What makes Coke and other soft drinks acidic? These drinks are carbonated, so a freshly-opened can should have a lot of dissolved carbonic acid, but the ingredients also lists phosphoric acid. Would flat Coke be significantly less acidic than carbonated Coke?
If you look at the list of ingredients, you will find that Coca-Cola and many other brands contain phosphoric acid (food grade - don't worry). This is probably the dominant cause of acidity. Citric acid is another common component. Of course, the carbon dioxide is also acidic, but weakly, so there should be very little difference between carbonated and flat.
Flat coke is less acidic than carbonated coke. I did a lab report on this topic, and found that Carbonated coke had a concentration of around 0.131M whereas flat coke had a concentration of 0.064M. I guess we can assume that the carbonic acid that's dissolved in Coke makes up a large proportion of the acidity.