Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

Calculate the amount of CaCl2 (molar mass = 111 g mol−1) which must be added to 500 g of water to lower its freezing point by 2 K, assuming CaCl2 is completely dissociated. - Chemistry

Sum

Calculate the amount of CaCl2 (molar mass = 111 g mol−1) which must be added to 500 g of water to lower its freezing point by 2 K, assuming CaCl2 is completely dissociated. (Kf for water = 1.86 K kg mol−1)

Solution

Given:

Molar mass of CaCl2 (MB) = 111 g/mol

Weight of water (wA)       = 500 g

Kf for water                      = 1.86 K kg/mol

ΔTf                                   = 2 K

Formula,

DeltaT_f=ixx(K_fxxw_Bxx1000)/(w_AxxM_B)

CaCl2 is an electrolyte which dissociates as,

CaCl2 → Ca2+ + 2Cl

Hence, i for CaCl2 = 3

Solution:

DeltaT_f=ixx(K_fxxw_Bxx1000)/(w_AxxM_B)

2 =3xx(1.86xxw_Bxx1000)/(500xx111)

w_B=(2xx500xx111)/(3xx1.86xx1000)

w_B=19.9 g

Amount of CaCl2 required = 19.9 g

Concept: Colligative Properties and Determination of Molar Mass - Depression of Freezing Point
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