An unknown nonelectrolyte and nonvolatile compound is dissolved in enough pure water to make 229 ml of solution. The vapour pressure above the solution drops from 233.7 torr to 229.1 torr when 82.3g of the compound is dissolved at constant temperature. Determine the molar mass of the unknown compound. Assume no change in volume occurs upon dissolution.
mols solute = 82.3 g/molar mass = ? Don't know molar mass.
delta P = Xsolute*P0solvent.
(233.7 - 229.1) = Xsolute*233.7
Xsolute =
Xsolute = 0.0197 = mols solute/(mols solute+moles solvent) where
mols solvent = 229 g H2O/18.02 = 12.71 and let n = moles solute. Then
n/(n+12.71) = 0.0197 and n of solute = 0.255
Then grams solute = mols solute/molar mass solute. You have grams and mols, solve for molar mass. Post your work if you get stuck.
This is math;not chemistry but i'm happy to show you.
n/(n+12.71)=0.0197
n = 0.0197 x (n + 12.71)
n = 0.0197n + 0.2504
1n - 0.0197n = 0.2504
1.0000n - 0.0197n = 0.2504
0.9803n = 0.2504
n = 0.2504/0.9803 = 0.255 :-)
Can you show steps how u got n of solute = 0.255
I'm not sure how to make n by itself in the equation n/(n+12.71)=0.0197
To determine the molar mass of the unknown compound, 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.
First, we need to calculate the change in vapor pressure caused by the addition of the unknown compound. The change in vapor pressure (∆P) is given by:
∆P = P°_solvent - P_solvent
where P°_solvent is the vapor pressure of the pure solvent and P_solvent is the vapor pressure of the solvent in the solution.
In this case, P°_solvent = 233.7 torr and P_solvent = 229.1 torr. Thus,
∆P = 233.7 torr - 229.1 torr
= 4.6 torr
Next, we can calculate the mole fraction of the solvent (water) in the solution. The mole fraction (X_solvent) is given by:
X_solvent = moles_solvent / moles_total
Since the volume doesn't change upon dissolution, we know that the total moles remain constant. Therefore, the mole fraction is equal to the volume fraction. In this case, the volume of the solution is 229 mL.
X_solvent = 229 mL / 229 mL
= 1
Now, we can use Raoult's law to relate the change in vapor pressure (∆P) to the mole fraction of the solvent (X_solvent). According to Raoult's law:
∆P = X_solvent * P°_solvent
Substituting the values we know:
4.6 torr = 1 * 233.7 torr
Simplifying, we find:
4.6 torr = 233.7 torr
Since this equation is not true, it suggests that the assumption of a nonelectrolyte and nonvolatile compound is incorrect. There must be some change in vapor pressure upon dissolution, indicating that the unknown compound is either an electrolyte or a volatile compound.
Therefore, we cannot determine the molar mass of the unknown compound based on the information provided.