poh. what is the [h + ]? if the [h + ] = 1 x 10 -7 m, the ph = 7.if the [h + ] = 1 x 10 -7 m, the ph...
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![Page 1: POH. What is the [H + ]? If the [H + ] = 1 x 10 -7 M, the pH = 7.If the [H + ] = 1 x 10 -7 M, the pH = 7. That works well with acids, but what about bases?](https://reader036.vdocuments.us/reader036/viewer/2022072017/56649efe5503460f94c13535/html5/thumbnails/1.jpg)
pOHpOH
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What is the [HWhat is the [H++]?]?
• If the [HIf the [H++] = 1 x 10] = 1 x 10-7-7M, the pH = 7.M, the pH = 7.
• That works well with acids, but what That works well with acids, but what about bases? Consider sodium about bases? Consider sodium hydroxide:hydroxide:
NaOH(aq) NaOH(aq) Na Na+ + + OH+ OH--
What is the [HWhat is the [H++]?]?
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We need a new formula!!We need a new formula!!
Since bases release hyroxides, OHSince bases release hyroxides, OH--, , we will calculate the pOH.we will calculate the pOH.
pOH = -log[OHpOH = -log[OH--]]
The formula is very similar to the pH The formula is very similar to the pH formula, in fact you use the calulator formula, in fact you use the calulator
the same way.the same way.
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Let’s consider waterLet’s consider water
Water is HWater is H22O, but let’s write it HOH.O, but let’s write it HOH.
Some water molecules naturally dissociate:Some water molecules naturally dissociate:
HOH HOH H H++ + OH + OH--
In fact, 1 x 10In fact, 1 x 10-7-7M of the water molecules M of the water molecules dissociatedissociate
When this happens, you end up with:When this happens, you end up with:
[H[H++] = 1 x 10] = 1 x 10-7-7M and the [OHM and the [OH--] = 1 x 10] = 1 x 10-7-7M M
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What is the pH?What is the pH?
• Therefore we can calculate that water Therefore we can calculate that water has a pH of 7.has a pH of 7.
• Using the new formula pOH = -log[OHUsing the new formula pOH = -log[OH --]; ]; we can calculate that water has a pOH we can calculate that water has a pOH of 7.of 7.
• Therefore, Therefore, pH + pOH = 14pH + pOH = 14. This works . This works for all solutions, not just water!for all solutions, not just water!
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Who’s on First?Who’s on First?
• Let’s consider our first base, NaOH. Let’s consider our first base, NaOH. What is the pOH of a 0.02M What is the pOH of a 0.02M NaOH(aq) solution?NaOH(aq) solution?
NaOH(aq) NaOH(aq) Na Na+ + + OH+ OH--
All of the NaOH will break apart All of the NaOH will break apart (dissociate), therefore the [Na(dissociate), therefore the [Na++] = ] = [OH[OH--] = 0.02M] = 0.02M
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pOH = -log[OHpOH = -log[OH--]]
• Now use the formula:Now use the formula:
pOH = -log[0.02] pOH = -log[0.02]
pOH = 1.7pOH = 1.7
If you are having trouble using your If you are having trouble using your calculator, ask your teacher for help.calculator, ask your teacher for help.
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But what is pOH?But what is pOH?
• Outside of chemistry class, pOH is not Outside of chemistry class, pOH is not used very much, but pH is used often.used very much, but pH is used often.
• So, what is the pH of 0.02M So, what is the pH of 0.02M NaOH(aq)?NaOH(aq)?
pH + pOH = 14pH + pOH = 14
pH + 1.7 = 14pH + 1.7 = 14
pH = 12.3pH = 12.3
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Does that make sense?Does that make sense?
• Should a 0.02M NaOH(aq) solution Should a 0.02M NaOH(aq) solution have a pH of 12.3? Sure!!have a pH of 12.3? Sure!!
• Remember that solutions with a pH Remember that solutions with a pH greater than 7 are basic and greater than 7 are basic and NaOH(aq) is a basic solution.NaOH(aq) is a basic solution.
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PracticePractice
• What is the pH of a 0.00045M What is the pH of a 0.00045M KOH(aq) solution?KOH(aq) solution?
• Work out the answer using your Work out the answer using your calculator and then click to see the calculator and then click to see the answer.answer.
• pOH = -log[0.00045] = 3.35pOH = -log[0.00045] = 3.35• pH + 3.35 = 14pH + 3.35 = 14• pH = 10.65pH = 10.65
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Practice AgainPractice Again
• What is the pH of a 0.073M What is the pH of a 0.073M LiOH(aq) solution?LiOH(aq) solution?
• Work out the answer using your Work out the answer using your calculator and then click to see the calculator and then click to see the answer.answer.
• pOH = -log[0.073] = 1.14pOH = -log[0.073] = 1.14• pH + 1.14 = 14pH + 1.14 = 14• pH = 12.86pH = 12.86
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One more practice One more practice problemproblem
• If a CuOH(aq) solution has a pH of 9.5, If a CuOH(aq) solution has a pH of 9.5, what is the concentration (molarity) of the what is the concentration (molarity) of the CuOH(aq)?CuOH(aq)?
• Work out the answer using your calculator Work out the answer using your calculator and then click to see the answer.and then click to see the answer.
• 9.5 + pOH = 14, therefore the pOH = 4.59.5 + pOH = 14, therefore the pOH = 4.5• 4.5 = -log[OH4.5 = -log[OH--], therefore [OH], therefore [OH--] = 3.16 x 10] = 3.16 x 10--
55
• Since one CuOH gives off one OHSince one CuOH gives off one OH--, the , the [CuOH] = 3.16 x 10[CuOH] = 3.16 x 10-5-5M.M.