Sulfonic acids are strong acids. They are commonly cited as being around a million times stronger than the corresponding carboxylic acid. For example, "p"-Toluenesulfonic acid and methanesulfonic acid have p"K"a values of −2.8 and −1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively. However, as a consequence of their strong acidity, their p"K"a values cannot be measured directly, and values commonly quoted should be regarded as indirect estimates with significant uncertainties. For instance, various sources have reported the p"K"a of methanesulfonic acid to be as high as −0.6 or as low as −6.5. Sulfonic acids are known to react with solid sodium chloride (salt) to form the sodium sulfonate and hydrogen chloride. This property implies an acidity within two or three orders of magnitude of that of HCl(g), whose p"K"a was recently accurately determined (p"K"aaq = −5.9).