4.0g of an unknown potassium halide (KX, where X is the halide) is dissolved in 100g of water. The solution freezes at -1.28 degrees Celsius. What is the identity of the halide?

Answer 1

freezingpt=-l.86*2*4/molmass*1/.1
molmass=l.86*8*10/1.28=116.3
mass of K is 39
so atomic mass of halide is 77, potassium bromide.

do the math a little more carefully than I did in my head.