Instruction

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If you had to calculate the molar

**mass**of any gas, would you take the value of the atomic weight of nitrogen and multiplied it to the index 2. In the end, would get 28 grams/mol. But how to calculate the molar**mass****of the mixture***of gases*? This problem is solved elementary. We only need to know what gases and in what proportion are part**of the mix**.2

Consider a specific example. Suppose you have a

*gas*th mixture, which consists of 5% (mass) of hydrogen, 15% nitrogen, 40% carbon dioxide, 35% oxygen and 5% chlorine. What is its molar mass? Use the formula for**the mixture**consisting of the x components: MSM = M1N1 + M2N2 + M3N3 +...+ MxNx, where M is the molar mass of the component, and N is its mass fraction (percentage concentration).3

Molar mass of

*gases*, you will learn the value of remembering the atomic weights of elements (here you will need a periodic table). Their mass fractions are known for this task. Substituting values into the formula and performing calculations, we obtain: 2*0,05 + 28*0,15 + 44*0,40 + 32*0,35 + 71*0,05 = 36,56 grams/mol. Here is the molar mass of the specified**mixture**.4

Can you solve the problem in a different way? Yes, of course. Suppose you have the exact same mixture contained in a sealed vessel of volume V at room temperature. As a laboratory to calculate its molar

**mass**? For this you will need to first weigh the vessel on an accurate scale. Denote its**mass**as M.5

Then with the help of a connected manometer, measure the pressure P inside the vessel. Then use a hose connected to a vacuum pump, siphon a little

**of the mixture**. It is easy to understand that the pressure inside the vessel will decrease. Cutting off the valve, wait about half an hour in order for the mixture inside the vessel again took the temperature of the surrounding air. Checking it with a thermometer, measure the pressure**of the mixture**pressure gauge. Label it P1. Weigh the vessel, mark the new**mass**as M1.6

Well, then remember the universal equation Mendeleev-Clapeyron. According to him, in both cases:- PV = MRT/m;- P1V = M1RT/m.Slightly modifying this equation, you get:- m = MRT/PV;- m = M1RT/P1V.

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It follows that m = (M – M1)RT/ (P – P1)And V. m – the same molar mass

**of the mixture***of gases*that you need to know. Substituting in the formula the known values, you will receive a response.# Advice 2: How to calculate the molar mass of the substance

Molar mass

**of a substance**is the mass of one mole, i.e. a quantity which is 6,022*10^23 elementary particles – atoms, ions, or molecules. Its unit of measurement is grams/mol.Instruction

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To calculate molar

**mass**, you'll need only a periodic table, basic chemistry knowledge and ability to calculate, of course. For example, a widely known substance – sulfuric acid. So it finds wide application in various industries that rightfully bears the name "blood chemistry." What is its molecular mass?2

Write the exact formula of sulfuric acid: H2SO4. Now take the periodic table and see what are the atomic masses of all elements included in its composition. These three elements – hydrogen, sulfur and oxygen. Atomic mass of hydrogen is 1, sulphur – 32, oxygen – 16. Therefore, the total molecular weight of sulfuric acid, including indices equal to: 1*2 + 32 + 16*4 = 98 .e.m (atomic mass units).

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Now let's recall another definition of the mole: is the amount of

**substance**whose mass in grams is numerically equal to its mass, expressed in atomic units. Thus, it appears that 1 mole of sulfuric acid weighs 98 grams. Here is its molar mass. The problem is solved.4

For example, suppose you specify the following conditions: there are 800 milliliters of a 0.2 molar solution (0.2 m), some salt, and it is known that dry this salt weighs 25 grams. You want to calculate its molar

**mass**.5

First recall the definition of a 1-molar (1M) solution. This solution, in 1 liter of which contains 1 mol of any

**substance**. Respectively, in 1 liter of 0.2 M solution would contain of 0.2 mole**of the substance**. But not 1 liter, and 0.8 liter. Therefore actually you have 0,8*0,2 = 0,16 mole**of the substance**.6

And then everything becomes easier. If 25 grams of salt according to the conditions of the problem are to 0.16 mole, what is the number equal to one pray? Performing the calculation in one step, you will find: 25/0,16 = 156,25 grams. Molar mass of salt is 156,25 grams/mol. The problem is solved.

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In the calculations you used the rounded value of the atomic weights of hydrogen, sulfur and oxygen. If you want to perform calculations with great precision, rounding is unacceptable.