Number 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. Given that the hydrocarbon is 85.8% carbon, this means that in one mole of the hydrocarbon (which we know has a mass of 84 g), 85.8% of this mass is due to carbon. So, the mass of carbon in one mole of the hydrocarbon is 0.858 * 84 g = 72.072 g.

Step 2: Calculate the number of moles of carbon in one mole of the hydrocarbon. The molar mass of carbon is 12 g/mol. So, the number of moles of carbon in one mole of the hydrocarbon is 72.072 g / 12 g/mol = 6 moles.

Step 3: Determine the number of moles of hydrogen in one mole of the hydrocarbon. We know that the total molar mass of the hydrocarbon is 84 g/mol, and we've just calculated that 72.072 g of this is due to carbon. Therefore, the remaining mass must be due to hydrogen. So, the mass of hydrogen in one mole of the hydrocarbon is 84 g - 72.072 g = 11.928 g.

Step 4: Calculate the number of moles of hydrogen in one mole of the hydrocarbon. The molar mass of hydrogen is 1 g/mol. So, the number of moles of hydrogen in one mole of the hydrocarbon is 11.928 g / 1 g/mol = 11.928 moles.

Step 5: Determine the number of hydrogen atoms in one mole of the hydrocarbon. We know that one mole of any substance contains Avogadro's number (6.022 x 10^23) of entities (in this case, atoms). So, the number of hydrogen atoms in one mole of the hydrocarbon is 11.928 moles * 6.022 x 10^23 atoms/mole = 7.18 x 10^24 atoms.

So, the number of hydrogen atoms per molecule of the hydrocarbon is 7.18 x 10^24 atoms.