Chemistry Reference
In-Depth Information
Look at the original formula for molality, and figure out how you would
isolate the unknown, which is the number of moles of solute.
Molality (m) =
number of moles of solute
kilograms of solvent
Can you see that you will want to multiply both sides of the equation by
“kilograms of solvent” as shown here?
number of moles of solute
kilograms of solvent
kilograms of solvent × molality =
× kilograms of solvent
Crossing out the “kilograms of solvent” on the right-hand side of the
equation, we get:
kilograms of solvent × molality = number of moles of solute
Rearranging the formula, and filling in the given information, we get:
number of moles of solute = kilograms of solvent × molality
number of moles of solute = 2.50 kg × 0.500 m = 1.25 moles of solute
For the next part, you'll need to find the molar mass of strontium chlo-
ride (SrCl
2
), by making use of the periodic table. The work is shown here:
Molar mass of strontium chloride (SrCl
2
)
Sr = 87.6 g/mole
Cl
2
=+ 71.0 g/mole
Molar mass of SrCl
2
=
158.6 g/mole
which rounds to 159 g/mole
Now you can determine the required mass of the solute by multiplying
the number of moles of solute by the molar mass of the solute.
mass of solute = number of moles × molar mass = 1.25 moles × 159 g/mole
= 198.75 g, which rounds to 199 g of SrCl
2
Here is a summary of this work:
Given: kilograms of solvent = 2.50 kg; molality of solution = 0.500 m
Find: A. number of moles of solute
B. molar mass of solute
C. mass of solute
A. number of moles of solute = kilograms of solvent × molaltiy (m)
= 2.50 kg × 0.500 m = 1.25 moles of SrCl
2
B. molar mass of SrCl
2
= sum of the molar masses of the parts.
= 87.6 g/mole + 71.0 g/mole = 158.6 g/mole,
which rounds to 159 g/mole
C. mass of solute = number of moles of solute × molar mass of solute
= 1.25 moles × 159 g /mole = 198.75 g,
which rounds to 199g
