You will need

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

Instruction

1

Relative molecular mass is a dimensionless value that indicates the mass of a molecule relative to 1/12 mass of carbon atom, measured in relative atomic units.

2

Example 1: Determine the relative molecular mass of CO2. One molecule of carbon dioxide consists of one atom of carbon and two atoms of oxygen. Look in the periodic table the atomic masses for these elements and write them down, rounding to the nearest whole number: Ar(C) = 12; Ar(O) = 16.

3

Calculate the relative mass of a molecule of CO2, adding the masses of the atoms composing it: Mg(CO2) = 12 + 2*16 = 44.

4

Example 2. How to Express the mass of one gas molecule in grams consider the example of carbon dioxide. Take 1 mol of CO2. Molar mass is numerically equal to the molecular CO2: M(CO2) = 44 g/mol. One mole of any substance contains of 6.02*10^23 molecules. This number is called the Avogadro constant and is denoted by Na. Find the mass of one molecule of carbon dioxide m(CO2) = M(CO2)/Na = 44/6,02*10^23 = 7,31*10^(-23) grams.

5

Example 3. You are given a gas with a density of 1.34 g/L. you want to find the mass of one molecule of gas. According to the law of Avogadro normal conditions one mole of any gas occupies a volume of 22.4 liters. Determine the mass of 22.4 liters, you will find the molar mass of gas: Mg = 22,4*1,34 = 30 g/mol

Now, knowing the mass of one mole, calculate the mass of one molecule analogously to example 2: m = 30/6,02*10^23 = 5*10^(-23) grams.

Now, knowing the mass of one mole, calculate the mass of one molecule analogously to example 2: m = 30/6,02*10^23 = 5*10^(-23) grams.

# Advice 2: 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.