The substance alpha chromium is found to crystallize in a cubic lattice, with an

Chemistry
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The substance alpha chromium is found to crystallize in a cubic lattice, with an edge length of 871.7 pm. If the density of solid alpha chromium is 7.561 g/cm3, how many Cr atoms are there per unit cell?


Your answer should be an integer:  atoms
May 29th, 2015

Hello John I am still working on it. please give me some more minutes

May 29th, 2015

this is how you go about the question.

Step1: since the density is given in g/cm3, use the edge length to calculate the volume of the cell in cm3, then use the density to calculate in one unit cell.

For a cubic cell the volume is the edge length cubed. 1pm=10^-12m =10^-10cm

Vcell= (871.7x10-10 cm)3 = 6.624x 10-22 cm3

M= D x V

Mcell= 7.561 g/cm3 x 6.624x 10-22 cm3

  =50.08x 10^-12g

Step 2: I am struggling with part two...

let me try to figure out

May 29th, 2015

Step1: since the density is given in g/cm3, use the edge length to calculate the volume of the cell in cm3, then use the density to calculate in one unit cell.

For a cubic cell the volume is the edge length cubed. 1pm=10^-12m =10^-10cm

Vcell= (871.7x10-10 cm)3 = 6.624x 10-22 cm3

M= D x V

Mcell= 7.561 g/cm3 x 6.624x 10-22 cm3

  =50.08x 10-22g

Step 2: use the molar mass of the element to calculate the mass of 1 atom. Then divide the mass of the unit cell by the mass of one atom to get the number of atoms in the cell.

We take the mass to be

M atom = (28.09g/mol) x (1 mol/ 6.022x 10-23 atoms = 4.665x10-23 g/atom

Number of atoms = (50.08x 10-22g/ cell) x (1 atom/ 4.665 x 10-23g)

  = 107.3 atoms

= 107 atoms

May 29th, 2015

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