Question

Analysis of vitamin C shows that it contains 40.92% carbon by mass, 4.58% hydrogen and 54.50% oxygen. Determine the empirical formula of vitamin C.

Answer 1

Hint: The empirical formulae is the ratio between the numbers of atoms of different elements which is present in the molecule of the given compound. Empirical formulas usually give the proportion of the elements present in the molecule of the given compound but nor the actual number of the atoms or how the atoms are arranged.

Complete step by step answer:
If mass of compound is given then empirical formula is calculated by changing the mass of every element to moles and then by dividing every mole value by the lowest value of mole computed and then the value of mole is rounded off to the closest whole number.

Given in the question:
100% of vitamin C contains 40.92% carbon by mass, 4.58% hydrogen and 54.50% oxygen
Or we can say that 100 g of vitamin C contains 40.92 g carbon by mass, 4.58 g hydrogen and 54.50 g oxygen.
The atomic mass of carbon = 12 $g/mol$
The atomic mass of hydrogen = 1 $g/mol$
The atomic mass of oxygen = 16 $g/mol$
Number of moles is calculated by the ratio of given mass to the molar mass.
Number of moles of carbon = $\dfrac{40.92}{12}$ = 3.4 mole
Number of moles of hydrogen = $\frac{4.58}{1}$ = 4.58 mole
Number of moles of oxygen = $\dfrac{54.50}{16}$= 3.40 mole
The molar ratio of carbon, hydrogen and oxygen is:
\[C:H:O=3.4:4.58:3.4=3:4:3\]
So, the correct answer is “Option C”.

Note: If the molecular formula of a compound is given then to calculate empirical formula from molecular formula we have to divide all the subscripts in the empirical formula by the simplest whole number. Molecular formula is the actual whole number ratio of the elements present in the molecule of the compound.