You use the mole ratio that exists between #"A"# and #"B"#.
Let's say your balanced equation looks like this
#"A" + color(red)(2)"B" -> "products"#
If you have 1 mole of #"A"#, you'd need #color(red)(2)# moles of #"B"# for the reaction to take place. This means that you have a #1:2# mole ratio between #"A"# and #"B"#.
So, if you have #x# moles of #"A"#, you'd have
#xcancel("molesA") * (color(red)(2)" moles B")/(1cancel("mole A")) = "2x moles B"#
So, if you know the number of moles of a species involved in the reaction, you can automatically use the stoichiometric coefficients to determine the number of moles of the rest of the species present.
If you have a balanced chemical equation that looks like this
#color(red)(2)"A" + C -> color(blue)(3)"B" + D#
You'd have a #color(red)(2):color(blue)(3)# mole ratio between #"A"# and #"B"#, so you can go from moles of #"A"# to moles of #"B"# by multiplying by 3/2
#"x"cancel("moles A") * (color(blue)(3)" moles B")/(color(red)(2)cancel("moles A")) = 3/2"x moles B"#
