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Kha hcLTH m bo mn HaThy Ngc Phng phptrung bnh
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PHNG PHP TRUNG BNHTI LIU BI GING
I. C IM CHUNG CA PHNG PHP TRUNG BNH1, Khi nim gi trTrung bnh- L
gi trtrung gian i din cho hn hp v l gi tr trung bnh ca tt c cc thnh
phn trong hn hp.
- Gi tr c tnh theo biu thc:ni i
ii 1
n
i
i
i 1
X .nX : i l-ng ang xt ca thnh phn th i trong hn hp
X vin : s mol ca thnh phn th i trong hn hp
n
- Trong trng hp tt c cc cht trong hn hp u c s mol bng nhau (n1 =
n2 = ... = nn) th X l trung bnhcng ca cc Xi:
1 2 3 nX + X + X + ... + X
Xn
- V X l gi tr trung gian, nn ta c tnh cht:
i i
i i
i i
min(X ) Xmin(X ) X max(X )
max(X ) X
: i l-ng nh nht trong tt cvi
: i l-ng ln nht trong tt c
2, Du hiu la chn gi trTrung bnh- i vi phng php Trung bnh, im mu
cht gii quyt c bi ton l phi chra c i lng thchhp nht ly gi tr trung
bnh.-Nhn chung, i lng c chn ly gi tr trung bnh phi tha mn phn ln
trong cc yu cu sau: (4 duhiu)3, ng dng gi trTrung bnh vo gii tonGi
tr Trung bnh c 2 ng dng quan trng trong gii ton:-S dng tnh cht ca
gi trTrung bnh xc nh CTPT cc cht- S dng gi trTrung bnh lm trung
gian i din tnh nhanh lng cht ca c hn hp ang xt (m khngcn quan tm n
lng cht c th ca tng thnh phn trong hn hp ).II. PHN LOI CC DNG BI TP
V PHNG PHP GII1, Bin lun xc nh cng thc phn tTy theo i lng c chn ly
gi tr trung bnh m ta c thphng php Trung bnh thnh cc dng bi sau:*
Dng 1:Phng php KLPT trung bnhp dng cho cc bi ton m cc cht trong hn
hp: (cc du hiu)VD1: Cho 1,67 gam hn hp gm hai kim loi 2 chu k lin
tip thuc nhm IIA (phn nhm chnh nhm II) tcdng ht vi dung dch HCl
(d), thot ra 0,672 lt kh H2 (ktc). Hai kim loi l:
A. Be v Mg B. Mg v Ca C. Sr v Ba D. Ca v Sr(Trch thi tuyn sinh H
C khi B2007)
VD2: Mt dung dch cha hai axit cacboxylic n chc k tip nhau trong
dy ng ng. trung ha dung dch
ny cn dng 40 ml dung dch NaOH 1,25M. C cn dung dung dch sau phn
ng thu c 3,68 gam hn hp muikhan. Cng thc phn t hai axit l:A.
CH3COOH; C3H7COOH B. C2H5COOH; C3H7COOHC. HCOOH; CH3COOH D.
CH3COOH; C2H5COOH
VD3:Cho hai axit cacboxylic A v B n chc (MA < MB). Trn 20 gam
dung dch A 23% vi 50 gamdung dch B20,64% c dung dch D. trung ho hon
ton dung dch D cn 200 ml dung dch NaOH 1,1M. Xc nh cngthc ca A v
B?* Dng 2:Phng php snguyn tC trung bnhp dng cho cc bi ton Ha hu c m
cc cht trong hn hp: (cc du hiu)VD1: C V lt kh A gm H2v hai olefin l
ng ng lin tip, trong H2 chim 60% v th tch. Dn hn hp Aqua bt Ni nung
nng c hn hp kh B. t chy hon ton kh B c 19,8 gam CO2 v 13,5 gam H2O.
Gi trca V v cng thc phn t ca hai olefin l:
A. 11,2 lt; C2H4 v C3H6 B. 6,72 lt; C3H6 v C4H8C. 8,96 lt; C4H8
v C5H10 D. 4,48 lt; C5H10 v C6H12
VD2:t chy hon ton 3,24 gam hn hp X gm hai cht hu c A, B trong B
hn A mt nguyn t C, thuc H2O v 9,24 gam CO2. Bit t khi hi ca X so vi
H2l 13,5. Xc nh cng thc phn t ca A v B?
