Use its molar mass!
Let's say that you're dealing with a generic reaction
#"A " + " 2B " -> " C"#
Notice that you have a
So, let's say that the reaction consumes
To convert the number of moles of
This means that
#3 color(red)(cancel(color(black)("moles C"))) * overbrace((x color(white)(.)"g")/(1color(red)(cancel(color(black)("mole C")))))^(color(blue)("the molar mass of C")) = (3x)" g"#
of the product, i.e.