How Do I Know The Molar Mass | The Science

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# How do I know the molar mass

Mass of 1 mole of a substance is called its molar mass and is denoted by the letter M. Units molecular weight - g / mol. The calculation of this value depends on the given conditions.

### You will need:

- periodic system of chemical elements DI Mendeleev (periodic table); - Calculator.

## Instruction how to find the molar mass

Step 1:

If we know the chemical formula of the substance, its molar mass can be calculated by means of the periodic table. The molar mass of a substance (M) is equal to its relative molecular mass (Mr). In order to calculate it, get in the periodic table the atomic masses of all the elements that make up the substance (Ar). Usually the number written in the lower right hand corner of the cell element by its serial number. For example, the atomic mass of hydrogen is 1 - Ar (H) = 1, the atomic mass of oxygen is 16 - Ar (O) = 16 atomic mass of sulfur is 32 - Ar (S) = 32.

Step 2:

To learn the molecular and molar mass of a substance, it is necessary to lay down the relative atomic masses of its constituent elements taking into account the number of atoms. Mr = Ar1n1 + Ar2n2 + ... + Arxnx. Thus, the molar mass of water (H2O) is the sum of the atomic weight of hydrogen (H), and 2 times the mass of atomic oxygen (O). M (H2O) = Ar (H)? 2 + Ar (O) = 1? 16 + 2 = 18 (g / mol). Molar mass of sulfuric acid (H2SO4) is the sum of the atomic weight of hydrogen (H) multiplied by 2, the atomic weight of sulfur (S) and the atomic mass of oxygen (O), multiplied by 4. M (H2SO4) = Ar (H)? 2 + Ar (S) + Ar (O)? 4 = 1? 32 + 2 + 16 × 4 = 98 (g / mol). The molar mass of simple substances, consisting of a single element, it is believed the same. For example, the molar mass of oxygen gas (O2) is equal to the atomic weight of the element oxygen (O), multiplied by 2 M (O2) = 16? 2 = 32 (g / mol).

Step 3:

If the chemical formula of the substance is unknown, but it is known and the amount of mass, molar mass can be found by the formula: M = m / n, where M - molar mass, m - mass of a substance, n - amount of the substance. For example, it is known that 2 mol of substance has a mass of 36 g, then it is the molar mass M = m / n = 36 r? 2 mole = 18 g / mol (most likely it is water H2O). If 1.5 mol of the substance has a mass of 147 g, then it is the molar mass M = m / n = 147g? 1.5 mol = 98 g / mol (most likely it is sulfuric acid H2SO4).