We can perform calculations involving pH, pOH, and other quantities for pure water when we’re not at 25°C. We’ll do one example here. Calculations Involving Water at Temperatures Other Than 25°C Example 1

Before we start the example, it’s very important to remember that when we’re not at 25°C When we’re NOT at 25°C,

We CANNOT use When we’re NOT at 25°C, We CANNOT use:

[H 3 O + ][OH – ]= 1.00 × 10 –14 When we’re NOT at 25°C, We CANNOT use: [H 3 O + ][OH – ]= 1.00 × 10 –14

When we’re NOT at 25°C, We CANNOT use: [H 3 O + ][OH – ]= 1.00 × 10 –14

And we cannot use pH + pOH = When we’re NOT at 25°C, We CANNOT use: [H 3 O + ][OH – ]= 1.00 × 10 –14 pH + pOH = 14.00

When we’re NOT at 25°C, We CANNOT use: [H 3 O + ][OH – ]= 1.00 × 10 –14 pH + pOH = 14.00

Here’s the example question. We’re given that the value of Kw at 20°C is 6.81 × 10 –15. The value of K w at 20°C is 6.81 × 10 –15.

a) What is the [H 3 O + ] in water at 20°C? Question “a” asks us to find the hydronium ion concentration in water at 20°C The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? The Kw, which is hydronium concentration times hydroxide concentration is 6.81 x The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Because water is neutral, [OH – ]=[H 3 O + ] The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? Because water is neutral [OH – ]=[H 3 O + ]

a) What is the [H 3 O + ] in water at 20°C? So we can substitute [H 3 O + ] in for [OH – ], and get [H 3 O + ] 2 = 6.81 × 10 –15 The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? Substitute [H 3 O + ] for [OH – ]

a) What is the [H 3 O + ] in water at 20°C? So [H 3 O + ] is the square root of 6.81 × 10 –15 The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Which is 8.25 × 10 –8 M The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? The second question asks us the pH in water at 20°C The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? pH is the negative log of the hydronium ion concentration, The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Which is the negative log of 8.25 x 10 -8, The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Which comes out to The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Question “c” asks for the hydroxide ion concentration in water at 20°C The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? We know that the hydronium ion concentration is 8.25 x M, and we need to find the hydroxide ion concentration. But be careful, we Cannot use 1.0 x divided by hydronium concentration here, because we’re not at 25°C! The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? [H 3 O + ]

a) What is the [H 3 O + ] in water at 20°C? However, because we’re dealing with pure water, we know that it is neutral. The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? [H 3 O + ] Neutral

a) What is the [H 3 O + ] in water at 20°C? Therefore hydroxide ion concentration is equal to hydronium ion concentration. The value of K w at 20°C is 6.81 × 10 –15. b) What is the pH in water at 20°C? c) What is the [OH – ] in water at 20°C? e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? [H 3 O + ] Neutral

a) What is the [H 3 O + ] in water at 20°C? Which is 8.25 x M The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? [H 3 O + ] c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? Question “d” asks us to find the pOH in water at 20°C. Remember, we cannot use 14 minus pH to find pOH because we’re not at 25°C. The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C?

a) What is the [H 3 O + ] in water at 20°C? However, because water is neutral, we know that the hydroxide concentration is equal to the hydronium concentration, The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? Neutral

a) What is the [H 3 O + ] in water at 20°C? Therefore, the pOH is equal to the pH The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = pH Neutral

a) What is the [H 3 O + ] in water at 20°C? Which is The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = pH = 7.084

a) What is the [H 3 O + ] in water at 20°C? We could also have said that pOH is the negative log of [OH-] The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = –log[OH – ] =–log(8.25 × 10 –8 ) = 7.084

a) What is the [H 3 O + ] in water at 20°C? Which is the negative log of 8.25 x The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = –log[OH – ] = –log(8.25 × 10 –8 ) = 7.084

a) What is the [H 3 O + ] in water at 20°C? Which is The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = –log[OH – ] = –log(8.25 × 10 –8 ) = 7.084

a) What is the [H 3 O + ] in water at 20°C? The last question asks if water at 20°C is acidic, basic, or neutral. The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = pH = 7.084

a) What is the [H 3 O + ] in water at 20°C? Here’s where we have to be careful. Notice the pH is above 7. We’re used to thinking that if the pH is above 7, then it is basic. However the 7 is only valid at 25°C. The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = pH = pH

a) What is the [H 3 O + ] in water at 20°C? Remember pure water is neutral, no matter what the temperature is. The value of K w at 20°C is 6.81 × 10 –15. e) Is water at 20°C acidic, basic, or neutral? Neutral d) What is the pOH in water at 20°C? c) What is the [OH – ] in water at 20°C? b) What is the pH in water at 20°C? pOH = pH = 7.084