Dissolve 1.5g of an unknown salt into 37g of water and find that the new vapor pressure at 75C is 277 torr. What is the Molar Mass of the unknown? How many grams of the unknown would have to be used in 100ml of water to make a 11.5% solution by mass of the unknown?

Psoln = XH2O*PoH2O

Look up Po for H2O at 75, substitute 277 torr for Psoln and solve for XH2O.
Then XH2O = (moles H2O)/(moles H2O + mols Unk).
Solve for moles Unknown
Then moles = grams/molar mass and solve for molar mass.
Probably you can pick it up here and obtain the % by mass solution. Post your work if you get stuck.

thoo

To find the molar mass of the unknown salt, we can use Raoult's law, which states that the vapor pressure of a solution is directly proportional to the mole fraction of the solute present.

First, we need to calculate the mole fraction of the solute. The mole fraction of the solute (X) is given by:

X = moles of solute / total moles of solution

In this case, we know the mass of solute (1.5g) and the mass of the solvent (37g). We can calculate the moles of solute using the molar mass.

1. Calculate the moles of solute:
moles of solute = mass of solute / molar mass

To find the molar mass, we use the ideal gas law equation:

PV = nRT

where P is the vapor pressure, V is the volume, n is the moles, R is the ideal gas constant, and T is the temperature in Kelvin.

2. Convert the temperature from 75°C to Kelvin:
T(K) = T(°C) + 273.15

3. Plug in the values into the ideal gas law to calculate the moles:
n = PV / RT

Since we know the vapor pressure (277 torr), the volume of the solvent (37g of water), the ideal gas constant (0.0821 L·atm/mol·K), and temperature (converted to Kelvin), we can find the moles of the solute.

Once we have the moles of the solute, we can calculate the mole fraction (X) and use it to determine the moles of the unknown salt in 100ml of water (given that it forms an 11.5% solution by mass).

4. Calculate the mole fraction of the solute:
X = moles of solute / total moles of solution
(total moles of solution = moles of solute + moles of solvent)

5. Calculate the grams of the unknown salt needed in 100ml of water:
mass of unknown salt = mass of solution x mass fraction of solute

mass of solution = 100ml of water (density of water is approximately 1g/mL)
mass fraction of solute = 11.5% (convert to decimal)

So, by following these steps, you can find the molar mass of the unknown salt and determine the grams of the unknown salt needed in a 100ml solution.