Urea (NH2)2CO is dissolved in 100.0 g of water. The solution freezes at -0.085 degrees C. how many grams of urea were dissolved to make this solution.
delta T = Kf*molality
Plug in 0.085 for delta T, I assume you know Kf for water, calculate molality.
Then molality x kg solvent x molar mass = grams.
You have all but molar mass which can be calculated.
0.085 degrees C = 0 degrees C (0.1 kg/60 g of (NH2)2CO
is this right
See above.
You don't need to post multiple times. It takes a LITTLE time to read, solve, and compose and answer for you.
well actually i double clicked the submit button and it posted it twice... sorry
Yes, you are on the right track.
To calculate the number of grams of urea dissolved in the solution, you need to use the formula:
delta T = Kf * molality
Here, delta T is the freezing point depression (-0.085 degrees C), and Kf is the freezing point depression constant for water.
Next, you need to calculate the molality of the solution. The molality (m) is defined as the number of moles of solute per kilogram of solvent. In this case, the solute is urea ((NH2)2CO), and the solvent is water.
To calculate the molality, you need to know the molar mass of urea. You can determine the molar mass by summing up the atomic masses of all the atoms in the chemical formula. In the case of urea, you have 4 hydrogen atoms (H), 2 nitrogen atoms (N), and 1 carbon atom (C), in addition to 3 oxygen atoms (O) which are not involved in the molality calculation.
The atomic masses are as follows:
H: 1.0079 g/mol
N: 14.0067 g/mol
C: 12.0107 g/mol
O: 15.999 g/mol
So, the molar mass of urea is (4 * 1.0079) + (2 * 14.0067) + 12.0107 = 60.0553 g/mol.
Now that you know the molar mass of urea, you can calculate the molality using the formula:
molality = (moles of solute) / (kilograms of solvent)
In this case, the kilograms of solvent is given as 100.0 g of water. Since 1 kg = 1000 g, the kilograms of solvent is 0.1000 kg.
You have the mass of the solute, which is the molar mass of urea times the number of moles of urea dissolved in the solution. Rearranging the formula, you can solve for the number of moles of urea:
moles of urea = (grams of urea) / (molar mass of urea)
To solve for grams of urea, you can rearrange the formula again:
grams of urea = (moles of urea) * (molar mass of urea)
Once you have the grams of urea, you can plug it into the molality formula to calculate the expected freezing point depression. If the calculated value matches the observed value of -0.085 degrees C, then your calculation is correct.