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$\ce{H3PO4}$ can donate 3 protons on the other hand $\ce{HNO3}$ can donate 1 proton. So that makes $\ce{H3PO4}$ stronger isn't it? But my book says $\ce{HNO3}$ is stronger without any explanation.

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    $\begingroup$ Think of it as the difference between throwing several small pebbles and one single cannonball. If you need to break down a wall, probably only the latter will do. $\endgroup$ – Nicolau Saker Neto Jul 20 at 10:14
  • $\begingroup$ Acid strength is a measure of how readily the first proton is donated. It is more readily donated by nitric acid than phosphoric. $\endgroup$ – Waylander Jul 20 at 10:26
  • $\begingroup$ Ok so nitrogen donates proton easily.Sir, can you please describe why the donation for nitric acid is easier than phosphoric acid? $\endgroup$ – Ghost Jul 20 at 11:51
  • $\begingroup$ Nitric acid pKa = −1.4, phosphoric acid pKa1 = 2.15; pKa2 = 7.09; pKa3 = 12.32 // In chemistry at first focus on the facts, the behavior of the chemicals. Ask why questions later. A lot of the why explanations in freshman chemistry are over simplifications. $\endgroup$ – MaxW Jul 20 at 13:10
  • $\begingroup$ chemistry.stackexchange.com/questions/96618/… $\endgroup$ – Mithoron Jul 20 at 17:27
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Draw the ions of respective acids. See which one becomes more stable. Nitric acid $\ce{N}$ is in a bad state with a positive charge. it's a highly eletronagative atom near oxygen becoming negative will become more stable. See also what happens with $\ce{H3PO4}$. $\ce{P}$ is not eletronegative as $\ce{N}$ so it doesn't care much as $\ce{N}$ for $\ce{O}$ becoming negative. See you will then understand the more stable ion of the respective acid want to get rid of the $\ce{H}$ atom badly meaning more acidic. Draw the ions you will understand then.

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