To find out the Kf for naphthalene, we have to discover the distinction within the freezing level of pure naphthalene and the answer of 1,Four-dichlorobenzene in naphthalene. For instance that we did this experiment, used 1. 00 g 1,Four-dichlorobenzene in 10. 00 g naphthalene, and located that the freezing temperature of pure naphthalene was 78. 2°C, whereas that of the answer was 75. Four°C. This offers us a ∆ Tf of 78. 2°C – 75. Four°C = 2. eight°C.
Utilizing the equation for freezing level melancholy and fixing for Kf, we have now… ∆Tf = Kfmsolute Kf = ∆ Tf/msolute the place msolute equals the molality of the solute. What’s the molality of the solute?
msolute=molality of solute = moles of solute/kg solvent
moles of solute = 1. 00 g 1,Four-DCB/146. 9 g/mol = 6. 81 x 10-Three moles 1,Four-DCB kg of solvent = 10. 00 g naphthalene/1000 g/kg = zero. 01 kg solvent msolute = 6. 81 x 10-Three moles 1,Four-DCB/zero. 01 kg naphthalene = zero. 681 m Kf = 2. eight°C/zero. 681 m = Four. 112 Ok·kg/mol
The precise worth for Kf for naphthalene is 7. 45 Ok·kg/mol, so we’re a good quantity off the mark.
It is a somewhat crude experiment, in order that’s to be anticipated.
Determination of the Molecular Mass/Molecular Formulation of Elemental Sulfur
Though this experiment did not go in addition to I’d have appreciated, there gave the impression to be some issues with the interpretation of the information as they had been obtained, so I assumed I would reveal how to do that calculation. Elemental sulfur has a molecular components of S8 (there are eight sulfur atoms in a molecule of sulfur, identical to there are 2 hydrogen atoms in a molecule of hydrogen). So, the aim of this experiment was actually simply to see how shut you might come to this reply.
Observe take a look at: reply key
To search out the molar mass (and, utilizing the atomic mass, the molecular components) for sulfur, we first want to search out the freezing level of an answer of sulfur in naphthalene. For instance we made such an answer with zero. 500 g powdered sulfur in 10. 00 g of naphthalene and we discovered that its freezing level was 76. 7°C. To search out the molecular mass, let’s first discover the molality of the answer by fixing the above equation for msolute. (NOTE: Opposite to what many of you thought, you may’t discover the molality of this resolution simply by utilizing the atomic mass of sulfur to search out the moles of sulfur… keep in mind that molecular sulfur has eight sulfur atoms, however for the needs of your experiment, you “do not know this” and are looking for that truth out. So, we’re fixing for the molar mass identical to the issue on the newest examination involving an unknown solute.)
∆ Tf =Kfmsolute msolute = ∆ Tf/Kf
Our ∆ Tf = 78. 2°C – 76. 7°C = 1. 5°C.
Let’s use the “actual” Kf as a substitute of the one we calculated so we’ll get a greater reply… msolute = 1. 5°C/7. 45 Ok·kg/mol = zero. 201 mol/kg (molal) Now, we all know the molality, and we all know the quantity of kilograms of solvent (zero. zero10 kg of naphthalene) – let’s discover the quantity of moles of sulfur current.
Moles of sulfur = molality x kg of solvent = zero. 201 m x zero. zero10 kg = zero. 00201 moles sulfur
Now, divide the mass of sulfur we took by this quantity of moles to get the molar mass of sulfur… zero. 500 g sulfur/zero. 00201 moles sulfur = 248. eight g/mol
That is the molar mass of elemental sulfur… divide it by the atomic mass of sulfur (32. 07 g/mol) to learn how many sulfur atoms there are in a sulfur molecule. We get 7. 76 ? eight atoms… is not it cool how one can get the best reply once you make up your personal knowledge?
