The molarity of urea in a solution prepared by dissolving 16 g of urea (MW = 60.0 g/mol) in 39 g of
H2O is __________ M. The density of the solution is 1.3 g/mL.
i get 8.9 but the practice test says the answer its 6.3
OH, ok thanks!
To find the molarity of the urea solution, we need to calculate the number of moles of urea and then divide it by the volume of the solution in liters.
1. Calculate the moles of urea:
Since we know the molecular weight of urea is 60.0 g/mol and we have 16 g, we can use the following formula:
moles of urea = mass of urea / molecular weight of urea
moles of urea = 16 g / 60.0 g/mol
moles of urea = 0.267 mol
2. Calculate the volume of the solution in liters:
We are given that the density of the solution is 1.3 g/mL, and we have 39 g of water.
We can use the following formula to calculate the volume in milliliters:
volume of solution = mass of solution / density of solution
volume of solution = 39 g / 1.3 g/mL
volume of solution = 30 mL
Since 1 L = 1000 mL, we need to convert the volume to liters:
volume of solution = 30 mL / 1000 mL/L
volume of solution = 0.03 L
3. Calculate the molarity of the urea solution:
molarity of urea = moles of urea / volume of solution
molarity of urea = 0.267 mol / 0.03 L
molarity of urea ≈ 8.9 M
Based on the calculations, it appears that the molarity of the urea solution is indeed approximately 8.9 M. It's possible that the practice test answer is incorrect or there might be some additional information missing from the question.
To find the molarity of urea in the solution, we need to calculate the number of moles of urea and the volume of the solution.
1. Calculate the number of moles of urea:
Given:
Mass of urea = 16 g
Molecular weight of urea (MW) = 60.0 g/mol
Number of moles of urea = Mass of urea / MW
Number of moles of urea = 16 g / 60.0 g/mol
Number of moles of urea = 0.267 mol
2. Calculate the volume of the solution:
Given:
Mass of water (H2O) = 39 g
Density of the solution = 1.3 g/mL
Volume of the solution = Mass of water / Density of the solution
Volume of the solution = 39 g / 1.3 g/mL
Volume of the solution = 30 mL
3. Convert mL to liters:
Volume of the solution = 30 mL x (1 L / 1000 mL)
Volume of the solution = 0.030 L
4. Calculate the molarity of urea:
Molarity (M) = Number of moles of urea / Volume of the solution
Molarity (M) = 0.267 mol / 0.030 L
Molarity (M) ≈ 8.9 M
Based on the calculations, the molarity of urea in the solution is approximately 8.9 M, which aligns with your initial calculation. It's possible that there was an error in the practice test answer key, or there might be additional context or information that could affect the calculation.
I think the problem is that the SOLUTION has a density of 1.3 g/mL. You are assuming the water is 1.3 g/mL and it isn't.
One way around that is to calculate the molality, then convert that to molarity.
Or you can do it in one step like this.
M = mols/L.
mols = 0.2667
total g soln = 16 from the urea + 39 from water = 55g and volume = 55/1.3 = 42.3 mL or 0.0423 L.
Then 0.2667/0.0423 = 6.3 M.