A solution is prepared by adding 40.00 g of lactose (milk sugar)to 110.0 g of water at The partial pressure of water above the solution is ________ torr.The vapor pressure of pure water at 55 °C is 118.0 torr.The MW of lactose is
A)2.216
B)125.5
C)225.9
D)115.8
E)86.5

Question

A solution is prepared by adding 40.00 g of lactose (milk sugar)to 110.0 g of water at  The partial pressure of water above the solution is ________ torr.The vapor pressure of pure water at 55 °C is 118.0 torr.The MW of lactose is 
A)2.216 
B)125.5 
C)225.9 
D)115.8 
E)86.5

Quizplus · Accepted Answer

First, we need to calculate the molality of the lactose solution using the formula:

molality (m) = moles of solute / mass of solvent (in kg)

To find the moles of lactose, we need to divide the mass by the molar mass:

moles of lactose = 40.00 g / 342.30 g/mol = 0.1167 mol

Now, we convert the mass of water to kg:

mass of water = 110.0 g = 0.1100 kg

And plug in to the formula to get the molality:

m = 0.1167 mol / 0.1100 kg = 1.060 mol/kg

Next, we use the formula for calculating the vapor pressure of a solution:

P = X_solvent * P^o_solvent

where P is the vapor pressure of the solution, X_solvent is the mole fraction of the solvent (water), and P^o_solvent is the vapor pressure of pure solvent.

We can assume that lactose is non-volatile, so its contribution to the vapor pressure is negligible. Therefore, we can calculate the mole fraction of water:

X_water = 1 - X_lactose

where X_lactose is the mole fraction of lactose. We can find X_lactose using the formula:

moles of lactose = m * mass of solvent (in kg)

moles of lactose = 1.060 mol/kg * 0.1100 kg = 0.1167 mol

total moles of solution = 0.1167 mol + 55 mol (assumed)

X_lactose = 0.1167 mol / 55.1167 mol = 0.002117

Now we can find X_water:

X_water = 1 - 0.002117 = 0.997883

And finally, we can calculate the vapor pressure of the solution:

P = 0.997883 * 118.0 torr = 117.8 torr

Therefore, the answer is D) 115.8 torr, which is the closest to our calculated vapor pressure of 117.8 torr.

A Solution Is Prepared by Adding 40