The freezing point reduction method measures the molecular weight, and why the relative molecular we

Under what conditions can the formula for measuring relative molecular weight by freezing point reduction be applicable?

The molar mass of solutes is determined by the freezing point reduction method using the dependence of dilute solutions. The formula has been simplified, so it can only be used under the condition of dilute solution, and the concentration of the solution deviates from the formula of dependence, the concentration is too low, and the change in dependence is not obvious. Therefore, the use of dependency can only roughly calculate the relevant indicators, but it is of good guiding significance to use the dependence to make qualitative judgments on the system.

Freezing point (when a solid pure solvent is precipitated): When the solution is cooled to the freezing point, the precipitation may be a pure solvent, or the solvent and solute may be precipitated together. When only pure solvent is precipitated, the freezing point TF of a dilute solution in equilibrium with a solid pure solvent is lower than that of a pure solvent at the same pressure.

The formula for the rise of the boiling point: t=k*b (k is the boiling point rise constant only related to the properties of the solvent itself, t is the temperature value of the boiling point of the solution after adding the molecular weight of the substance to be measured, b is the molar concentration of the mass, b=n (solute) m (solvent)), well, pay attention, the temperature rise value t can be measured by the thermometer, the solvent mass m can be measured in advance, the added solute mass can also be measured before joining, k value can be known by looking up the table, so the molar number n of the added solute can be obtained, with n and m, Naturally, the molecular weight m can be obtained.

Freezing point reduction method: formula: t=k*b (t is the temperature value of the freezing point of the solution after adding the substance of the molecular weight to be measured, k value is the solidification constant, b and the above are the same meaning, the method is the same as the above, not much to say).

PS: If the molecular weight of high-temperature and volatile substances such as proteins is determined, the freezing point reduction method is used.

The freezing point method is a method for determining the relative molecular weight of a substance, and its principle is to calculate the relative molecular weight of a substance by measuring the freezing point of the solution at different concentrations. However, this method has the problem of large error, mainly in the following aspects:

1.Solvent purity: The freezing point method requires the use of high-purity solvents, and it is difficult to completely avoid the influence of impurities and moisture on the results in practice.

2.Experimental conditions: Experimental conditions such as temperature, pressure, and stirring rate can affect the experimental results, and these parameters are difficult to fully control.

3.Crystalline morphology: Polycrystalline or monocrystalline crystals may be formed during crystallization, and they have different properties such as surface lease products and densities, which can lead to errors.

4.Correction coefficient: Due to the mutual miscibility between the sample and the solvent, and each has a certain degree of inertia of rising and cooling, the correction coefficient needs to be introduced in the calculation process for correction. However, this correction factor is often also incorrect.

In summary, although the freezing point method can be used to quickly determine the relative molecular mass of a substance, it is easy to operate. However, due to the interference and limitations of various factors, the accuracy needs to be further improved and improved.

The freezing point reduction method is a classic experiment in physical chemistry and inorganic chemistry laboratory teaching, and manual stirring is required to make the temperature of the system uniform during the whole experiment. Since it is manual, during the operation, it is impossible to avoid the stirring rod and thermometer.

The friction can cause the thermometer to fluctuate left and right, which has exceeded the absolute error of temperature.

The results of the freezing point reduction method for molecular weight determination are still relatively rough, and the average molecular weight is measured.

It is also not very practical to use this method to directly determine the molecular weight of drug molecules.

Nowadays, there are many advanced analytical methods, such as mass spectrometry, which can measure molecular weight very accurately. Therefore, the freezing point reduction method is no longer used in scientific research.

But on the other hand, it can be used to study the state of drug molecules in the solution of Lihuai solution, whether dissociation, association, solvation, etc. occur. Because this reduction in the freezing point of the solvent is only related to the number of particles of the solute in the solvent.

1 What approximations have been made in the derivation of the calculation formulas in the principle, and how to judge the rationality of these assumptions in this experiment? 2 What is the design idea of the Beckman thermometer, the temperature measuring instrument used in this experiment? Please propose an alternative temperature measurement plan.

3 What is the effect of too much supercooling on the results when determining the freezing point of the solution? What are the degrees of freedom for solution systems and pure solvent systems?

4 What is the function of the outer sleeve?

Answer: The molecular weight of a compound is an important physicochemical parameter. Freezing point reduction is a simple and accurate method for determining the molecular weight of a substance. The dilute solution has a dependence, and the reduction of the freezing point is a manifestation of the dependence.

The relationship between the freezing point reduction of the dilute solution (for the system where the precipitate is a pure solvent) and the fraction of the substances in the solution is: δ=TF* -TF= kfMB, where TF* is the freezing point of the pure agent, TF is the freezing point of the solution, MB is the mass molar concentration of solute B in the solution, and is the mass molar freezing point reduction constant of the solvent, and its value is only related to the properties of the solvent.

Question: Isn't this exactly the same as on the Internet?

The amount of naphthalene substance is: n=

128g/mol

Dissolved naphthalene in camphor, can also dissolve the same amount of the organic matter, that is, the temperature of the organic matter is reduced by 13 when the organic matter is dissolved in the camphor state, then the temperature of the dissolved organic matter in the camphor is reduced by 13, and the amount of the substance of the organic matter that the dissolved cavity is playing is:

Dissolved in the camphor, the freezing point decreases, then the amount of dissolved organic matter is:

The molar mass of this organic matter is: m=

121 g mol, the amount of the organic substance is: n = 121 g mol

The amount of CO2 is:

44g/mol

Then in the organic matter: n(c)=, h2

o The amount of the substance is:

18g/mol

Then n(h)=, the number of c and h atoms in the organic molecule is: n(c)=712, n(h)=

6, then the number of oxygen atoms contained in the organic molecule is: 121?12×7?1 62, the molecular formula of this organic matter is: C7H6O2

Answer: The molecular formula of this organic substance is C7H6O2