Chemistry molar mass problem?
An organic substance has 40% C, 6.7% H, 53.3% O. A solution of .650 g of the solid in 27.8g of the solvent diphenyl gave a freezing point depression of 1.56 degrees C. calculate the molar mass and molecular formula. Kf= 8.0 C/m? Any help would be great
Comments
first we calculate the empirical formula
moles of C in 100 g of compound = 40 / 12 = 3.33 moles
moles of H = 6.7 / 1.008 = 6.64 moles
moles of O = 53.3 / 16 = 3.33
molar ratio = C : H : O = 1 : 2 :1
empirical formula is CH2O
ΔTf = Kf · mB
ΔTf, the freezing point depression, is defined as Tf (pure solvent) − Tf (solution) = 1.56
Kf, the cryoscopic constant, = 8
mB is the molality of the solution,
substituting we get
1.56 = 8 x mB
molality = 1.56 / 8 = .195 moles / kg
we have .650 g of solute / 27.8g of solvent = .650 x 1000 / 27.8 gm per 1000g
= 23.38 g
so .195 moles = 23.38 g ; 1 mole has mass of 23.38 /.195 = 119.90 g
molecular weight = 119.9g / mole
empirical "mass" = mass ( CH2O) = 12 +2 +16 = 30 which is 1/4 molecular weight
molecular formula is C4H8O4
nice problem.. I hope you can follow the reasoning ... P:
through fact 2.66 divided via 0.0380 is 70. To get the respond, you may desire to learn that with the intention to artwork out MR (molar mass), you may desire to take Mass and divide it via moles (there's a formula triangle that enables with remembering the three formula)