Molar mass

[skmackie]

Anyone know how I should go at this problem?

A solution of an unknown substance is made by dissolving 712.0g. of the unknown solid in 150.0g. water. The vapor pressure of the solution is measured at 60 degrees C and is found to be 119.55mm Hg. If the vapor pressure of pure water is 149.44 mm Hg at 60C, calculate the molar mass of the solute.

Any help appreciated,

Sheryl

[chemistrygosu]

Originally Posted by skmackie

Anyone know how I should go at this problem?

A solution of an unknown substance is made by dissolving 712.0g. of the unknown solid in 150.0g. water. The vapor pressure of the solution is measured at 60 degrees C and is found to be 119.55mm Hg. If the vapor pressure of pure water is 149.44 mm Hg at 60C, calculate the molar mass of the solute.

You need to use Raoult's Law.

Psoln= Psolv * Xsolv

Start out by rearranging the equation to

Xsolv= (Psoln)/(Psolv)

Where Psoln is the pressure of solution. Psolv is the pressure of the solvent which is water and and X is the mole fraction of water.

First calculate the mole fraction of water.

Xsolv= (119.55mm Hg) / (149.44 mm Hg)

that equals .7999

Next calculate the number of moles of water.

150.0g H20*(1 mol / 18g) = 8.333 mol H2O

Mole fraction is defined as number of moles of water over the total moles.

X= Mole water/(Total moles of solution+ Moles of water)

re arrange and we get

Total moles = Moles of water/Xsolv

Plug our numbers for moles of water and mole fraction of the solution in and we get.

8.333 mol / .7999 = 10.41 mol

now we minus the total moles from the moles of solvent.

10.41 mol - 8.333 mol

2.08 mol solute.

so grams divided by moles eqauals molar mass so we get

712.0g/ 2.08 mol solute = 341.7 g/mol is the molar mass.

[Jack lerunstrensta]

I have a question. In my engaging experiment of the hidden properties of an unknown substance, i ran into this new formula which maintained the same proportion of properties as another element located on the periodic table. After vigorous days and nights of research, i have concluded this element is no other... than H20. It was in a liquid form, and i could see my reflection through this substance. Iis H20 safe for the human body? someone needs to help me.. or there will be problems.

:wink: