Question

A drop of water has a volume of approximately 0.05 mL. How many water molecules does it contain? The density of water is 1.0g/mL.

Would love a step by step process of how you found it!

Thanks as always!

Answer 1

Approx. `1.7xx10^21` `"inividual water molecules...."`

Explanation:

If I asked you how many `"dozen"` biscuits there were in `"1 gross of biscuits"` (`1*"gross"=144`) I think you would able tell me `12` dozen pdq. This is precisely the same sort of question, except that the mole, `6.022xx10^23*mol^-1` is an impossibly large number...

We know that the molar quantity is given by the quotient..

`"Moles of stuff"="Mass of stuff"/"Molar mass of stuff"`

Now here we not given the mass of stuff but the volume of stuff....and we know or should know that for water conveniently `rho_"water"=1*g*mL^-1`..

And so we just reconfigure the given quotient...

`"no. of water molecules"=(0.05*mLxx1.00*g*mL^-1)/(18.01*g*mol^-1)xx6.022xx10^23*mol^-1`...

This gives a number as required (it is still a large number), and you note that it is dimensionless in that all the units have cancelled out...

`"number of water molecules"=1.67xx10^21`

If that looks intimidating, remember all I did was take the quotient....

`"mass of stuff"/"molar mass"`...and multiply this by the given `"Avogadro's number"` ....which you do not have to remember. Good luck.