A salt is known to be an alkali metal fluoride. A quick approximate freezing point determination indicates that 4 g of salt dissolved in 100 g of water produces a solution which freezes at about -2.8 degrees celsius. What is the formula of the salt? (assume i=2)

Question

anonymous · Answer

can someone please do this for me i tried working it out but i can't figure it out

anonymous · Answer

did you use
dT = Kf * m * i 
first you get out molality 
m = dT/Kf*i
and then amount of solute is
m = n(solute) / m(water)
n (solute) = m(molality) * m(water)
and then 
n= m/M -> M = m/n