Ti liu hc tp chia s B tr kin thc thi i hc

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Phn 1: Phng php tng gim khi lng Trong chuyn ny, thy s h tr cho cc em cch tnh ton nhanh bng phng php tng gim khi lng. MT S GI : iu kin p dng: Nn p dng cho cc bi ton - Mi lin h khi lng trc (mT) v khi lng sau (mS) - tng gim khi lng Lut tng, gim:

- Nu mT mS Khi lng Sau tng mt lng m = mS - mT

- Nu mT mS Khi lng Sau gim mt lng m = mT - mS Cng thc chung: m Sau = m Trc m Tng: + m

Gim : - m

Tng qut: aA bB, c A Bm a M b M nP

Vi : nP = .

CC BI TP P DNG Bi 1: X phng ho hon ton 17,24 gam cht bo cn va 0,06 mol NaOH. C cn dung dch sau phn ng thu c khi lng x phng l A. 17,80 gam B. 18,24 gam C. 16,68 gam D. 18,38 gam

(HKB-2008) Gi :

- CTTQ ca cht bo l 3 3 5(RCOO) C H

- Phn ng x phng ha cht bo: 3 3 5(RCOO) C H + 3 NaOH Ot

3 RCOONa + C3H5(OH)3

- Nh phng php tng gim khi lng ta c cng thc cn nh sau:

mx phng = mcht bo + 28. nP ( Vi: n P = 1

3n NaOH p )

Gii: m x phng = 1

17,24 28.( 0,06) 17,83

(gam) p n: A

Bi 2: Trung ha 5,48 gam hn hp gm axit axetic, phenol v axit benzoic, cn dng 600 ml dung dch NaOH 0,1M. C cn dung dch sau phn ng, thu c hn hp cht rn khan c khi lng l A. 8,64 gam. B. 6,84 gam. C. 4,90 gam. D. 6,80 gam.

(HKA-2008) Gi : - Cc cht axit axetic, phenol , axit bezoic khi phn ng vi NaOH u sinh mui nc v c tnh cht sau

mMui = mcht hu c + 22. nP ( Vi: n P = n NaOH p )

Gii: m Mui = 5,48 22.(0,6 0,1) 6,8 (gam) p n: D

Bi 3: 40,3 gam triete X ( este 3 chc ) ca glyxerol vi cc axit bo tc dng va vi 6 gam NaOH. S gam mui thu c l:

A. 38,1 B. 41,7 C. 45,6 D.45,9

B TR KIN THC THI I HC

PHN 1

Gio vin: NGUYN TN TRUNG

1

H s cn bng mol Cht tng ng

Ti liu hc tp chia s B tr kin thc thi i hc

Hocmai.vn Ngi trng chung ca hc tr Vit Tng i t vn: 1900 58-58-12 - Trang | 2 -

( Trch mu 2010 )

Gii: m Mui = 1 6

40,3 28.( ) 41,73 40

(gam) p n: B

Bi 4: Khi oxi ho hon ton 2,2 gam mt andehit n chc thu c 3 gam axit tng ng. Cng thc andehyt l

A. C2H5CHO B. CH3CHO C. HCHO D. C2H3CHO

(HKB-2007) Gi : - Do bi cho khi lng trc v sau nn ta p dng phng tng gim khi lng s tnh c s mol andehyt

d dng, t s tnh c M, c M l t nhin bit c phng n tr li no l p n. - Trong phn ng oxi ha andehyt n bng oxi ta c maxit = m andehyt + 16. nP ( Vi: n P = n andehyt p )

Gii: Kt hp cc gi trn ta c biu thc tnh M nh sau:

andehyt2,2

M 443 2,2

16

vC Andehyt cn tm l: CH3CHO p n: B

Bi 5: -aminoaxit X cha mt nhm (-NH2). Cho 10,3 gam X tc dng vi HCl (d), thu c 13,95 gam mui khan. Cng thc cu to thu gn ca X l A. H2NCH2COOH B. H2NCH2CH2COOH

C. CH3CH2CH(NH2)COOH D. CH3 CH (NH2)COOH

(HKA -2007) Gi : - Amin n + HCl: RNH2 + HCl RNH3-Cl - Theo phng php tng gim khi lng c:

mmui = m amin + 36,5. nP ( Vi: n P = n amin p )

- Trong bi ton ny nhm chc ( - COOH) khng phn ng vi HCl, nn cng thc trn p dng c cho aminoaxit.

Gii: Tng t cc bi p dng trn ta c: a min oaxit10,3

M 10313,95 10,3

36,5

vC p n: C

Bi 6: Cho 10 gam amin n chc X phn ng hon ton vi HCl (d), thu c 15 gam mui. S ng phn cu to ca X l A. 8. B. 7. C. 5. D. 4.

(HKA -2009) Gi : - tr li c s ng phn, ta phi tm CTPT X. - Nn p dng phng php tng gim khi lng tnh M, t tm c CTPT X.

- Theo cng thc bi 5 ta c: a min X10

M 7315 10

36,5

vC

- Ta nn c sn gi sau:

Mamin n no CTPT (s ng phn)

31

CH5N

(1 p)

45

59

73

C2H7N

(2 p)

C3H9N

(4 p)

C4H11N

(8p)

Ti liu hc tp chia s B tr kin thc thi i hc

Hocmai.vn Ngi trng chung ca hc tr Vit Tng i t vn: 1900 58-58-12 - Trang | 3 -

- Nu cc em s thy mt quy lut tht d nh v s ng phn ca cc amin trn. Cc em nhn xt i s thy! - Vi cc gi trn sau khi tnh c M =73 ta d dng thy c p n l A. Bi 7: Cho 5,76 gam axit hu c X n chc, mch h tc dng ht vi CaCO3 thu c 7,28 gam mui ca axit hu c. Cng thc cu to thu gn ca X l (Cho H = 1; C = 12; O = 16; Ca = 40) A. CH3-CH2-COOH B. HCC-COOH

C. CH2=CH-COOH D. CH3COOH

(CKA -2007) Gi : - Theo c im

- Ta c phn ng : 2 RCOOH + CaCO3 (RCOO)2Ca + CO2 + H2O p dng phng php t ng gim khi lng cho phn ng trn ta c:

mmui = m axit + 38. nP ( Vi: n P = 1

2n axit p )

- Ta tnh c M X theo biu thc sau:

axit X5,76

M 727,28 5,76

238

vC p n: C.

- cu ny d sai st khi tnh s mol axit, v cc em b qun s 2. - Cc em cn : n axit p = 2 nP Phng php ny cn gii nhanh cc bi ton v c. Cc em th nh! Bi 8: Ha tan hon ton 2,81 gam hn hp gm Fe2O3, MgO, ZnO trong 500 ml axit H2SO4 0,1M ( va ). Sau phn ng, hn hp mui thu c khi c cn dung dch c khi lng l: A. 6,81 gam B. 4,81 gam C. 3,81 gam D. 5,81 gam

(HKA -2007) Gi : - Khi oxit tc dng vi H2SO4 ch thu c mui v nc. Ta d dng chng minh c cng thc sau mmui = m axit + 80. nP ( Vi: n P = n axit p )

- Theo cng thc trn ta c mmui = 2,81 80 (0,5 0,1) 6,81 (gam) p n: A.

Thy hy vng cc em vn dng tt phng php ny cho rt nhiu bi ton cn li.

Gio vin: Nguyn Tn Trung

Ngun : Hocmai.vn