The empirical formula is
 C5H11
Explanation:
We have decane, the alkane with a chemical formula of  
C10H22
 .
The empirical formula of a substance has the smallest ratio between the elements its composed of.
Here, we got the carbon to hydrogen ratio as  
10:22
 . So, we need to find the greatest common factor between the two numbers.
10=2*5
22=2*11
 As you can see here, the greatest common factor between the two numbers is  
2
 , and so we divide both numbers by  
2
 .
10/2=5
22/2=11
And so, the new formula becomes  
C5H11