1.) A solution of potassium chloride (KCl) is prepared by dissolving 9.63 g of t

Question

1.) A solution of potassium chloride (KCl) is prepared by dissolving 9.63 g of the salt in 1.00 kg water (H2O). What is the freezing point of this solution? (The freezing point of pure water is 0°C (exactly) at 1 atm pressure; the molal freezing-point-depression constant for water is 1.86°C/m.)

2.) A certain element exists in a face-centered cubic structure. The atomic radius of an atom of this element is 159 pm. Calculate the density of this element in g/cm3. (Assume the element has a molar mass of 64.6 g/mol.)

3.) For the following reaction, write expressions for the rate of formation of each of the products. 3 A + 3 B + 2 C 3 D + 4 E + F

4.) For the following generalized reaction, if the rate of change of D is 1.76103 M/s in a given time period, what is the rate of change of B in the same time period? 2 A + B C + 3 D

Explanation / Answer

solution of potassium chloride (KCl) is prepared by dissolving 9.63 g of the salt in 1.00 kg water (H2O). What is the freezing point of this solution? (The freezing point of pure water is 0°C (exactly) at 1 atm pressure; the molal freezing-point-depression constant for water is 1.86°C/m.)

moes of KCl in 9.63 gm= mass/molar mass= 9.63/74.5 =0.13, molality, m= moles of salt/ kg of solvent water= 0.13/1=0.13

depression in freezing point= i*kf*m, i= Van;t Hoff factor for KCl =2 ( it dissociates in K+ and Cl-)

depression in freezing point= 2*1.86*0.13= 0.48

Freezing point= freezing point of water- freezing point depression= 0-0.4836=-0.4836 deg.c

2. radius, r = 159   cm, edge length= 4*r/sqrt(2)= 4*159*10-10/1.414= 450*10-10 cm volume= (450*10-10)3= 91.1*10-24 cm3. there are 4 atom in unit cell of FCC. Mass of one atom= atomic weight/ avagadro no = 64.6/(6.023*1023 )= 10.73*10-23 gm

mass of single unit cell = 4*10.73*10-23 gm =42.92*10-23 gm

density of unit cell= mass/volume= 42.92*10-23 /91.1*10-24=4.71 g/cc

3.

-dCA/3dt= -dCB/3dt= -dCC/2dt= dCD/3dt= dCE/4dt= dCF/dt, 3, 3,2, 3,1 and 4 are coeficients of reactatants. A, B, C, D, E and F respectively. dC/dt= rate of reaction.

-ve sign indicates consumption of reactant and +ve sign indicates formation.

4. For the following generalized reaction, if the rate of change of D is 1.76 103 M/s in a given time period, what is the rate of change of B in the same time period? 2 A + B C + 3 D

-dCB/dt= dCD/3dt, given dCD/dt= 1.76*10-3 m/s

-dCB/dt= 1.76*10-3/3= 5.9*10-4 M/s

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