An aqueous solution containing 32.9 g of an unknown molecular (nonelectrolyte) compound in 130.0 g of water has a freezing point of -1.2 ∘C.
Calculate the molar mass of the unknown compound.
Express your answer using two significant figures.
Please help, thank you.
delta T = i*Kf*m
1.2 = 1*1.86*m
Solve for m = molality.
Then molality = moles/kg solvent = moles/0.130 kg
Solve for moles.
Then moles = grams/molar mass.
Plug in for moles from above and grams from the problem and solve for molar mass.
Round to two significant figures. Post your work if you get stuck.
Im sorry I still have no idea how to do this.
1.2 = 1*1.86*m
= 1.2 = 1.86*m
1.2/1.86 = 0.645... m
Molality = 0.64
But multiply it by 0.13 and you get 0.084 and I know thats too low
waiiiit, did you mean 32.9/130 = 0.253 ???
Your first step was right. m = 0.645 but I would have kept all the numbers until finishing before rounding anything.
Your second step was right. moles = 0.0839 but I didn't round off. Your error is that is NOT the end of the problem. The number of moles = 0.0839. I had another step.
mols = grams/molar mass
0.0839 = 32.9/molar mass or molar mass = 32.9/0.0839 = 392 which when rounded to two significant figures is 390. Your mistake? Apparently you equation # mols (that 0.0839 as the molar mass. It isn't. You just didn't do the last step. I would be interested in knowing why. Please tell me what about my three-step solution confused you. Good luck.
To calculate the molar mass of the unknown compound, we need to use the freezing point depression equation:
ΔT = K_f * m
where:
ΔT = freezing point depression (-1.2 ∘C)
K_f = freezing point depression constant for water (∼1.86 ∘C/m)
m = molality of the solution (moles of solute per kg of solvent)
First, let's calculate the molality (m) of the solution:
m = moles of solute / kg of solvent
To determine moles of solute, we need to convert the given mass of the unknown compound into moles. We can use the molecular weight (molar mass) for this calculation. Let's call the number of moles of the unknown compound "n":
n = mass of compound / molar mass of compound
Now, let's calculate the molality:
m = n / mass of water (in kg)
To convert the mass of water from grams to kilograms, divide by 1000:
mass of water (in kg) = mass of water (in g) / 1000
Finally, we can substitute the values into the freezing point depression equation and solve for the molar mass of the unknown compound:
ΔT = K_f * (n / mass of water (in kg))
Rearranging the equation:
molar mass of compound = (n / (ΔT / K_f)) * mass of water (in kg)
Now we can substitute the given values:
ΔT = -1.2 ∘C
K_f = 1.86 ∘C/m
mass of compound = 32.9 g
mass of water = 130.0 g
Calculations:
1. Convert the mass of water to kilograms:
mass of water (in kg) = 130.0 g / 1000 = 0.130 kg
2. Calculate the moles of the unknown compound:
n = 32.9 g / molar mass of compound
3. Calculate the molality:
m = n / mass of water (in kg)
4. Substitute the values into the rearranged equation:
molar mass of compound = (n / (ΔT / K_f)) * mass of water (in kg)
By solving this equation, we can determine the molar mass of the unknown compound.