Question

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

Answer 1

`"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.