With singular nouns the rule is to add an apostrophe plus an 's'.
the mother's bad temper
the dog's bone
Singular nouns ending in 's'
In the past, with singular nouns that end in 's', an apostrophe and an 's' was always added. Today it is acceptable to have only the apostrophe after the 's'.
The class's textbooks or The class' textbooks
Peter Davis's CDs or Peter Davis' CDs
With plural nouns only an apostrophe is required.
two weeks' pay
the girls' books