6.5 xc nh khi lng ring v t trng

 

Khi lng ring

Khi lng ring ca mt cht nhit t (rt) l khi lng mt n v th tch ca cht , xc nh nhit t:

 

M: Khi lng ca cht, xc nh nhit t.

V: Th tch cht, xc nh nhit t.

Trong h n v quc t S.I, n v ca khi lng ring l kg/cm3. Trong ngnh dc thng xc nh khi lng ring nhit 20 oC (r20) c tnh n nh hng ca sc y ca khng kh (tc l quy v gi tr xc nh trong chn khng) v dng n v kg/l hoc g/ml.

T trng tng i

T trng tng i ca mt cht l t s gia khi lng ca mt th tch cho trc ca cht , v khi lng ca cng th tch nc ct, tt c u cn 20oC.

T trng biu kin

i lng t trng biu kin c dng trong cc chuyn lun ethanol, ethanol 96% v long hn..., l khi lng cn trong khng kh ca mt n v th tch cht lng. T trng biu kin c biu th bng n v kg m-3 v c tnh ton theo biu thc sau y:

 

T trng biu kin = 997,2 x

 

Trong l t trng tng i ca cht th.

997,2 l khi lng cn trong khng kh ca 1 m3 nc, tnh bng kg.

Xc nh t trng ca mt cht lng

Xc nh t trng ca mt cht lng theo phng php c ch r trong chuyn lun. Nu chuyn lun khng ch r dng phng php no xc nh t trng mt cht lng, c th dng mt trong cc phng php sau y:

Phng php dng picnomet: Cn chnh xc picnomet rng, kh v sch. vo picnomet mu th iu chnh nhit thp hn 20oC, ch khng c bt kh. Gi picnomet nhit 20oC trong khong 30 pht. Dng mt bng giy lc thm ht cht lng tha trn vch mc, lm kh mt ngoi ca picnomet, cn ri tnh khi lng cht lng cha trong picnomet. Tip mu th i, ra sch picnomet, lm kh bng cch trng ethanol ri trng aceton, thi khng kh nn hoc khng kh nng ui ht hi aceton, sau xc nh khi lng nc ct cha trong picnomet nhit 20oC nh lm vi mu th. T s gia khi lng mu th v khi lng nc ct thu c l t trng cn xc nh.

Phng php ny cho kt qu vi 4 ch s l thp phn.

Phng php dng cn thy tnh Mohr-Westphal: t cn trn mt phng nm ngang. Mc phao vo n cn, t phao chm trong nc ct nhit 20oC v chnh thng bng bng cc con m t cc v tr thch hp, thu c gi tr M. Ly phao ra, thm kh ri t li phao chm trong cht lng cn xc nh t trng, cng nhit 20oC, ch sao cho phn dy treo chm trong cht lng mt on bng on chm trong nc ct. Chnh li thng bng bng cc con m t v tr thch hp, thu c gi tr M1. T s M1/M l t trng cn xc nh.

Phng php ny cho kt qu vi 3 ch s l thp phn.

Phng php dng t trng k: Lau sch t trng k bng ethanol hoc ether. Dng a thu tinh trn u cht lng cn xc nh t trng. t nh nhng t trng k vo cht lng sao cho t trng k khng chm vo thnh v y ca dng c ng cht th. Chnh nhit ti 20oC v khi t trng k n nh, c kt qu theo vng khum di ca mc cht lng. i vi cht lng khng trong sut, c theo vng khum trn.

Phng php ny cho kt qu vi 2 hoc 3 ch s l thp phn.

Xc nh t trng ca m, sp, nha, nha thm:

Dng picnomet: u tin cn chnh xc picnomet rng (M1) ri cn picnomet y nc ct 20oC (M4). Sau nc i, lm kh picnomet, dng ng ht hoc phu cung nh, cho vo trong picnomet mu th c un chy khong 1/3-1/2 th tch ca picnomet. 1 gi trong nc nng, khng y nt. Lm ngui n 20oC, y nt. Lau kh mt ngoi picnomet ri li cn (M2). Cui cng thm nc ct n vch, lau kh mt ngoi picnomet ri li cn (M3). Ch khng c bt kh cn li gia lp nc v mu th.

Tnh kt qu theo cng thc: