Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mūtābĭlĭtas, ātis, f. [mutabilis], changeabieness, mutability (class.; syn.: mobilitas, inconstantia, levitas): mentis, * Cic. Tusc. 4, 35, 76; Mart. Cap. 8, § 871.