You remember Dr. Laude saying
something about how adding salt to water
increases the boiling point. If you have 1 cup
(8 fl. oz. = 250 mL = 250 g) of H2O, how
much NaCl should you add in order to raise
the boiling point to 105�C?
ΔTb = molality * Kb * i
molality=moles solute/kg solvent
http://chemistry.about.com/od/solutionsmixtures/a/boilingpointele.-NxZ.htm
142.5
To calculate the amount of NaCl needed to raise the boiling point of water to 105 degrees Celsius, we need to use the concept of boiling point elevation.
The equation we can use is:
∆Tb = Kb * m * i
Where:
∆Tb is the change in boiling point
Kb is the molal boiling point elevation constant for water (0.512 ºC/m)
m is the molality of the sodium chloride (NaCl)
i is the van 't Hoff factor, which represents the number of particles formed when the substance dissolves (for NaCl, i = 2)
To begin, we need to calculate the molality (m) of the NaCl solution. Molality is defined as the number of moles of solute (NaCl) divided by the mass of the solvent (water, in this case).
Given that we have 250 grams (or 250 mL) of water, we can convert this to moles:
moles = mass / molar mass
moles = 250 g / 18.015 g/mol (molar mass of H2O)
moles = 13.874 mol H2O
Now we can calculate the molality (m):
molality (m) = moles of solute / mass of solvent (in kg)
molality (m) = 1 / (mass of H2O in kg)
molality (m) = 1 / (250 g / 1000 g/kg)
molality (m) = 1 / 0.25 kg
molality (m) = 4 mol/kg
Next, we can calculate the change in boiling point (∆Tb):
∆Tb = Kb * m * i
∆Tb = 0.512 ºC/m * 4 mol/kg NaCl * 2 (since NaCl dissociates into 2 particles when dissolved)
∆Tb = 4.096 ºC
Finally, we can calculate the amount of NaCl needed to raise the boiling point of water by this amount:
∆Tb = Kb * m * i
4.096 ºC = 0.512 ºC/m * (moles of NaCl / 0.25 kg) * 2
4.096 ºC = 2.048 * (moles of NaCl / 0.25 kg)
moles of NaCl = (4.096 ºC * 0.25 kg) / 2.048
Now, using the molar mass of NaCl (58.44 g/mol), we can calculate the amount of NaCl needed:
mass of NaCl = moles of NaCl * molar mass of NaCl
mass of NaCl = (moles of NaCl) * (58.44 g/mol)
Therefore, by substituting the value of moles of NaCl calculated above, we can determine the mass of NaCl needed.