Raoult's Law and Ionic or Organic Solutions
At 55.0 C, what is the vapor pressure of a solution prepared by dissolving 62.8 g of LiF in 247 g of water? The vapor pressure of water at 55.0 C is 118 mmHg. Assume complete dissociation of the solute.
Psoln = XH2OPoH2O
Use the g LiF and g H2O to determine mole fraction water. Since LiF ionizes completely, you will have X for Li ion, X for F ion, and X for water.
To determine the vapor pressure of the solution prepared by dissolving LiF in water, we can use Raoult's Law, which states that the vapor pressure of a solvent above a solution is directly proportional to the mole fraction of the solvent in the solution.
Here's how you can calculate it step-by-step:
Step 1: Calculate the mole fraction of water:
To find the mole fraction of water, we need to know the moles of both water and LiF.
Moles of water = mass of water / molar mass of water
Molar mass of water (H₂O) = 18.015 g/mol
Moles of water = 247 g / 18.015 g/mol
Step 2: Calculate the mole fraction of LiF:
The problem states that LiF completely dissociates in water, meaning that every mole of LiF will produce one mole of Li⁺ ions and one mole of F⁻ ions. Since we have the mass of LiF, we can calculate the moles of LiF by using the molar mass of LiF.
Molar mass of LiF = 25.94 g/mol + 18.998 g/mol = 43.938 g/mol
Moles of LiF = 62.8 g / 43.938 g/mol
Step 3: Calculate the total moles of solute and solvent:
The total moles of solute and solvent can be found by adding the moles of LiF and moles of water.
Total moles = Moles of solute (LiF) + Moles of solvent (water)
Step 4: Calculate the mole fraction of water:
The mole fraction of water can be calculated by dividing the moles of water by the total moles.
Mole fraction of water = Moles of water / Total moles
Step 5: Calculate the vapor pressure of the solution:
Now that we have the mole fraction of water, we can use Raoult's Law to find the vapor pressure of the solution.
Vapor pressure of solution = Mole fraction of water * Vapor pressure of pure water
Substituting the given values:
Mole fraction of water = calculated in step 4
Vapor pressure of pure water = 118 mmHg (given)
Vapor pressure of solution at 55.0°C = Mole fraction of water * 118 mmHg
By performing these calculations using the given data and the steps outlined above, you can find the vapor pressure of the solution.