How many molecules of water are there in 36 grams of H_2O?

1 Answer
Aug 2, 2016

1.2xx10^(24) "molecules"

Explanation:

To go from the mass of water to molecules of water, we have to do two things:

  1. Convert mass of H_2O to moles of H_2O using the molar mass of H_2O as a conversion factor

  2. Convert moles of H_2O to molecules of H_2O using Avogadro's number (6.02xx10^(23)) as a conversion factor

color(brown)("Step 1:"
Before we start, I should note that the molar mass of H_2O is 18.01 g/(mol).

We can go from mass to moles using dimensional analysis. The key to dimensional analysis is understanding that the units that you don't need any more cancel out, leaving the units that are desired:

36cancelgxx(1mol)/(18.01cancelg) = 2.00 mol

color(red)("Step 2:"
We'll use the following relationship:
www.sprinklernewz.uswww.sprinklernewz.us

Using the moles of H_2O that were just obtained, we can use Avogrado's number to perform dimensional analysis to cancel out units of mol to end up with molecules:

2.00cancel"mol"xx(6.02xx10^(23) "molecules")/(1cancel"mol") = 1.204xx10^(24) "molecules"

color(blue)("Thus, 36g of water is equivalent to" 1.2xx10^(24) "molecules"