Question

calculate the empirical and molecular formula of the compounds having following percentage composition
Na=36.5%, H=0.8% , P=24.6% , O=38.1%
The molecular mass of the compounds is 126 amu. Also name the compounds​

Answer 1

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Answer 2

The empirical formula of the compound is  Na2HPO3

The molecular formula of the compound is  Na2HPO3.

The name of the compound is Disodium hydrogen phosphite.

Given: Percentage of Sodium = 36.5 %,

          Percentage of Hydrogen = 0.8 %,

          Percentage of Phosphorus = 24.6 %,

          Percentage of Oxygen = 38.1 %,

          The molecular mass of the compound = 126.

To Find: The empirical and molecular formula and the name of the compound.

Solution:

• We need to find the moles of each of the constituents first and then divide it with the smallest molar quantity to get the empirical formula.

• Next, we need to find the formal mass and divide the total molecular mass with it to get n which shall give us our molecular formula.

The total molecular mass of the compound = 126.

Percentage of Sodium (Na) = 36.5

So, the number of moles of Sodium = 36.5 / 23

                                                           ≅ 1.58

Percentage of Hydrogen = 0.8

So, the number of moles of Hydrogen = 0.8 / 1

                                                                ≅ 0.8

Percentage of Phosphorus = 24.6

So, the number of moles of Phosphorus = 24.6 / 31

                                                            = 0.81

Percentage of Oxygen = 38.1

So, the number of moles of Oxygen = 38.1 / 16

                                                     = 2.38

Now, the smallest molecular quantity = 0.8

Now, we shall divide the number of moles of all the constituent elements with this smallest molecular quantity to get the empirical formula,

So, for Sodium, value = 1.58 / 0.8 ≅ 2

    For Hydrogen, value = 0.8 / 0.8 ≅ 1

    For Phosphorus, value = 0.81 / 0.8 ≅ 1

    For Oxygen, value = 2.38 / 0.8 ≅ 3

So, the empirical formula of the compound is Na2HPO3.

Formal mass of the compound = 23 × 2 + 1 × 1 + 31 + 16 × 3

                                                   = 126

So, the value of n = Total molecular mass / Formal mass

                      = 126 / 126

                      = 1

So, the molecular formula of the compound is  Na2HPO3.

Hence, the empirical formula of the compound is  Na2HPO3

The molecular formula of the compound is  Na2HPO3.

The name of the compound is Disodium hydrogen phosphite.

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