![]() ![]() Using this equation, we find that if the pH of a solution is 7, then = 10 -7 M. For this, we need to invert the logarithm from the first equation, by raising 10 to the power of the negative pH. ![]() In some situations, we know the pH and need to convert to the molar concentration of H + ions. Now if the acid does not completely dissociate, and they give you the K a of the acid, you need to calculate pH from K a, which is covered in this article. Then use a calculator to plug the 0.2 M into the pH formula. The pH formula tells us that the pH is the negative log of the hydrogen ion concentration – which is 0.2 molar. So the concentration of hydrogen ions is 0.2 M. (HCl is a strong acid, so completely dissociates). This means for each mole of hydrochloric acid, there is 1 mole of H + ions. In this, knowing the hydrogen ion concentration is straight forward.įor example: Assume a 0.2 molar solution of HCl completely dissociates in solution. ![]() Sometimes, a problem will tell you that an acid completely dissociates into ions in solution. If you know the concentration of hydrogen ions, then calculating the pH is just plugging in to the pH equation. For the pH equation, the concentration of hydrogen ions is always a molar concentration, that is, moles of H + per liter. Because pH is on a log scale that means that increasing the pH by 1 corresponds to multiplying the concentration of H + ions by 10! So even though the difference between pH 6 and pH 7 might sound small, it’s actually quite sizeable. The pH is a logarithmic measure of the concentration of hydrogen ions in a solution. pH is defined as the negative log base 10 of the hydronium concentration. The most basic or alkaline substances will have a classification between 7 and 14. The lower the pH, the more H + ions will be present and the stronger the acid. These numbers allow the classification of substances based on their pH the most acidic substances will be close to 0, while the most basic or alkaline substances will be close to 14. The pH scale starts from the number 0 and ends at the number 14.
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