Final answer:
The claim about Mexican gun ownership is false, as Mexico has strict gun laws and lower rates of gun ownership. The factual statement about the Treaty of Guadalupe Hidalgo resulting in Mexico losing more than half its territory is true.
Step-by-step explanation:
The statement that as of 2012, Mexicans own about 15,000,000 guns, or about 13.5 guns per 100 people, is false. The ownership of firearms in Mexico is substantially lower than in some other countries, such as the United States. Mexico has strict gun laws, and there is only one legal gun store in the country, run by the military. Gun ownership rates are lower as a result. This fact was not provided in the reference material, but it is widely known.
According to the provided information, as a result of the Treaty of Guadalupe Hidalgo, Mexico lost more than half of its territorial land mass. This statement is true. The Treaty resulted in Mexico ceding a significant portion of its territory, including parts of what are now the states of California, Nevada, Utah, Arizona, New Mexico, Colorado, Wyoming, and Texas, to the United States in 1848. This had a profound impact on the demographics and political landscape of both nations.