What is the molecular formula of oxalic acid if its molar mass is 90 g/mol? its percent composition is 26.7% c, 2.2% h, and 71.1% o?

First we have to find the empirical formula, empirical formula gives the simplest ratio of whole numbers of components in a compound.
We calculate the formula for 100 g of compound 
                                carbon            hydrogen            oxygen 
mass                         26.7 g               2.2 g               71.1 g
number of moles     26.7/12             2.2/1                 71.1/16 
                                = 2.22              = 2.2                  = 4.44
divide by least number of moles 
                                2.22/2.2 = 1.00  2.2/2.2 = 1       4.44/2.2 = 2.01
numbers rounded off are-
C - 1
H - 1
O - 2
empirical formula - CHO₂
mass of one empirical unit - 12 + 1 + (16x2) = 45
number of empirical units - 90/45 = 2
Therefore molecular formula - 2(CHO₂)
Molecular formula - C₂H₂O₄