The density of potassium bromide, KBr, is 2.75 g/cm3 and it has the same structure as NaCl. A cubic unit cell consists of 4 K+ and 4 Br- ions. The atomic weights of K and Br are 39.1 and 79.9 respectively. Calculate the edge length of the unit cell a.
A. 0.702 nm
B. 0.564 nm
C. 0.354 nm
D. 0.416 nm
E. 0.660 nm
2007-08-05 18:30:10 · 1 answers · asked by waterlooguy 2 in Science & Mathematics ➔ Chemistry
molecular mass of KBr = 39.1+79.9=119
So a mole which has 6.022*10^23 molecules has a weight of 179 g
One molecule has a weight of1.976*10^-22g
in the cell there are 4 molecules, the mass of the cell is
1.976*10^-22 * 4 =7.9*10^-22g
its volume is 7.9*10^-22/2.75 =2.875*10^-22cm3
the volume = edge^3
so the edge has a length of (2.875*10^-22)^(1/3)=6.6*10^-8cm^=0.66 nm
Correct answer E
2007-08-05 18:49:31 · answer #1 · answered by maussy 7 · 1⤊ 0⤋