# Molar Mass of Unknown Solute and Mole Fraction of Gases

• Chemistry
• Soaring Crane
In summary, the conversation discusses the calculation of the molar mass of an unknown substance and the mole fraction of benzene in a solution. The molar mass is found using the equation pi = MRT, and the mole fraction of benzene is calculated using the equation X_gas = P_gas (benzene) / P_total. Both calculations are found to be correct.
Soaring Crane
1) Find the molar mass of an unknown substance if 227.5 mL of an aqueous solution containing 2.785 grams of the unknown generates an osmotic pressure of 588.0 mm Hg at 20.9 Celsius. Report your answer to 3 significant figures.

pi = MRT

pi = 588.0 mm Hg = 588 mm Hg/760 mm Hg = 0.7736842 atm
T = 20.9 C = 20.9 C + 273.15 = 294.05 K

molar mass = g solute / mol solute

pi = R*T*(mol solute/L solution)

mol solute = (pi*Volume_solution)/(R*T) = (0.7736842 atm * 0.2275 L)/(294.05 K * 0.08206) = 0.007294448 mol

m. mass = 2.785 g / 0.007294448 mol = 381.797 g/mol = 382 g/mol ?

2) Calculate the mole fraction of benzene in the vapor phase from a solution that contains a mole fraction of benzene of 0.557 when mixed with toluene if the vapor pressure of pure benzene is 183 mmHg and that of toluene is 59.2 mm Hg.

X_b = 0.557-------------------VP_b = 183 mm Hg
X_t = 1 - 0.557 = 0.443-------VP_t = 59. 2 mm Hg

Total (vapor) pressure = (X_b*P_b) + (X_t*P_t)
P_t = 0.557*183 mm Hg + 0.443*59.2 mm Hg
P_t = 101.931 mm Hg + 26.2256 mm Hg = 128.1566 mm Hg

X_gas = P_gas (benzene) / P_total = 101.931 mm Hg / 128.1566 mm Hg = 0.795 ?

Thank you.

I think the approach to a) is correct, but let me get GCT or Ouabache to take a look.

#1 is correct

Is #2 correct?

Thanks again.

#2 is correct too.

yeah, both of them seem fine.

Nice job Soaring Crane.. I also tried them and come up with those solutions.

## What is molar mass?

Molar mass is the mass of one mole of a substance. It is usually expressed in units of grams per mole (g/mol).

## How do you calculate molar mass?

The molar mass can be calculated by adding up the atomic masses of all the atoms present in a molecule. The atomic masses can be found on the periodic table.

## What is the significance of knowing the molar mass of an unknown solute?

Knowing the molar mass of an unknown solute is important for various chemical calculations, such as determining the concentration of a solution or predicting the behavior of a reaction.

## What is mole fraction?

Mole fraction is a unitless measure of the concentration of a component in a mixture. It is calculated by dividing the moles of a specific component by the total moles of all components in the mixture.

## How is mole fraction used in the study of gases?

In the study of gases, mole fraction is used to determine the partial pressure of a specific gas in a mixture. This is important in understanding the behavior of gases in different conditions.

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