How many moles are there in 397 grams of #Na_2SO_4#?
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A mole is a unit-molecular weight (grams/mole in this case) of an Avogadro's number of molecules or atoms. It is calculated for a molecule by adding up the individual atomic weights of its constituent atoms.
For #Na_2SO_4# this is #2 x 23 = 46# for the sodium, plus #4 x 16 = 64# for the oxygen, plus #32# for the sulfur. That adds up to a molecular weight of #142 (g/"mole")#. Now we divide the given number of grams by this value to derive the number of moles.
#(397/142)(g/(g/"mol")) = 2.80 "moles"#
Molar mass of #Na_2SO_4 =142.04g/(mol)#
Use the equation,
#n_(Na_2SO_4)=m/M#
where,
#n=# number of moles
#m=#mass (in grams)
#M=#molar mass (in #(grams)/(mol)#)
#n_(Na_2SO_4)=(397g)/(142.04(g/(mol)))#
#n_(Na_2SO_4)=2.79 mol#
