[dancer20121810]

Well this is AP chem I hope it still fits under this forum
I have two questions left on my webassign and i don't know how to figure them out.

1. A forensic chemist is given a white solid that is suspected of being pure cocaine (C17H21NO4, molar mass = 303.35 g/mol). She dissolves 1.22  0.01 g of the solid in 15.60  0.01 g benzene. The freezing point is lowered by 1.32  0.04°C.
   
     What is the molar mass of the substance?
           I know this is 303
     Assuming that the percentage of uncertainty in the calculated molar mass is the same as the percentage of uncertainty in the temperature change, calculate the uncertainty in the molar mass.
           I assumed you just run the min and max of uncertainty through the same steps, but I did that and it didn't work so i don't know what to do

2. A solid mixture contains MgCl2 and NaCl. When 0.5000 g of this solid is dissolved in enough water to form 1.000 L of solution, the osmotic pressure at 25°C is observed to be 0.3970 atm. What is the mass percent of MgCl2 in the solid? (Assume ideal behavior for the solution.)
          I know pi=MRTi
                   so M=pi/RTi
                        M=.397/(.0821*298*5)
                        M=.0032453466
                 and since its 1 L mol = .0032453466
          but after this I dont know what to do

[rabolisk]

1. How do you know that the molar mass of the substance is 303?

As far as uncertainty goes, if you assume that the relative uncertainty is same, then you are already given the "percentage of uncertainty in the temperature change", which is just 0.04/1.32 * 100. You multiply that by whatever molar mass you get to get the uncertainty.

2. i does not equal 5. You solved for a case in which a solid dissolved into 5 different particles. This problem is a mixture of two solids, which add up to 0.500 grams, whose osmotic pressure is 0.3970 atm (298 K, 1 L), and dissolve into different number of particles. i.e., a very tough problem.

The best way to solve this is to break up the osmotic pressure into two parts, one due to NaCl and one to due to MgCl₂. Set up a system of equations, and solve.

[dancer20121810]

1.Multiply that by what molar mass? the 303 I already know?

2. How would I split it up? Do I set them both to 0.3970 or do I divide that in half?

[dancer20121810]

And  I solved it by doing m=delta T/k
                                  m=1.32/5.114
                                  m=.258
                                  mol=m*kg
                                  mol=.258*.0156
                                  mol=.004
                                  mm=g/mol
                                  mm=1.22/.004
                                  mm=302.9

 

[rabolisk]

1. Yes, multiply it by 302.9 to get the uncertainty.

2. You set the sum to 0.3970. Remember that these are the conditions.
  i. The osmotic pressure due to a mols of NaCl and b mols MgCl₂ is 0.3970 atm. (298 K, 1.0 L solution)
  ii. The mass of a mols of NaCl and b mols of MgCl₂ add to 0.5000 g.

The rest is algebra.