molecules in a spoonful of water!

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Now divide that spoonful of water with this infinitely huge (is that word enough?) number to get the size of one molecule of water. Realize how small a molecule is ? Thank you.

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