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Kha hcLTH m bo mn HaThy Ngc Phng phptrung bnh
Hocmai.vnNgi trng chung ca hc tr Vit Tng i t vn: 1900 58-58-12 -
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* Dng 3:Phng php snguyn tHtrung bnhp dng cho cc bi ton Ha hu c m
cc cht trong hn hp: (cc du hiu)VD:t chy hon ton 0,672 lt hn hp A gm
hai hirocacbon (ktc) c cng s nguyn tcacbon thu c 2,64gam CO2 v 1,26
gam H2O. Mt khc, khi cho A qua dung dch [Ag(NH3)2]OH ng trong ng
nghim thy c ktta bm vo thnh ng nghim. Cng thc phn t cc cht trong A
l:
A. C2H4; C2H6 B. C2H2; C2H6 C. C3H4; C3H8 D. C3H4; C3H6* Dng
4:Phng php snguyn tC v snguyn tHtrung bnh
p dng cho cc bi ton Ha hu c m cc cht trong hn hp: (cc du
hiu)VD1:t chy hon ton 2,24 lt hn hp hai hirocacbonl ng ng lin tip
ca nhau, thu c 7,168 lt CO2.T khi hi ca hn hp ny vi H2 l 22,4. Bit
cc thtch kh o cng iu kin. Xc nh cng thc phn tca 2 hirocacbon
cho?VD2:t chy hon ton 2,24 lt hn hp hai hirocacbonl ng ng lin tip
ca nhau, thu c 7,84 lt CO2. Tkhi hi ca hn hp ny vi H2 l 25,2. Bit
cc thtch kh o cng iu kin. Xc nh cng thc phn t ca 2hirocacbon cho?*
Dng 5:Phng php bt bo hatrung bnh- Mt s khi nim c bn v bt bo ha.- p
dng cho cc bi ton Ha hu c m cc cht trong hn hp: (cc du hiu)VD: Cho
4,48 lt hn hp X (ktc) gm hai hirocacbon mch htc dng va vi 700 ml
dung dch Br2 0,5M.Sau khi ton b lng kh b hp th ht th khi lng bnh
tng thm 5,3 gam. Cng thc phn t ca hai
hirocacbon l:A. C2H2 v C2H4 B. C2H2 v C3H8 C. C3H4 v C4H8 D.
C2H2 v C4H6
* Dng 6:Phng php snhm chc trung bnh hoc ha tr trung bnhp dng cho
cc bi ton Ha hu c m cc cht trong hn hp: (cc du hiu)VD: Nitro ho
benzen bng HNO3c thu c 2 hp cht nitro l A v B hn km nhau 1 nhm NO
2. t chyhon ton 2,3 gam hn hp A, B thu c CO2, H2O v 255,8 ml N2
(27
0C v 740mmHg). Tm cng thc phn tca A, B?p s: C6H5NO2 v
C6H4(NO2)2.
Hng dn gii:Cch 1:Phng php Snhm chc trung bnhCch 2:T l nguyn
ttrung bnh2, Tnh cc gi trchung ca c hn hp thng qua gi trtrung
bnhVD:em ha hi 6,7 gam hn hp X gm CH3COOH, CH3COOC2H5, CH3COOCH3 v
HCOOC2H5thu c 2,24lt hi (ktc). Khi lng nc thu c khi t chy hon ton
6,7 gam X l:
A. 4,5 gam B. 3,5 gam C. 5,0 gam D. 4,0 gamMrng cc bin i vphng
php trung bnh
Hn hp khv hi A gm: hi ruu etylic, hi ru metylic v kh metan. em t
chy hon ton 20 cm3hn hp A th thu c 32 cm3 kh CO2. Thtch cc kh, hi o
trong cng iu kin v nhit v p sut. Ktlun no di y l ng:
A. Hn hp A nng hn kh Propan B. Hn hp A nhhn khng khC. Hn hp A
nng hn khng kh D. Hn hp A nng tng ng khng kh
Gio vin: V Khc Ngc
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