1) Gold (Au) crystallizes in a cubic close-packed (face-centered cubic unit cell
ID: 551702 • Letter: 1
Question
1) Gold (Au) crystallizes in a cubic close-packed (face-centered cubic unit cell). The unit cell edge length is 144pm. What is the density of the gold crystal?
2) In an experiment to determine the molar mass of dichlorobenzene C6H4Cl2 using freezing point depression of solvent cyclohexane, a student uses 10mL of cyclohexane (density 0.779g/mL) and obtains an average freezing point of 5.75 degrees Celsius. She graphically determines the new freezing point to be 2.8 degrees Celsius by setting equation of 2 lines equal to each other. What is her Tf? If she uses a total of 0.2301g of dichlorobenzene what is her calculated molecular weight for the solute? What is the % error from the actual molecular weight of solute? Is this a reasonable result?
3) The rate constant of a first order reaction is 4.36x10-2s-1 at 298K. What is the rate constant at 250K if the activation energy for the reaction is 40.2kJ/mol? (HINT: Use modified form of Arrhenius equation (at 2 temperatures) ).
Explanation / Answer
1)
Here a = 144 pm
a = 1.44*10^-8 cm
Molar mass = 197.0 g/mol
since the cubic cell is Face Centred Cubic, the value of Z=4
d = (Z*M)/(a^3*NA)
d = (4*197)/((1.44*10^-8)^3*(6.022*10^23))
d = 438 g/cm^3
Answer: 438 g/cm^3
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