What is the mass, in grams, of #1.24# moles of water, #"H"_2"O"# ?

1 Answer
Mar 29, 2018

Answer:

#"22.3 g"#

Explanation:

Your tool of choice here is the molar mass of water. The molar mass of a compound tells you the mass of exactly #1# mole of that compound.

#color(blue)(ul(color(black)("the molar mass of H"_2"O" = "the mass of 1 mole of H"_2"O")))#

Now, water has a molar mass of #"18.015 g mol"^(-1)#. This tells you that #1# mole of water has a mass of #"18.015 g"#. So every time your sample of water has a mass of #"18.015 g"#, you can say for a fact that it contains #1# mole of water.

In your case, the sample contains #1.24# moles of water. To find the mass of the sample, use the molar mass of water as a conversion factor.

You need to go from moles to grams, so rearrange the molar mass like this

#"18.015 g mol"^(-1) => "18.015 g"/("1 mole H"_2"O") " "color(white)((color(blue)(larr \ "what you need"))/(color(blue)(larr \ "what you have"))#

So now all you have to do is to multiply the number of moles given to you by this conversion factor.

#1.24 color(red)(cancel(color(black)("moles H"_2"O"))) * "18.015 g"/(1 color(red)(cancel(color(black)("mole H"_2"O")))) = color(darkgreen)(ul(color(black)("22.3 g")))#

The answer is rounded to three sig figs, the number of sig figs you have for the number of moles of water.