Answer
See the answer below.
Work Step by Step
a) Number of moles of He:
$PV=nRT$
$n=1\ atm×12.5\ L/0.082057\ L.atm/K.mol/(21.5+273)\ K$
$n=0.517\ mol$
Mass: $0.517\ mol×4.0026\ g/mol=2.07\ g$
b,c d)
Since only volume and number of moles change:
$V/n=constant$
$12.5\ L/0.517\ mol=26\ L/n_f$
$n_f=1.08\ mol$
From Dalton's law:
He: $0.517/1.08×100\%=48\ mol\%$
$O_2$: $(1.08-0.517)/1.08×100\%=52\ mol\%$
$P_{He}=1\ atm×48/100=0.48\ atm$
$P_{O_2}=1\ atm×52/100=0.52\ atm$
