What is molar weight? It is weight of one mole of a substance, that is its such quantity in which contains as much atoms, how many in 12 grams of carbon. The molar mass of complex substance is calculated, putting the molar mass of the elements entering it. For example, NaCl - table salt well familiar to all of us. What its molar weight? Having glanced in Mendeleyev's Table, receive the answer: 23 + 35.5 = 58.5. The task to determine the molar mass of gas is often set. How it can be done?
Instruction
1. Knowing a gas formula, its molar weight can be calculated by elementary calculation. Take carbon dioxide. Its formula – CO2. Its molar weight is as follows means: 12 + 32 (the molar mass of oxygen taking into account the 2 index) = 44.
2. And if you have to calculate the molar mass of the gas unknown to us which is in some closed volume, for example, hermetically closed cylinder? Here we will be come to the rescue by the universal equation of Mendeleyev-Klapeyrona describing a condition of "ideal gas". Certainly, any gas does not meet conditions "ideal", but with a pressure and temperature which are insignificantly differing from normal, this equation is very convenient for calculations. And the error received at calculations is very insignificant and it can safely be neglected.
3. The universal equation has such appearance: PV = MRT/m, a gder – gas pressure in Pascals; V – its volume in cubic meters; M – actual mass of gas; m – its molar weight; R – universal gas constant; T – gas temperature in degrees Kelvin.
4. You will see that molar weight is calculated on a formula MRT/PV. For example, it is necessary to find the molar mass of gas if it is known that 3 kilograms of this gas are in a sealed container of 1.7 cubic meters with a pressure of 100000 Pas and temperature of 27 degrees Celsius.
5. Substitute the known values in this formula, certainly, without having forgotten to transform to one system of sizes at first. Otherwise there will be a full nonsense. 3.0*8.31*300/170000=0.04399 kg/mol.
6. And as the molar mass of substance is measured in grams on mol, increase the received result by 1000 and receive the answer: the molar mass of the gas which is under such circumstances is equal 43.99 grams/mol or taking into account rounding - 44 grams/mol. That is it is the same carbon dioxide.