Question

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

Answer 1

`1.51 * 10^(24)`

Explanation:

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.