How To Determine The Molecular Weight Of The Substance | The Science

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# How to determine the molecular weight of the substance

Molecular Weight - is the weight of a molecule of a substance expressed in atomic units. Often the problem arises: to determine the molecular weight. How can I do that?

## Instruction how to determine the molecular weight of the substance

Step 1:

If you know the formula of the substance, then the problem is solved elementary. It takes only the periodic table. For example, you want to find the molecular weight of calcium chloride. Write a formula of substance: CaCl2. In the periodic table, set the atomic weight of each element included in its composition. For calcium, it is equal (rounded) 40 for chlorine (as rounded) - 35.5. Given the index 2 Found: 40 + 35.5 * 2 = 111 amu (Atomic mass units).

Step 2:

And what about when the precise formula of the substance is unknown? Then you can act in different ways. One of the most effective (at the same time simple) - so-called "osmotic pressure method". It is based on the phenomenon of osmosis, consisting in that solvent molecules can pass through the semipermeable membrane, while the solute molecules can not penetrate it. The magnitude of the osmotic pressure can be measured and is directly proportional to the concentration of analyte molecules (i.e. their number per unit volume of solution).

Step 3:

Some familiar universal equation of Mendeleev-Clapeyron equation, which describes the state of the so-called "ideal gas". It has the form: PVm = MRT. Van't Hoff formula is very similar to it: P = CRT, where P - the osmotic pressure, C - molar concentration of solute, the R - universal gas constant, T - temperature in degrees Kelvin. This similarity is not accidental. As a result of the work of van't Hoff became clear that the molecules (or ions) in solution behave as if they are in the gas (at the same volume).

Step 4:

By measuring the magnitude of the osmotic pressure, it is possible to calculate the elementary molar concentration: C = P / RT. Then, knowing the mass of the substance as a liter of solution to find its molecular weight. Suppose it was found empirically that the molar concentration of the aforementioned substances is 0.2. Thus in one liter of solution contained 22.2 grams of that substance. What is its molecular weight? 22.2 / 0.2 = 111 amu - Exactly the same as in the previously mentioned calcium chloride.