How to count number of moles

How to count number of moles

The mol is such amount of substance in which is 6.022*10^23 elementary particles (molecules, atoms, or ions). The mentioned size carries the name "Avogadro's number" - by name the famous Italian scientist. Weight asking any substance, expressed in grams, it is in number equal to the mass of its molecule in atomic units. How it is possible to count molal quantity?


1. For example, the task is set: to define how many moths contain in 150 grams of sodium saltpeter (that is sodium nitrate). First of all, write a formula of this substance – NaNO3.

2. Determine its molecular weight, knowing atomic masses of elements and taking into account index 3 at oxygen. It will turn out: 14+23+48=85 and. e. m (atomic units of mass). Therefore, one mol of nitrate sodium will make 85 grams. And you have 150 grams of substance. Thus, you find: 150/85=1.765 asking. The task is solved.

3. And if, for example, such conditions are set: there are 180 liters of oxygen at the room temperature and atmospheric pressure. How many will it be moths? And there is nothing difficult. It is only necessary to remember that 1 mol of any gas under normal conditions occupies volume, approximately equal 22.4 liters. Having divided 180 into 22.4, receive required size: 180/22.4=8.036 asking.

4. Let's assume, temperature significantly exceeded room, and pressure much more surpassed atmospheric. At the same time the mass of oxygen, and volume of a vessel in which he was imprisoned is known to you. How to find the number of moles of gas in that case?

5. Here you will be come to the rescue by the universal equation of Mendeleyev-Klapeyrona. However, it is removed for the description of conditions of ideal gas which oxygen, of course, is not. But it can be used in calculations: the error will be very insignificant. PVm = MRT, where P – gas pressure in pascals, V – its volume in cubic meters, m – molar weight, M – weight in grams, R – a universal gas constant, T – temperature in degrees Kelvin.

6. It is easily possible to see that M/m = PV/RT. And the size M/m just is also the number of moles of gas under existing conditions. Having substituted the known sizes in a formula, you receive the answer.

7. And if you deal with alloy? How then to calculate the number of moles of each component in a sample? For the solution of such task, it is necessary to know the mass of a sample and exact composition of alloy. For example: widespread cupronickel – copper alloy with nickel. Let's assume, you have a German silver product weighing 1500 grams containing 55% of copper and 45% of nickel. Decision: 1500*0.55=825 grams of copper. That is, 825/63.5=13 moths of copper. Respectively, 1500–825=675 grams of nickel. 675/58.7 = 11.5 moths of nickel. The task is solved

Author: «MirrorInfo» Dream Team