Your stomach acid, vinegar and lemons are all quite acidic. Stomach acid is a combination of compounds, not just one type of acidic compound: hydrochloric acid, potassium chloride and sodium chloride are combined together in the stomach and then depending on the amount of food in your stomach, make the acidity of your stomach from about 1.5-3.5. Vinegar and lemon juice each have their own compound that makes them acidic. Vinegar has acetic acid and can have a range of pHs depending on the type of vinegar, but regular white vinegar is around 2.2. Lemons have citric acid that makes them acidic and their acidity is around 2.