Answer
Molecular formula of oxalic acid is $C_{2}H_{2}O_{4}$ .
Work Step by Step
Strategy: 1- Find the empirical formula mass.
2- by dividing molecular mass with empirical formula mass, we can find the number that empirical formula mass must be multiplied to find the molecular formula.
To find the empirical formula mass:
26.7% C, 2.2% H, and 71.1% O. 100 g oxalic acid contains: 26.7 g C, 2.2g H, and 71.1g O. These amount converted to moles are:
26.7 g C x $\frac{1 mol (C) }{12g (C)}$= 2.2 mol C
2.2 g H x $\frac{1 mol (H) }{1g (H)}$ = 2.2 mol H
71.1 g O x $\frac{1 mol (O) }{16g (O)}$ = 4.4 mol O
Now we divide with the smallest number of moles, that is 2.2 and we get: 1.0 for C, 1.0 for H, and 2 for O. So the empirical formula for oxalic acid is $CHO_{2}$.
Mass of empirical formula : 1 x 12 + 1 x 1 + 2 x 16 = 45 amu.
2- We are given the molecular mass of 90 amu, so we can find the number empirical formula mass must be multiplied to find the molecular mass:
n = $\frac{Molecular mass}{empirical (formula-mass)}$
n = $\frac{90}{45}$ = 2
So molecular formula should be $(CHO_{2})_2$ or molecular formula is $C_{2}H_{2}O_{4}$ .
