when a hydrocarbon is completely burnt in oxygen ,11.0g of carbon 4 oxide and 4.5g of water are formed determine the molecular formula of the hydrocarbon if its relative molecular mass is 84 (C=12, O=16,H=1)
To determine the molecular formula of the hydrocarbon, we need to first find the empirical formula.
1. Find the moles of each element present.
Moles of carbon = 11.0g / 44.01 g/mol = 0.250 mol
Moles of hydrogen = (4.5g / 18.02 g/mol) * 2 = 0.500 mol (multiplied by 2 since there are 2 hydrogen atoms in each water molecule)
Moles of oxygen = (11.0g + 4.5g) / 32.00 g/mol = 0.46875 mol
2. Divide by the smallest number of moles to get the mole ratios.
Mole ratio of carbon : hydrogen : oxygen = 0.250 mol : 0.500 mol : 0.46875 mol
= 1 : 2 : 1.875 (divide by 0.250 mol)
3. Convert these ratios to the nearest whole number to get the empirical formula.
Empirical formula = CH2O
4. Determine the molecular formula using the relative molecular mass given.
The empirical formula has a mass of 30 (12+2+16), and the relative molecular mass is 84. Therefore, the molecular formula must be a multiple of (84/30) = 2.8.
Molecular formula = Empirical formula x 2.8
= C2.8H5.6O2.8
We need to round these numbers to the nearest whole number so that we can't have fractional subscripts in a chemical formula.
Molecular formula = C3H7O3 (rounded to nearest whole number)
Therefore, the molecular formula of the hydrocarbon is C3H7O3.