How many water molecules are in a block of ice containing 2.50 mol of water?

1 Answer
Aug 10, 2016

#1.51 * 10^(24)#


This is a pretty straightforward example of how to sue Avogadro's number to figure out the number of molecules present in a given sample.

In your case, the sample is said to contain #2.50# moles of water. Now, a mole is simply a very, very large collection of molecules. In order to have #1# mole of a molecular compound you need your sample to contain #6.022 * 10^(23)# molecules of that compound.

#color(blue)(|bar(ul(color(white)(a/a)"1 mole" = 6.022 * 10^(23)"molecules" color(white)(a/a)|))) -># Avogadro's number

This is what Avogadro's number is all about -- the number of molecules needed to form #1# mole of a molecular compound.

So, now that you know how many molecules are needed to have #1# mole of water, use Avogadro's number as a conversion factor to calculate the number of molecules present in your sample

#2.50 color(red)(cancel(color(black)("moles H"_2"O"))) * (6.022 * 10^(23)"molec. H"_2"O")/(1color(red)(cancel(color(black)("mole H"_2"O")))) = color(green)(|bar(ul(color(white)(a/a)color(black)(1.51 * 10^(24)"molec. H"_2"O")color(white)(a/a)|)))#

The answer is rounded to three sig figs.