How do you calculate the molality of a solution

here are the questions:
a.15.7 g NaCl in 100.0 g H2O
b.20.0 g CaCl2 in 700.0 g H2O
c.3.76 g NaOH in 0.850 L H2O

Molality is defined as moles of solute per kilogram of solution. To get the moles of solute for each you'll divide the number of grams by the formula weight. So for (a) we first get the formula weight of NaCl from the periodic table, which is: 58.5 grams/mole

which we divide the given grams of NaCl by this amount, like so: 15.7/58.5= .268 moles.

Now we just divide this number by the kilograms of solution given above, like so: .268/.1= 2.68 molal

Do this for each of the mixtures and you'll have your answers :) good luck!

determine moles of each solute, then divide moles of solute by liters
a)= 15.7 g NaCl/ molec. wt NaCl= ? moles NaCl
? moles NaCl/0.1L= molarity of NaCl solution
b) & c) done the same way