Unformatted text preview: NAME: Quiz 3 7.6. A sample of methane gas of mass 25 g at 250 K and 18.5 atm expands isothermally until its pressure is 2.5 atm. Calculate the change in entropy of the gas given that Pile= Vf/Vi, AS = n R
In (Vp/Vi), and R = 8.3145 J/mol K (where Pi and Pf are the ﬁnal and initial pressures of the gas
and Vf and Vi are the ﬁnal and initial volumes of the same gas). The molecular weight of
methane gas is 16.04 g/mol. AS = n R In (Pi/Pf) = (25 g/16.04 g/mol) x (8.3145 J/ mol K) x In (18.5 atm/2.5 atrn)
= + 26 J/k. 7.15. With the knowledge that AHsystem for a reversible process is 100 kJ/mol, ﬁnd the entropy
change at 298K at constant pressure. A: constant P: as", = ‘M—m = —-"
\ Note: also constant
J T of 298 K
100, 000 — J
Thergﬂnm, as“, =—"""= 335.57
8.15. Find the AGf at T=298K and P2=3atm. AGf at STP = 27%. Use
Ang = —Rln [g] and AG = AH — TAS . State any simpliﬁcations or assumptions (hint:
examine pressure dependence contributions of the different terms).
AS“ = —Rh1[%] and M} = AH -Tﬁ.S . State any simpliﬁcations or assumptions (hint:
1 examine pressure depemience contributions of the different terms). In AG = 5H —TASJ the :15 term is much more pressure dependent than the 111-! term, so
AG : -TAS _ Using thaﬁrsr 1:155” equation; one gets a ﬁnal equation of so). = soft“) + RTln[%]. Plugging in the numbersﬁnr P1=Inrm, P;=3a:m, T=29£Kj
I R=3.31'45 Jr’molﬁ', and AGIW = -smanit gives aﬁna! answer of 35:24.33!
me I ma | ...
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