umber of hydrogen atoms per molecule of a hydrocarbon A having 85.8%85.8% carbon is (Given : Molar mass of A=84 g mol−1A=84 g mol-1 )

Step 1: Calculate the mass of carbon in one mole of the hydrocarbon A.

Given that the percentage of carbon in the hydrocarbon A is 85.8%, the mass of carbon in one mole of A is 85.8% of the molar mass of A.

So, mass of carbon = 85.8/100 * 84 g/mol = 72.072 g/mol

Step 2: Calculate the number of moles of carbon in one mole of the hydrocarbon A.

The molar mass of carbon is 12 g/mol. So, the number of moles of carbon in one mole of A is the mass of carbon divided by the molar mass of carbon.

So, moles of carbon = 72.072 g/mol / 12 g/mol = 6 moles

Step 3: Calculate the mass of hydrogen in one mole of the hydrocarbon A.

The molar mass of the hydrocarbon A is 84 g/mol. The mass of hydrogen is the molar mass of A minus the mass of carbon.

So, mass of hydrogen = 84 g/mol - 72.072 g/mol = 11.928 g/mol

Step 4: Calculate the number of moles of hydrogen in one mole of the hydrocarbon A.

The molar mass of hydrogen is 1 g/mol. So, the number of moles of hydrogen in one mole of A is the mass of hydrogen divided by the molar mass of hydrogen.

So, moles of hydrogen = 11.928 g/mol / 1 g/mol = 11.928 moles

Step 5: Calculate the number of hydrogen atoms in one molecule of the hydrocarbon A.

The number of hydrogen atoms in one molecule of A is the number of moles of hydrogen times Avogadro's number (6.022 x 10^23).

So, number of hydrogen atoms = 11.928 moles * 6.022 x 10^23 = 7.18 x 10^24 atoms

However, this is the number of hydrogen atoms in one mole of A. To find the number of hydrogen atoms in one molecule of A, we divide this number by Avogadro's number.

So, number of hydrogen atoms per molecule of A = 7.18 x 10^24 atoms / 6.022 x 10^23 = 12 atoms

Therefore, the number of hydrogen atoms per molecule of the hydrocarbon A is 12.