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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Sis290025 on January 25, 2006, 10:14:51 PM

Title: What is the vapor pressure and temperature of a solution?
Post by: Sis290025 on January 25, 2006, 10:14:51 PM

1. What is the temperature at which the solution of KI will freeze if the solution contains 46.83 grams of KI dissolved in 687.4 grams of water. Kf of water is 1.86 C/m.
Write answer to four significant figures.
 
deltaT_f = K_f*m*i

KI is ionic so i = 2?

m = mol solute/kg solvent = 0.2821118 mol KI / 0.6874 kg H2O = 0.41040418 m

To get mol KI:

46.83 g KI * (1 mol KI/165.998 g KI) = 0.2821118 mol KI

Back to first formula:
x - 0 C = (1.86 C/m)*2*(0.41040418 m)
x = 1.52670355 C = 1.527 C???

2. Calculate the vapor pressure of a solution containing 43.3 grams of CH3OH (P*=140 mm Hg) and 31.6 grams of CH3CH2OH (P*=83.5 mm Hg).

Use P_total = X_a(P_a) + X_b*(P_b) ??

First, CH3OH:

43.3 g CH3OH * (1 mol/32.04 g) = 1.3514357 mol CH3OH

Now, CH3CH2OH:

31.69 g CH3CH2OH * (1 mol/46.07 g ) = 0.687866 mol CH3CH2OH

Find mol fractions of each:

X_CH4O = (1.3514357 mol CH3OH)/(0.687866 mol CH3CH2OH + 1.3514357 mol CH3OH) = 0.66269523

Thanks.

Title: Re:What is the vapor pressure and temperature of a solution?
Post by: Donaldson Tan on January 29, 2006, 02:00:33 AM

1. You did question 1 correctly

2. "P_total = X_a(P_a) + X_b*(P_b)" is valid for ideal solution. Generally, it is fair to assume ideality homework-wise.

Title: Re:What is the vapor pressure and temperature of a solution?
Post by: Sis290025 on February 01, 2006, 11:38:34 PM

When is a solution not ideal, and what formula do you for this?