What is the chemical formula of citric acids? What happens when they enter our stomach(does it convert our stomach's HCl acid into a base?)?(Please give an explanation with chemical reactions)

Citric acid is a single organic compound that is a weak acid, it is found in all citrus fruits like lemons, oranges, limes, grapefruit, etc.

You can use pubchem to get information about its chemical properties and those of other chemicals. The molecular formula of citric acid is C6H8O7.

Link for citric acid: http://pubchem.ncbi.nlm.nih.gov/compound/citric_acid

Citric acid would not react with HCl (Hydrochloric acid) which is a strong inorganic acid. Instead it will be absorbed into the body in the intestines.

What do you mean by changing HCl into a base?