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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 8

Lesson 1: Introduction to acids and bases

# Worked examples: Calculating [H₃O⁺] and pH

In this video, we'll solve for [H₃O⁺] and pH in two different worked examples. First, we'll walk through the possible approaches for calculating [H₃O⁺] from pOH. Then, we'll find the pH of pure water at 50°C from the value of the autoionization constant at 50°C. Created by Jay.

## Want to join the conversation?

• So pH + pOH does not always sum to 14?
• Correct. That only applies at 25°C when the autoionization constant of water, Kw, is equal to 1.0 x 10^(-14). If we use Jay's example here at 50°C, then pH + pOH should equal ~13.28.

Hope that helps.
• so at the end (last problem), the PH value for neutral (equal concentration of H+ and OH-) water @ 50°C is 6.64. I've always assumed that only PH=7 is neutral. Does the neutral PH value of everything depend on temperature? In other words, does the [H+]=[OH-] concentration equilibrium change depending on the temperature?
• So no matter the temperature, the same rule applies for neutral water in that the number of hydronium ions equals the number of hydroxide ions. A pH (little p) of 7 only means neutral water at 25°C, but for other temperatures this means a pH above or below 7. This is due to the changing value of water's self-ionization constant, Kw, with temperature. At 25°C we know it to be 1.0 x 10^(-14), but at something like 50°C it's about 5.5 x 10^(-14). When we do the math using the Kw at 50°C to find the pH and pOH we find that they are both lower than 7, simply meaning that there more hydronium and hydroxide ions in neutral water at higher temperatures.

Hope that helps.
• So if the neutrality point depends on temperature, does that mean the entire pH scale depends on temperature?
(1 vote)
• Yeah, exactly. A neutral pH is only true at 25°C because Kw 1.0x10^(-14). If the temperature changes then the Kw also changes which means what constitutes a neutral pH also changes. A neutral pH of 7 is used because 25°C is average room temperature in most places.

Hope that helps.
• Excuse me, how do you convert the 10^-4.75 to 1.8 x 10^-5? For reference, it's around the timestamp . Thank you!
• If you mean how does he solve the equation around that time, he's using antilog. An antilog is how you would undo a logarithm by making both sides of the equation exponents to a number equal the value of the logarithm's base, in this case 10.

After that, 10^(-4.75) is just a math operation so we can have a calculator do for us.

Hope that helps.