Molar weight is a mass of one of any substance, that is its such quantity in which contains 6.022*10^23 elementary particles. In number molar weight coincides with molecular, expressed in atomic units of mass (and. e. m), but dimension at it another – gram/mol.
Instruction
1. If you should have calculated the molar mass of any gas, you would take the size of atomic mass of nitrogen and increased it by index 2. As a result would receive 28 grams/mol. But how to calculate the molar mass of mix of gases? This problem is solved elementary. It is only necessary to know what gases and in what proportion are a part of mix.
2. Review a concrete example. Let's assume, you have gas mix which consists of 5% (mass) of hydrogen, 15% of nitrogen, 40% of carbon dioxide, 35% of oxygen and 5% of chlorine. What its molar weight? Use a formula for the mix consisting from x components: Msm = M1N1 + M2N2 + M3N3 + … + MxNx, where M – the molar mass of a component, and N – its mass fraction (percentage concentration).
3. You learn the molar mass of gases, having remembered sizes of atomic scales of elements (here you need Mendeleyev's Table). Their mass fractions are known under the terms of a task. Having substituted sizes in a formula and having made calculations, receive: 2*0.05 + 28*0.15 + 44*0.40 + 32*0.35 + 71*0.05 = 36.56 grams/mol. Here such is the molar mass of the specified mix.
4. Whether it is possible to solve a problem in a different way? Yes, sure. Let's assume, you have just the same mix concluded in a tight vessel of V at the room temperature. How it is possible to calculate in the laboratory way its molar weight? For this purpose you need to weigh at first this vessel on exact scales. Designate its weight as M.
5. Then by means of the connected manometer measure P pressure in a vessel. Then by means of the hose connected to the vacuum pump pump out a little mix. It is easily possible to understand that pressure in a vessel will decrease. Having blocked the gate, wait about half an hour in order that mix in a vessel accepted ambient temperature again. Having checked it by means of the thermometer, measure mix pressure by the manometer. Designate it by P1. Weigh a vessel, designate the new weight as M1.
6. And further remember the universal equation of Mendeleyev-Klapeyrona. According to it, in both cases: - PV = MRT/m; - P1V = M1RT/m. Having a little altered this equation, you receive: - m = MRT/PV; - m = M1RT/P1V.
7. From this it follows that m = (M – M1)RT/(P – P1)V. And m is that molar mass of mix of gases which you should learn. Having substituted the known sizes in a formula, you receive the answer.