Chemistry applicant quiz

We've been asking our undergraduate applicants if they can answer this chemistry question.


If a tap dripped one molecule of water every second from the beginning of the Universe, the quantity of water collected by now would occupy:

a. A tiny drop
b. A swimming pool
c. An ocean
d. The entire planet Earth

Here we'll explain the answer, and how we worked it out.

The answer is option a – a tiny drop.

It is estimated that the Universe was born 13.8 billion years ago. So, the number of seconds since then is: 13800000000 x 365 x 24 x 60 x 60 = 4.4 x 1017.

Over this time, the tap will have dropped 4.4 x 1017 water molecules, which sounds a lot.

Using Avogadro's number, we can equate this to moles:

  • 6.0 x 1023 water molecules equates to 1 mole, so
  • 4.4 x 1017 water molecules equates to 0.7 x 10-6 moles, or 0.7 µmoles

Converting this to grams by multiplying by water’s molecular mass gives us 12.6 µg and assuming a density of ~1 g/ml then this equates to 0.013 microlitres – literally a fraction of a drop!

This calculation shows just how small individual molecules are, and therefore how challenging it is to understand their properties and how they interact with each other.

However, here in Sheffield, Dr Tim Craggs has shown that by using the photophysical properties of dyes, they can measure distances within individual molecules of DNA and proteins. This helps his team to understand how the shape of these complex molecules dictates their properties.

In fact, fourth year students John Cully and James Baxter (pictured) built the microscope that makes these measurements for their final year research project:

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