How many molecules?

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The problem :: Your signature weighs 5 microgrammes. The gel pen ink has a molecular weight of 1275. How many molecules of ink are in your signature?

The Solution

Number of molecules = mass / molecular mass * Ao

where Ao is Avogadro's number = 6.022 × 10²³

5 microgrammes is 0.000005 g. The (g) molecular weight of the ink is 1275. So

0.000005 / 1275 * 6.02214199 × 10²³ = 2.362 × 10¹⁵ molecules

More like this

Now try these - you will need to look up the molecular / atomic masses of the elements / compounds.

1)    0.5g of Water contains how many molecules?

2)    5mg of Zinc contains how many molecules?

3)    You need 15 micrograms of potassium per day to stay healthy. Find out what potassium does in the body, and estimate how many molecules of potassium you need each day.

Answers in a week or two. This topic was prompted by Paul S.

Keith Burnett
Last modified: 2003-11-10