You will need

- - the periodic system of chemical elements D. I. Mendeleev;
- calculator.

Instruction

1

In order to cope with the task, you need to use a formula to determine the relative density:

D (air) = Mr (gas)/ Mr (air) where:

D (air) – relative density;

Mr (gas) – relative molecular mass of a gaseous substance;

Mr (air) - relative molecular mass of air.

All three parameters units of measure do not have.

Mr (air) = 29 (a constant), hence the formula would be:

D (air) = Mr (gas)/ 29.

D (air) = Mr (gas)/ Mr (air) where:

D (air) – relative density;

Mr (gas) – relative molecular mass of a gaseous substance;

Mr (air) - relative molecular mass of air.

All three parameters units of measure do not have.

Mr (air) = 29 (a constant), hence the formula would be:

D (air) = Mr (gas)/ 29.

2

Similar is the formula to determine the relative density of hydrogen, with the exception that instead of air is hydrogen. And, therefore, no account is taken of the relative molecular mass of hydrogen.

D (hydrogen) = Mr (gas)/ Mr (of hydrogen);

D (hydrogen) is the relative density;

Mr (gas) – relative molecular mass of a gaseous substance;

Mr (hydrogen) is the relative molecular mass of hydrogen.

Mr (hydrogen) = 2, hence the formula would be:

D (air) = Mr (gas)/ 2.

D (hydrogen) = Mr (gas)/ Mr (of hydrogen);

D (hydrogen) is the relative density;

Mr (gas) – relative molecular mass of a gaseous substance;

Mr (hydrogen) is the relative molecular mass of hydrogen.

Mr (hydrogen) = 2, hence the formula would be:

D (air) = Mr (gas)/ 2.

3

Example No. 1. Calculate the relative density of ammonia in the air. Ammonia has the formula NH3.

First, find the relative molecular mass of ammonia, which can be calculated according to the table of D. I. Mendeleev.

Ar (N) = 14 Ar (H) = 3 x 1 = 3, hence

Mr (NH3) = 14 + 3 = 17

Substitute the data obtained in the formula for determination of relative density of air:

D (air) = Mr (ammonia)/ Mr (air);

D (air) = Mr (ammonia)/ 29;

D (air) = 17/ 29 = 0, 59.

First, find the relative molecular mass of ammonia, which can be calculated according to the table of D. I. Mendeleev.

Ar (N) = 14 Ar (H) = 3 x 1 = 3, hence

Mr (NH3) = 14 + 3 = 17

Substitute the data obtained in the formula for determination of relative density of air:

D (air) = Mr (ammonia)/ Mr (air);

D (air) = Mr (ammonia)/ 29;

D (air) = 17/ 29 = 0, 59.

4

Example No. 2. Calculate the relative density of ammonia by hydrogen.

Substitute the numbers into the formula to determine the relative density of hydrogen:

D (hydrogen) = Mr (ammonia)/ Mr (of hydrogen);

D (hydrogen) = Mr (ammonia)/ 2;

D (hydrogen) = 17/ 2 = 8, 5.

Substitute the numbers into the formula to determine the relative density of hydrogen:

D (hydrogen) = Mr (ammonia)/ Mr (of hydrogen);

D (hydrogen) = Mr (ammonia)/ 2;

D (hydrogen) = 17/ 2 = 8, 5.

# Advice 2: How to determine the density of hydrogen

Hydrogen (from the Latin for "Hydrogenium" - "feed water") is the first element of the periodic table. Widely distributed, exists in three isotopes – protium, deuterium and tritium. Hydrogen represents a light colorless gas (14.5 times lighter than air). In mixtures with air and oxygen is highly explosive. Used in chemical, food industry, and also as rocket fuel. Research is being conducted on the possibility of using

**hydrogen**as fuel for automotive engines.**The density****of hydrogen**(or any gas) can be defined in different ways.Instruction

1

First, on the basis of a universal definition of density – the amount of substance per unit volume. In that case, if pure hydrogen is in a sealed vessel, the gas density is determined by the elementary formula (M1 – M2)/V, where M1 is the total mass of the vessel with the gas, M2 – mass of empty vessel, and V is the internal volume of the vessel.

2

If you want to determine the density

P – gas pressure

V is the volume

R – the universal gas constant

T is the gas temperature in degrees Kelvin

M – molar mass of gas

m – the actual weight of gas.

**of hydrogen**, with such initial data, as its temperature and pressure, then comes to the aid of the universal equation of state ideal gas law or the equation Mendeleev – Clapeyron: PV = mRT/M.P – gas pressure

V is the volume

R – the universal gas constant

T is the gas temperature in degrees Kelvin

M – molar mass of gas

m – the actual weight of gas.

