Question

On analysis a substance was found to have the following percentage composition K=31.84%, Cl=28=98% and O=39.18%. Calculate the molecular formula if its molecular mass is 122.5​

Answer 1

Explanation:

As always with these problems, we assume an 100⋅gmass of compounds, and then interrogate the molar quantities of the constituent atoms...

Moles of potassium≡31.84⋅g39.10⋅g⋅mol−1=0.814⋅mol.

Moles of chlorine≡28.98⋅g35.45⋅g⋅mol−1=0.817⋅mol.

Moles of oxygen≡39.18⋅g16.00⋅g⋅mol−1=2.45⋅mol.

We divide thru by the lowest molar quantity to give an empirical formula of KClO3 (or close enuff to!), i.e. potassium chlorate.

And the molecular formula is always a multiple of the empirical formula.....(39.1+35.45+3×16)n⋅g⋅mol−1=122,5⋅g⋅mol−1. Clearly, n=1, and the molecular formula is KClO3, potassium chlorate.

Answer 2

The molecular formula of the compound is  KClO3.

Given: Percentage of Potassium = 31.84 %,

         Percentage of Chlorine = 28.98 %

         Percentage of Oxygen = 39.18 %,

         The molecular mass of the compound = 122.5.

To Find: The molecular formula 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 = 122.5

Percentage of Potassium (K) = 31.84

So, the number of moles of Potassium = 31.84 / 39

                                                          ≅ 0.82

Percentage of Chlorine (Cl) = 28.98

So, the number of moles of Chlorine = 28.98 / 35.5

                                                               ≅ 0.82

Percentage of Oxygen (O) = 39.18

So, the number of moles of Oxygen = 39.18 / 16

                                                    = 2.45

Now, the smallest molecular quantity = 0.82

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 Potassium, value = 0.82 / 0.82 ≅ 1

   For Chlorine, value = 0.82 / 0.82 ≅ 1

   For Oxygen, value = 2.45 / 0.82 ≅ 3

So, the empirical formula of the compound is KClO3.

Formal mass of the compound = 39 + 35.5 + 16 × 3

                                                  = 122.5

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

                     = 122.5 / 122.5

                     = 1

Hence, the molecular formula of the compound is  KClO3.

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