You will need

- For calculations grab a pen, a calculator and a periodic table.

Instruction

1

Unit of molecular mass is 1/12 the mass of an atom of carbon, which is conventionally taken as 12. Molecular weight is numerically equal to the sum of the relative atomic mass of all atoms included in the molecule, and it is very easy to calculate.

2

According to Avogadro's law equal volumes of gases at constant pressure and temperature will contain equal numbers of molecules. Of it was later derived the equation Mendeleev-clayperon. Now you'll need to use it, but it is only valid for gases! Substitute in the formula the known pressure and temperature, the result is molecular

As you can see, this method requires a lot of data, but the error of such calculations is minimal.

**mass**of gas:M=(m∙R∙T)/(P∙V),where M is the desired molecular weight, m – mass of the substance, R is the universal gas constant (accept for 8,31 j/mol*K), T is the temperature in Kelvins, P is pressure in Pascals, V is the volume in cubic meters.As you can see, this method requires a lot of data, but the error of such calculations is minimal.

3

The following method is much easier. If you only know the mass m of the substance and its chemical number ν, we substitute these numbers into the formula:M=m/ν,

where m is the mass of the substance (usually in grams), and ν is the amount of substance in moles.

where m is the mass of the substance (usually in grams), and ν is the amount of substance in moles.

4

And there is the easiest option if you know the chemical formula of the substance. Take the periodic table, a view of the molecular weight of each element of the composition. For example, hydrogen, it is equal to 1, oxygen – 16. And to find the molecular mass of all matter (take for example water, which consists of two hydrogen molecules and one oxygen molecule) simply add up the masses of all its constituent elements. For water: M(H2O) = 2M(H)+M(O) = 2•1+16 = 18. E. M.

Useful advice

As you can see, to find the molecular weight can be very simple. The main thing is not to be confused with the molar mass of substances – they are numerically equal but have different units of measurement and physical meaning.

# Advice 2 : How to determine the molar mass of the substance

In order to find the molar

**mass****of the substance**, determine its chemical formula and using the periodic table calculate its molecular**mass**. It is numerically equal to molar mass**of the substance**in grams per mole. If you know the mass of one molecule**of the substance**, put it in grams and multiply by 6,022•10^23 (Avogadro's number). Molar**mass**of gas can be found using the ideal gas equation of state.You will need

- periodic table, pressure gauge, thermometer, scales.

Instruction

1

Determination of the molar mass

**of a substance**from the chemical formula. Locate elements in the periodic table of Mendeleev, which correspond to atoms from which a molecule**of the substance**. If a molecule**of a substance is**monatomic, then it will be its molar mass. If not, find the atomic**mass**of each element and add the masses. The result will be the molar mass**of a substance**expressed in grams per mole.2

Determination of the molar mass

**of the substance**mass of a single molecule. In that case, if we know the mass of one molecule, put it into grams, then multiply by the number of molecules in one mole of any**substance**, which is 6,022•10^23 (Avogadro's number). Get the molar**mass****of the substance**in grams per mole.3

Determination of the molar mass of the gas. Take a container which can be hermetically sealed with a predetermined volume, which translate to cubic meters. With a pump siphon his gas, on the scales and weigh the empty cylinder. Then fill it up with gas, the molar mass is measured. Again weigh the container. The difference in the mass blank and the injected gas cylinder will be equal to the mass of gas, to Express it in grams.

Using the manometer, measure the pressure of the gas inside the cylinder, to do this, connect it to the hole for gas injection. You can use cylinder with built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in Pascals.

Using the manometer, measure the pressure of the gas inside the cylinder, to do this, connect it to the hole for gas injection. You can use cylinder with built-in pressure gauge to quickly monitor pressure indicators. Measure pressure in Pascals.

4

Wait some time for the gas temperature inside the container equal to the ambient temperature and measure it with a thermometer. Temperature from degrees Celsius translate to the Kelvin, which add to the measured value of the number 273.

The mass of gas multiply by the temperature and universal gas constant (8,31). The resulting number is consistently split on the values of pressure and volume (M=m•8,31•T/(P•V)). The result will be the molar mass of the gas in grams per mole.

The mass of gas multiply by the temperature and universal gas constant (8,31). The resulting number is consistently split on the values of pressure and volume (M=m•8,31•T/(P•V)). The result will be the molar mass of the gas in grams per mole.