3

An ideal gas is a mathematical model of gas in which the potential energy of interaction between molecules compared to their kinetic energy can be neglected. In the model of an ideal gas between the molecules are forces of attraction or repulsion and collision of the particles with other particles or vessel walls are absolutely elastic.

4

Of course, neither hydrogen nor another gas is not ideal, but this model allows calculating with high accuracy at conditions close to atmospheric pressure and room temperature. For example, given the task: find the density

**of hydrogen**at a pressure of 6 atmospheres and a temperature of 20 degrees Celsius.5

First, set all the initial value SI (6 atmospheres = 607950 PA, 20 degrees C=293 degrees K). Then write the equation Mendeleev-Clapeyron law PV = mRT/M. Convert it in the form: P = mRT/MV. Since m/V is density (the ratio of the mass of a substance to its volume), you get: the density

**of hydrogen**= PM/RT, where all the necessary data for the solution we have. You know the amount of pressure (607950), temperature (293), universal gas constant (8,31), the molar mass**of hydrogen**(0,002).6

Substituting these numbers into the formula you get: the density

**of hydrogen**under specified conditions of pressure and temperature is 0.499 kg/cubic meter, or about 0.5.# Advice 3: How to calculate density

**Density**is one of the characteristics of matter, such as mass, volume, temperature, area. It is the ratio of mass to volume. The main task is to learn how to calculate this value and know what it depends on.

Instruction

1

**The density**of a substance is numerically the ratio of mass to volume of a substance. If you want to determine the density of a substance and you know its mass and volume, finding the density will not make you work. The easiest way to find the density in this case is p = m/V. It is measured in kg/m^3 in SI units. However, not always a given these two values, so you should know some ways by which you can calculate the density.

2

**The density**has different values depending on the type of substance. In addition, the density changes from salinity and temperature. With decreasing temperature, the density increases, and with the lowering of the degree of salinity decreases and the density. For example, the density of the red sea is still considered high, and in the Baltic sea it is already less. All of you have noticed that if the water to fuel, it POPs up. All this is due to the fact that oil has a lower density than water. Metals and stone substances, on the contrary sink because their density is higher. On the basis of the density of bodies there was a theory about their swimming.

3

Thanks to the theory of the swimming of bodies there was a formula that I can find the density of a body, knowing the density of water, the volume of the body and the volume of its immersed part. This formula has the form:Vпогруж. part / V of the body = p / p liq.It follows that the density of the body can be found as follows:R body = V submersible. parts * fluid R / V body.This condition is satisfied on the basis of the tabular data and the given volume V submersible. part V of the body.

# Advice 4: How to calculate the relative molecular mass of the substance

Relative molecular mass is a dimensionless quantity indicating how many times the mass of molecules more than 1/12 the mass of an atom of carbon. Accordingly, the mass of a carbon atom is 12 units. To determine the relative molecular mass of chemical compounds can be folded mass of atoms that comprise a molecule.

You will need

- - handle;
- paper for records;
- calculator;
- - the periodic table.

Instruction

1

Write the chemical formula of the compound, relative molecular mass which you want to calculate. For example, phosphoric acid H3PO4. From the formula you can see that the molecule of the acid has three hydrogen atoms, one phosphorus atom and four oxygen atoms.

2

Look in the periodic table cell elements that comprise this molecule. The values of relative atomic masses (Ar) for each substance listed in the lower left corner of the cell. Rewrite them, rounding to the nearest whole number: Ar(H) – 1; Ar( P) – 31; Ar(O) is 16.

3

Determine the relative molecular mass of compound (Mr). To do this, multiply the atomic mass of each element by the number of atoms in the molecule. Then add the resulting values. For phosphoric acid: Mr(н3ро4) = 3*1 + 1*31 + 4*16 = 98.

4

Relative molecular mass is numerically equal with the molar mass of the substance. Some tasks use this connection. Example: a gas at a temperature of 200 K and a pressure of 0.2 MPa has a density of 5.3 kg/MZ. To determine its relative molecular mass.

5

Use the equation Mendeleev-clayperon for an ideal gas: PV = mRT/M, where V is gas volume, m3; m is the mass of the ate of gas, kg; M is the molar mass of gas, kg/mol; R – universal gas constant. R=8.314472 м2кг-2 K-1 Mol-1; T – gas temperature, K; P is the absolute pressure, PA. We Express this dependence of the molar mass: M = mRT/(PV).

6

As you know, the density formula p = m/V, kg/m3. Substitute it into the expression: M = рRT/P. Determine the molar mass of gas: M = 5,3*8,31*200/(2*10^5) = 0,044 kg/mol. Relative molecular mass of gas: Mr = 44. You can assume that it is carbon dioxide: Mr(CO2) = 12 + 16*2 = 44.