A 3.0 molal solution of naphthalene in cyclohexane boils at 89.4 C. What is the boiling point of pure cyclohexane? Although solid naphthalene is slightly volatire, assume its volatility is zero in this calculation. The constant kb for cyclohexane is +2.80 Km^-1.
can someone help?
delta T = Kb m
You know K and m so delta T can be calculated. b.p.(soln)= normal b.p. + delta T. Check my thinking.
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Your thinking is correct. To calculate the boiling point of pure cyclohexane, we need to calculate the change in boiling point (delta T) caused by the presence of the solute (naphthalene) in the solution.
The formula to calculate delta T is:
delta T = K_b * m
Given that the molality (m) of the solution is 3.0 molal and the K_b value for cyclohexane is +2.80 K/m, we can substitute these values into the formula to find delta T:
delta T = (2.80 K/m) * (3.0 molal) = 8.40 K
The boiling point of the solution is elevated by 8.40 degrees Celsius compared to the boiling point of pure cyclohexane.
To calculate the boiling point of pure cyclohexane, we can simply subtract the delta T from the boiling point of the solution:
Boiling point of pure cyclohexane = Boiling point of solution - delta T
Boiling point of pure cyclohexane = 89.4°C - 8.40°C = 81.00°C
Therefore, the boiling point of pure cyclohexane is 81.00°C.
Yes, your thinking is correct! To calculate the boiling point of pure cyclohexane, you can use the formula:
delta T = Kb * m
Where:
delta T is the change in boiling point
Kb is the boiling point elevation constant for the solvent (cyclohexane), which is given as +2.80 K*m^-1
m is the molality of the solution, which is given as 3.0 mol/kg
Substituting the given values into the formula:
delta T = 2.80 K*m^-1 * 3.0 mol/kg
Now, calculate the value of delta T using multiplication:
delta T = 8.40 K
Once you have calculated delta T, you can find the boiling point of the solution (soln) using the formula:
b.p.(soln) = normal b.p. + delta T
The normal boiling point of cyclohexane can vary slightly depending on the source, but it is typically around 80-82 degrees Celsius. Let's assume it is 80 degrees Celsius.
Substituting the values into the formula:
b.p.(soln) = 80 degrees Celsius + 8.40 degrees Celsius
Now, calculate the boiling point of the solution:
b.p.(soln) = 88.40 degrees Celsius
Therefore, the boiling point of pure cyclohexane is approximately 88.40 degrees Celsius.