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 find molar concentration

In the school course of chemistry is found for a term such as molar concentration. It is present in chemistry textbooks intended for University students. To know what is molar mass and how to calculate it, you need as pupils and students who want to just successfully pass a chemistry exam, and those who decided to choose this science as their future profession.

Instruction

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In experiments in analytical chemistry are very often carried out the sampling. In each of the analyses, among other parameters, determine the number of taken substances. In most problems of analytical chemistry have to deal with concepts such as the mole, amount of substance, molar mass and concentration. Chemical concentrations expressed in several ways. There are molar mass and volume concentration.Molar concentration is the ratio of the amount of a substance to the volume of the solution. This concept is found in the course of chemistry on 10 and 11 class. It is expressed as a formula:c (X) = n(X) /V, where n (X) is the number of solute X; V - volume of the solution.Most often, the calculation of the molar concentration is made in respect of solutions, because solutions are composed of water and solute

**concentration**to be determined. The unit of molar concentration is mol/L.2

Knowing the formula of molar concentration, it is possible to prepare a solution. If we know the molar concentration, to obtain the solution of the following formula is used:Cb =mb/Mb * VpПо this formula calculate the mass mb of the substance, and Vp does not change (Vp =const). Then a substance of a certain mass is slowly mixed with water and get the solution.

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In analytical chemistry the solution of tasks on the solutions, the molar concentration and mass fraction of a substance are interrelated with each other. The mass fraction wb of a solute is the ratio of its mass to the mass mb of the solution mp:wb =mb/mp, where mp =mb+H2O (solution consists of water and a solute)Molar concentration is equal to the product of the mass fraction on the density of the solution divided by the molar mass:сb = wb Pp-pa/ Mb

# Advice 3: How to find the molar mass of air

Molar mass is the mass of one mole of a substance, i.e. the value showing what quantity of a substance contains 6,022*10 (in degree 23) of particles (atoms, molecules, ions). But if we are not talking about pure substance and mixtures of substances? For instance, vital to people the air, because it represent a mixture of a great many of gases. How to calculate its molar mass?

You will need

- - accurate laboratory scales;
- - round-bottomed flask with a ground joint and stopcock;
- - vacuum pump;
- - pressure gauge with two valves and connecting hoses;
- thermometer.

Instruction

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First of all, consider the admissible computational error. If you don't need high accuracy, limit yourself to only the three most significant components: nitrogen, oxygen and argon, and take a "rounded" value of their concentrations. If you need more accurate result, use in calculations and carbon dioxide and can do without rounding.

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Suppose you want the first option. Write the molecular masses of these components and their mass concentration in the air:

- nitrogen (N2). Molecular mass 28, mass concentration of 75,50%;

- oxygen (O2). Molecular mass 32, the mass concentration of 23.15%;

- argon (Ar). Molecular mass 40, mass concentration of 1.29%.

- nitrogen (N2). Molecular mass 28, mass concentration of 75,50%;

- oxygen (O2). Molecular mass 32, the mass concentration of 23.15%;

- argon (Ar). Molecular mass 40, mass concentration of 1.29%.

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To facilitate calculations, rounded values of the concentrations:

- for nitrogen – up to 76%;

- for oxygen – up to 23%;

- for argon gas – to 1.3%.

- for nitrogen – up to 76%;

- for oxygen – up to 23%;

- for argon gas – to 1.3%.

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Make a simple calculation:

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.

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The obtained value is very close to that specified in the handbooks: of 28.98 g/mol. The discrepancy is due to rounding.

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Unable to determine the molar mass of air and with the help of simple laboratory experience. To do this, measure the mass of the flask with her air.

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Write down the result. Then, by connecting the hose of the flask to the pressure gauge, open the faucet and turned on the pump, begin to pump the air out of the flask.

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Wait for a while (so the air in the flask was heated to room temperature), note the reading of manometer and thermometer. Then, closing the valve on the bulb, disconnect the hose from the pressure gauge, and weigh the flask with the new (reduced) amount of air. Write down the result.

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Next you will come to the aid of the universal equation Mendeleev-Clapeyron:

PVm = MRT.

Write it down in a somewhat modified form:

∆PVm = ∆MRT, and you are aware of, and change of air pressure ∆P and the change of the air mass ∆M. the Molar mass of air m is calculated, is elementary: m = ∆MRT/∆PV.

PVm = MRT.

Write it down in a somewhat modified form:

∆PVm = ∆MRT, and you are aware of, and change of air pressure ∆P and the change of the air mass ∆M. the Molar mass of air m is calculated, is elementary: m = ∆MRT/∆PV.

Useful advice

The equation Mendeleev-Clapeyron describes the state of an ideal gas, which air is, of course, is not. But the values of pressure and temperature close to the normal, the error is so small that can be neglected.

# Advice 4: How to calculate molar concentration

Molar concentration is the value which indicates how many moles of the substance is 1-m liter of solution. For example, it is known that in a liter of solution is exactly of 58.5 grams of common salt – chloride of sodium. Molar because this substance is 58,5 g/mol, we can say that in this case you have odnokolernyh salt solution. (Or, in record, 1M solution).

You will need

- - table of solubility of substances.

Instruction

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The solution of this problem depends on the specific conditions. If you know the exact material mass and the exact volume of the solution, the solution is very simple. For example, 15 grams of barium chloride contains 400 milliliters of solution. What is its molar concentration?

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Start with what you remember the exact formula of the salt: BaCl2. On the periodic table, determine the atomic masses of the elements included in its composition. And, given the index 2 from chlorine, get molecular weight: 137 + 71 = 208. Therefore, the molar mass of barium chloride 208 g/mol.

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And in terms of the tasks, in the solution contains 15 grams of this substance. How much is it in moles? Dividing 15 by 208, will receive: approximately 0,072 mole.

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Now you need to take into account that the solution volume to 1 liter, and 0.4. 0,072 dividing by 0.4, we get the answer: 0,18. That is, you have approximately 0.18-molar solution of barium chloride.

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Let's build the solution. Suppose you began to dissolve in 100 milliliters of water at room temperature already mentioned, familiar to you, table salt is sodium chloride. Added you in small portions, stirring constantly and waiting until complete dissolution. And then came the moment when a small portion is not dissolved until the end, despite the intense mixing. You need to determine what is the molar concentration of the resulting solution.

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First of all, you need to find the table of solubility of substances. They are in the majority of chemical directories, you can also find this information on the Internet. You can easily determine that at room temperature the saturation limit (i.e., solubility limit) of sodium chloride is 31.6 grams/100 grams of water.

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For this task, you dissolved salt in 100 milliliters of water, but its density is almost equal 1. So we conclude: in the resulting solution contains approximately 31.6 grams of sodium chloride. Small undissolved excess, as well as some volume change upon dissolution of the salt can be neglected, the error will be small.

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Respectively, in 1 liter of solution would contain 10 times more salt – 316 grams. Given that the molar mass of sodium chloride, as mentioned in the beginning, is 58.5 g/mol, you will easily find the answer: 316/58,5 = a 5.4-molar solution.

# Advice 5: 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.