Question

How many moles of `MgS_2O_3` are in 241 g of the compound?

Answer 1

`"1.77 moles"`

Explanation:

The idea here is that you can use a compound's molar mass, which tells you the mass of exactly `1` mole of the compound, as a conversion factor to go from grams to moles or vice versa.

In this case, magnesium thiosulfate has a molar mass equal to

`M_ ("M MgS"_2"O"_3) = color(blue)("136.433 g")color(white)(.)color(darkorange)("mol"^(-1))`

This tells you that every `color(darkorange)("1 mole")` of magnesium thiosulfate has a mass of `color(blue)("136.433 g")`.

You can thus say that your sample will contain

`241 color(red)(cancel(color(black)("g"))) * (color(orange)("1 mole")color(white)(.)"MgS"_2"O"_3)/(color(blue)(136.433)color(red)(cancel(color(blue)("g")))) = color(darkgreen)(ul(color(black)("1.77 moles MgS"_2"O"_3)))`

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