Question

emperical formula of a compound is AB. if its empirical formula weight is 1/3 of its vapour density, calculate the molecular formula of the compound

Answer 1

Your diffractometer program will suggest you some cells and symmetries, however, with your structure solution program you can check for the cell with the highest symmetry fitting your dataset (beware of twinning :)) and you will see, which of the candidates is the best.Well there is Z and Z'. Z is the number of formula units in your unit cell.  Z' is the number of asymmetric units. This is  Z divided by the lowest multiplicty in your list of Wyckoff positions. Sometimes, the formula unit is easy to see, as in salts or molecular compounds and many other simple structures with small unit cells. As the size of the unit cell grows, however, Z becomes somewhat arbitrary, depending on the unit you define.

Answer 2

Answer:

A6B6

Explanation:

Molecular formula = n× empirical formula = n×AB

n = molecular formula mass/ Empirical formula mass

= 2× V.D /empirical formula

Now Empirical formula= 1/3 × V.D

n= 2×V.D ÷ 1/3 xV.D = 6

Molecular formula = n (AB) = 6(AB)= A6B6