IRS Pub. 501 contains the rules you are looking for ...
Publication 501.
In Pub. 501, look under "Filing Status ... Head of Household" ... Table 4 ("Who Is a Qualifying Person Qualifying You To File as Head of Household?").
*Basically, if you can claim your mother as an exemption, she is a Qualifying Person for the Head of Household analysis.
Look under "Exemptions" ... Table 5 ... "Qualifying Relative"
*Basically, the rules for whether your mother is a "Qualifying Relative" are these:
[i]1. The person cannot be your qualifying child or the qualifying child of anyone else.
2. The person either: [list](a) must be related to you in one of the ways listed under Relatives who do not have to live with you, or
(b) must live with you all year as a member of your household (and your relationship must not violate local law).
3. The person's gross income for the year must be less than $3,300.
4. You must provide more than half of the person's total support for the year.[/i] (From Table 5)
[/list]
The key question seems to be how much income does your mother get from SS?