I have o.236 g for urea and 0.235g for NaCl.
And the question is
On a gram basis, which is more effective in depressing the freezing point, urea or NaCl? On a mole basis, which Is more effective?
I am confused on what depressing the freezing point me and idk how to explain the answer
The Freezing point depression equation is ΔT = i*Kf*m
Where m=molality=moles of solute/kg of solvent (H20)
i= van 't Hoff factor
Kf=constant
So, lets get rid of some of this stuff and equate ∆T= moles of solute/kg of solvent.
the one with the greater number of moles, assuming the kg of H20 stays constant, will cause an increase in ∆T.
I found molality of urea but I am not sure how to do after
Sorry wrong post
For NaCl, Van 't Hoff factor=2. Find the molality like you did for Urea. But remember, the question is asking how does ∆T change with different mass or mole ratios.
grams of substance*( 1 mole/molecular weight)= moles of substance
So nacl is effective in both cases
I believe that is a valid assumption.
My teacher say I don't have to show any calculation so how do I explain it using the graph I have for both??
That is what i was conveying in my second post; you need to show how the freezing point changes. Looking at the equation, you will see that it depends on moles not mass. The compound with the higher number of moles, assuming H2O is constant for both, will the greater effect on the freezing point.
To determine which substance is more effective in depressing the freezing point, we first need to understand what "depressing the freezing point" means.
When a solute (in this case, urea or NaCl) is dissolved in a solvent (such as water), it disrupts the orderly arrangement of the solvent molecules, making it more difficult for them to form the solid crystal lattice structure. Consequently, the freezing point of the solvent is lowered. The greater the depression of the freezing point, the more effective the solute is.
Now let's analyze the two substances, urea and NaCl, both on a gram basis and on a mole basis:
1. Gram basis:
To compare the substances on a gram basis, we need to calculate the molality of each solute.
Molality (m) is defined as the moles of solute per kilogram of solvent. We calculate it using the formula:
m = (moles of solute) / (mass of solvent in kg)
In this case, since we are comparing the substances, the mass of the solvent is the same.
a. Urea:
Given: mass of urea = 0.236 g
To calculate the molality of urea, we need the molar mass of urea. The molar mass of urea (CO(NH2)2) is approximately 60.052 g/mol.
moles of urea = (0.236 g) / (60.052 g/mol)
b. NaCl:
Given: mass of NaCl = 0.235 g
To calculate the molality of NaCl, we need the molar mass of NaCl. The molar mass of NaCl is approximately 58.44 g/mol.
moles of NaCl = (0.235 g) / (58.44 g/mol)
Now that we have the number of moles for each solute, we can compare the molality of urea and NaCl. The solute with the higher molality will be more effective in depressing the freezing point.
2. Mole basis:
To compare the substances on a mole basis, we need to consider the ratio of moles of solute to moles of solvent.
a. Urea:
Given: moles of urea (calculated earlier)
b. NaCl:
Given: moles of NaCl (calculated earlier)
Now, let's compare the mole ratios of urea and NaCl. The solute with the greater mole ratio will be more effective in depressing the freezing point.
After calculating the molality and mole ratios, you can compare the values to determine which substance is more effective at depressing the freezing point, both on a gram basis and a mole basis.