A 250.0 mL sample of an aqueous solution at 25 degrees Celsius contains 35.8 mg of an unknown nonelectrolyte compound. If the solution has an osmotic pressure of 11.25 mmHg, what is the molar mass of the unknown compound?
a.) 237 g/mol
b.) 484 g/mol
c.) 351 g/mol
d.) 699 g/mol
e.) 160 g/mol
I thought I did the math right, but I guess I didnt, because I just cant seem to get the right answer. Help me please?
pi = MRT
Solve for M
Then M = mols/L, solve for mols.
Then mols = grams/molar mass; solve for molar mass.
Don't forget pi is in atm so that's 11.25/760 = ?atm.
Also T is in K = 298 K and
R is 0.08206. I ran through it and obtained one of the answer choices.
Post your work and I'll find the error.
What do you mean solve for molar mass? I think that is the problem that I am having.
nevermind, I got it. Is the answer 160 g/mol?
To find the molar mass of the unknown compound, we can use the equation for osmotic pressure:
π = MRT
Where:
π = osmotic pressure
M = molarity (in mol/L)
R = ideal gas constant (0.0821 L·atm/mol·K)
T = temperature (in Kelvin)
First, let's convert the mass of the compound to moles:
m = 35.8 mg = 0.0358 g
molar mass of compound = m / n
Next, let's calculate the molarity of the solution:
V = 250.0 mL = 0.2500 L
Molarity (M) = n / V
We can rearrange the formula for osmotic pressure to solve for molarity:
M = π / (RT)
Now, let's substitute the given values:
π = 11.25 mmHg
R = 0.0821 L·atm/mol·K
T = 25 degrees Celsius = 298 K
M = (11.25 mmHg) / (0.0821 L·atm/mol·K * 298 K)
Next, convert mmHg to atm:
1 mmHg = 0.00131579 atm
M = (11.25 * 0.00131579 atm) / (0.0821 L·atm/mol·K * 298 K)
M = (0.0147634 atm) / (24.4378 L·atm/mol·K)
Now, substitute Molarity back to get molar mass:
Molarity (M) = n / V
n = M * V
molar mass of the unknown compound = m / n
molar mass of the unknown compound = (0.0358 g) / (M * 0.2500 L)
Finally, substitute the value we calculated earlier for M:
molar mass of the unknown compound = (0.0358 g) / ((0.0147634 atm) / (24.4378 L·atm/mol·K) * 0.2500 L)
Now, calculate this expression to obtain the molar mass, and compare it to the given answer choices to find the correct one.