Exxon Mobil Corp (XOM.N) will pay $32.2 million to the United States to resolve claims that some of its units knowingly underpaid royalties owed on natural gas, the Justice Department said on Monday. The government alleged that Mobil Natural Gas Inc, Mobil Exploration & Producing U.S. Inc and affiliates underreported the value of natural gas taken from federal and American Indian leases between March 1, 1988 and Nov. 30, 1999.