Advice 1: How to find molecular weight

Relative molecular mass of a substance shows how many times a molecule of the substance is heavier than 1/12 of an atom of carbon. It can be found in the case, if known, its chemical formula using the periodic table of the elements. Otherwise, use other methods to find the molecular weight, given that it is numerically equal to molar mass of a substance expressed in grams per mole.
How to find molecular weight
You will need
  • periodic table of chemical elements;
  • - sealed container;
  • - scales;
  • - pressure gauge;
  • thermometer.
If you know the chemical formula of a substance determine its molecular weight using the periodic table of chemical elements of Mendeleev. To do this, define the elements that go into the formula of the substance. Then, find their relative atomic masses, which are recorded in the table. If the atomic mass of the table presents a fractional number, round it to the nearest integer. If the chemical formula contains a few atoms of this element, multiply the mass of one atom in their number. The obtained atomic masses fold and get the relative molecular mass of the substance.
For example, to find molecular mass of H2SO4, find the relative atomic masses of elements that go into the formula, respectively, hydrogen, sulphur and oxygen Ar(H)=1, Ar(S)=32 Ar(O)=16. Given that hydrogen in a molecule of 2 atoms, and oxygen 4 atoms, calculate the molecular mass of the substance Mr(H2SO4)=2•1+32+4∙16=98 atomic mass units.
In that case, if we know the amount of substance in moles ν and the mass m of a substance, expressed in grams, determine its molar mass for this mass divide by amount of substance M=m/ν. It will be numerically equal to its relative molecular mass.
If we know the number of molecules N, and the known mass m, find its molar mass. It will be equal to molecular mass, find the mass in grams to number of molecules of a substance in the mass, and multiply the result by Avogadro's constant PA=6,022^23 1/mol (M=M * N/ PA).
To find the molecular mass of the unknown gas, find its mass in a sealed container of known volume. To do this, siphon the gas out of it, creating there the vacuum. Weigh the cylinder. Then pump the gas back and again find its mass. The difference between the mass blank and the injected cylinder and is equal to the mass of gas. Measure the pressure inside the cylinder with a manometer in Pascals and temperature in Kelvins. To do this, measure the temperature of the surrounding air, it will be equal to the inside temperature in Celsius to convert it to Kelvins, add 273 to the obtained value.

Determine the molar mass of the gas by finding the product of temperature T, gas mass m and the universal gas constant R (8,31). The number you divide by values of pressure P and volume V, measured in m3 (M=m•8,31•T/(P•V)). This number will correspond to the molecular mass of the gas of interest.

Advice 2: How to find the molar mass of air

Molar mass is the mass of one mole of a substance, i.e. the value showing what quantity of a substance contains 6,022*10 (in degree 23) of particles (atoms, molecules, ions). But if we are not talking about pure substance and mixtures of substances? For instance, vital to people the air, because it represent a mixture of a great many of gases. How to calculate its molar mass?
How to find the molar mass of air
You will need
  • - accurate laboratory scales;
  • - round-bottomed flask with a ground joint and stopcock;
  • - vacuum pump;
  • - pressure gauge with two valves and connecting hoses;
  • thermometer.
First of all, consider the admissible computational error. If you don't need high accuracy, limit yourself to only the three most significant components: nitrogen, oxygen and argon, and take a "rounded" value of their concentrations. If you need more accurate result, use in calculations and carbon dioxide and can do without rounding.
Suppose you want the first option. Write the molecular masses of these components and their mass concentration in the air:

- nitrogen (N2). Molecular mass 28, mass concentration of 75,50%;
- oxygen (O2). Molecular mass 32, the mass concentration of 23.15%;
- argon (Ar). Molecular mass 40, mass concentration of 1.29%.
To facilitate calculations, rounded values of the concentrations:

- for nitrogen – up to 76%;
- for oxygen – up to 23%;
- for argon gas – to 1.3%.
Make a simple calculation:

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.
The obtained value is very close to that specified in the handbooks: of 28.98 g/mol. The discrepancy is due to rounding.
Unable to determine the molar mass of air and with the help of simple laboratory experience. To do this, measure the mass of the flask with her air.
Write down the result. Then, by connecting the hose of the flask to the pressure gauge, open the faucet and turned on the pump, begin to pump the air out of the flask.
Wait for a while (so the air in the flask was heated to room temperature), note the reading of manometer and thermometer. Then, closing the valve on the bulb, disconnect the hose from the pressure gauge, and weigh the flask with the new (reduced) amount of air. Write down the result.
Next you will come to the aid of the universal equation Mendeleev-Clapeyron:

PVm = MRT.

Write it down in a somewhat modified form:

∆PVm = ∆MRT, and you are aware of, and change of air pressure ∆P and the change of the air mass ∆M. the Molar mass of air m is calculated, is elementary: m = ∆MRT/∆PV.
Useful advice
The equation Mendeleev-Clapeyron describes the state of an ideal gas, which air is, of course, is not. But the values of pressure and temperature close to the normal, the error is so small that can be neglected.
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