The boiling point of water at 735 mmHg is 99.07C. What percent by mass of NaCl should be present in a water soln to raise the boiling point to 100.0C? [Kb for water is 0.512 C kg solvent (mol solute )-1]
To find the percent by mass of NaCl needed to raise the boiling point of water to a given temperature, we'll use the formula for boiling point elevation:
ΔT = Kb * m
where ΔT is the boiling point elevation, Kb is the molal boiling point elevation constant for the solvent, and m is the molality of the solute.
We first need to calculate the molality of the solute (NaCl) in the water solution. Molality (m) is defined as the amount of moles of solute per kilogram of solvent. In this case, the solvent is water.
Given that the boiling point elevation (ΔT) is 100.0°C - 99.07°C = 0.93°C and the molal boiling point elevation constant for water (Kb) is 0.512 °C kg solvent (mol solute)^-1, we can rearrange the formula to solve for m:
m = ΔT / Kb
Substituting the values, we get:
m = 0.93 °C / (0.512 °C kg solvent (mol solute)^-1)
Now, we can find the molality (m) in units of moles of solute per kilogram of water.
Next, we need to convert the molality (m) to the mass percent of NaCl in the solution. Mass percent is defined as the mass of solute divided by the mass of the solution, multiplied by 100.
Here's how you proceed to calculate the mass percent:
1. Convert the molality (m) to moles of NaCl per kilogram of water by multiplying it by the molar mass of NaCl, which is approximately 58.5 g/mol.
2. Calculate the mass of NaCl in grams based on the above moles.
3. Determine the mass of the water solution by considering the total mass of NaCl and the water.
4. Finally, calculate the mass percent of NaCl by dividing the mass of NaCl by the total mass of the solution and multiplying by 100.
By following these steps, you can find the percent by mass of NaCl needed in the water solution to raise the boiling point to 100.0°C.
This is what you need to do.
First, use delta T = i*Kb*m and solve for m. i = 2 for NaCl and you have the other numbers in the problem.
Convert molality to percent by mass. Post your work if you get stuck.