Molarity is moles of solute (compound) in Litres of solution
M = mol / litres
Molality is moles of solute in kg of solvent (not solution, solvent only)
m = mol / kg
To work this out I first pretended I had 100 ml. Work out how many moles of Pb(NO3)2 are in 100 ml of 0.726 M Pb(NO3)2
M = mole / L
therefore mole = M x L
moles Pb(NO3)2 = 0.726 M x 0.1 L
= 0.0726 moles in 100 ml
You are told the density is 1.202 g/ml, therefore
weight of 100 ml of solution = 100 ml x 1.202 g/ml
=120.2 g
Some of this weight is attributted by the Pb(NO3)2, there are 0.0726 moles of Pb(NO3)2 in 100 ml, so work out the mass of this using
moles = mass / molecular weight
molecular weight Pb(NO3)2 = 207.2 + (2 x 14.01) + (6 x 16.00) = 331.22 g/mol
So mass of Pb(NO3)2 in 100 ml = 0.0726 mol x 331.22 g/mol
= 24.0466 g of Pb(NO3)2 in 100 ml of 0.726M solution
To find the mass of the water subtract mass of Pb(NO3)2 from total mass.
120.0 g - 24.0466 g = 96.1534 g
So in 100 ml of 0.726 M Pb(NO3)2 there are 0.0726 moles (24.0466 g) of Pb(NO3)2 and 96.1534 g of water.
Molality = moles / kg (remember moles Solute / kg SOLVENT (water))
= 0.0726 moles / 0.0961534 kg
= 0.755 m
Hence, the molal concentration or molality of the solution is 0.755 m.
Thank You.
