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PHNG PHP GI TR TRUNG BNH

1. Khi lng mol trung bnh

V d: Cho 31,84 gam hn hp hai mui natri ca hai halogenua k tip X v Y (MX< MY),

vo dung dch AgNO3 d c 57,34 gam hai kt ta. Tm hai mui halogenua?

p n:NaBr; NaI

Cch gii lp h phng trnh.

t 2 mui halogen ln lt l NaX v NaY, s

mol tng ng l a v b.

Phng trnh ha hc:

NaX + AgNO3 AgX+ NaNO3 (1)

NaY + AgNO3 AgY+ NaNO3 (2)

H phng trnh ton:

mhh= a(23 +X) + b(23+Y) =31,84

m = a(108 +X) + b(108+Y) = 57,34

Gii h ta c: a + b = 0,3

a X + b Y = 24,94

t X < Y X < 83,13 < Y

Hai mui halogenua k tip ph hp NaBr; NaI

Cch gii theo phng php tr trung bnh:

Thay hai mui halogenua NaX v NaY bng

mt mui tng ng: NaR (vi X

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X X 3 M=36

K 39 36 - X

Theo :3

36 X>

1

9 X >9 Vy 9 < X < 36.

Kim loi kim ph hp vi X l Natri.(23)

3. Bi tp khng dng iu kin c bn c a v phng php khi lng mol

trung bnh

V d : Ho tan ht 28,35 gam hn hp A gm 2 mui 3HCO v 23CO

ca 1 kim loi kim

bng dung dch HCl c 5,04 lt kh (ktc). Tm kim loi kim?

p n: Kali

Cch gii lp h phng trnh:Cc phng trnh ha hc:

MHCO3 + HCl MCl + H2O + CO2 (1)

M2CO3 + 2HCl 2 MCl + H2O + CO2 (2)

2

5,04

22,4COn = = 0,225

t s mol MHCO3 v M2CO3 ln lt l x v

y, ta c h phng trnh:

(M+61) x + (2M+60) y = 28,35

2COn = = x + y = 0,225

M =14,625

0,225

y

y

+

+ 33 < M < 65

0 < y < 0,225

Kim loi kim ph hp l: Kali

Cch gii theo phng php tr trung bnh:Thay AHCO3 v A2CO3 bng mt cht tng

ng MCO3 vi A+H(1) < M< A+A

Phng trnh ha hc:

MCO3 + 2 HCl MCl2 + H2O +CO2

60 22,4

28,35 5,04

M+= M = 66

vi A + 1 < M = 66 < A+A 33

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MO + 2 HCl MCl2 + H2O (1)

M + 2HCl MCl2 + H2 (2)

t s mol MO v M ln lt l x v y, ta c h

phng trnh:

(M+16) x + My = 73,3

(M +71)* (x+y) = 104

55x + 71y = 30,7

M =73,3 16

30,7 5171

xx

x

+

0 < x < 0,5582

115,32 < M < 169,52.

Kim loi kim th ph hp l: Ba

ng: XMO vi 0 < x < 1

Phng trnh ha hc:

XMO + 2HCl MCl2 + 2 XH O

Theo phng trnh ha hc v ra ta c

16 73,3

104 73, 371 16

M x

x

+=

Thay 0 < x < 1

115,32 < M < 169,52.

Kim loi kim th ph hp l Ba

V d 2/4: Ha tan hon ton 12,5 gam hn hp gm 1 kim loi kim v oxit ca n vo

nc thu c 20 gam hidroxit kim loi. Tm kim loi kim?

p n:Natri

Cch gii lp h phng trnh:

Cc phng trnh ha hc:

M2O + H2O 2MOH (1)

2M + 2 H2O 2MOH + H2 (2)

t s mol M2O v M ln lt l x v y, ta c

h phng trnh:

(2M+16) x + My = 12,5

(M +17)* (2x+y) = 20

18 x + 17 y = 7,5

M =12,5 16

7, 5 182

17

xx

x

+

Vi: 0 < x < 0,4167

7 < M < 28,(3).

Kim loi kim ph hp l: Natri

Cch gii theo phng php tr trung bnh:

Thay hn hp M v M2O bng mt cht tng

ng:X

MO vi 0