Answer
See explanation.
Work Step by Step
a. If the average spacing is L, the volume occupied by each atom is $L^3 $.
K and Br occur in equal amounts, so the density is the average mass of K and Br, divided by this volume.
$$m_{average}=9.895\times10^{-26}kg$$
Now calculate the density in terms of L and solve.
$$2.75\times10^3kg/m^3=\frac{9.895\times10^{-26}kg}{L^3}$$
$$L=3.30\times10^{-10} m$$
b. This is larger than the spacing in NaCl, but K and Br have larger atomic numbers than Na and Cl, respectively, and they are larger atoms.
