The pH Scale - Video Tutorials & Practice Problems
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1
concept
The pH Scale
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So here we're going to say that the Ph scale is used to classify the acidity or the baity of a quiz solutions. And we're going to say that the Ph scale normally ranges from 0 to 14 under normal conditions. Now, when we say normal conditions, we mean the temperature is around room temperature which is approximately 25 °C and a molarity of one. If we go outside of molarity of one, then the Ph scale can be outside the range of 0 to 14. Now, here with the Ph scale, we have 0 to 14. We can see here that at ph of seven, we are classified as being neutral. If it's below seven, then it becomes acidic. And the lower we go in terms of a Ph, the more acidic we become above seven were basic. The higher we go up, the more basic we become. So here we're gonna say the stronger the acid, the lower the Ph and the stronger the base, the higher the PH could be. Here, we have examples of acidic, acidic, neutral and basic types of solutions. In the first one, we have vinegar here, vinegar is acidic. Since it's acidic. That would mean that its acidic component, which is your H plus ion would be greater in amount than the concentration of oh minus, which is typically reserved for basic compounds. Here, the ph of vinegar is around three. Now neutral. What we can think of here is purified water. Its ph is equal to seven for it to be neutral. The hydro ion concentration H plus and hydroxide ion concentration. Ohmoh minus have to be roughly equal to each other. So they're equal. And then finally basic here we have ammonia cleaner or to be basic, the amount of hydroxide ion concentration has to be greater than the H plus concentration. Here for this example, the ph of this ammonia cleaner is around 12. So remember under normal conditions where temperatures around room temperature and the molarity is up to one molar. Our ph scales from 0 to 14. Remember below seven is acidic, above seven is basic at seven is neutral.
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example
The pH Scale Example
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Given the H plus concentration of the following solutions, which one is the least acidic. So here we have nitric acid with the H plus concentration of 1.2 molar hcl is 0.025 molar. So uric acid is 0.27 molar. And then we have perchloric acid at 0.019 molar. We want the least acidic. This would translate to the one with the lowest concentration of H plus ions. The numbers that we see the smallest concentration for H plus ions would, would be with our perchloric acid solution. So here our answer would be option d it has the lowest H plus concentration which would translate to the being the least acidic solution out of the options present.
3
concept
pH and pOH
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We can determine Ph or POH of a solution if concentrations, which means molarity of either the H plus and or oh minus concentrations are known and vice versa. So basically, if we take a look here, we have our Ph and our POH formulas. First of all P just is a stand in for negative log. So when I say ph, I really mean negative log of your H plus concentration here, we could rework this in which we can determine the concentration of H plus from our known Ph value. So here we would take the inverse law, we divide both sides by minus one. They give me negative P equals log of concentration of H plus. You would take the inverse log of both sides doing that would help you realize that H plus concentration equals 10 to the negative Ph. So this is a way of us going between Ph and H plus back and forth. So if I know my H plus concentration, I can use it to find my Ph, if I know my PH I can use it to find my H plus concentration. The same method can be applied to POH with POH again, remembering P equals negative log. POH just means negative log of my oh minus concentration by dividing both times by minus one. And taking the inverse log we can see that oh minus concentration is equal to 10 to the negative P oh So again, if I know my oh minus concentration, I can find POH or if I know my POH constant POH value, I can use it to find my oh minus concentration. So, just keep in mind the relationship between Ph and POH with their concentrations of H plus and oh minus.
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example
The pH Scale Example
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A solution is prepared by dissolving hydro cyanic acid in 2 L of water. If the H plus concentration was found to be equal to 0.34 moles, calculate the ph of this hydro cyanic acid solution. All right. So remember Ph is equal to the negative log of our H plus concentration. Another name for concentration is molarity. So here we're gonna say molarity equals moles over liters. We're gonna take the moles of H plus ion given to us divided by the two leaders. So it gives me 0.17 molar. We're then just going to say now ph equals the negative log of 0.17 molar. Here when we work that out, that's gonna give me 0.77 at the ph of this solution. Here. The numbers a little bit high because we were able to generate quite a bit of H plus ion for 0.17 molar is a pretty large number. Remember when we're dealing with the ph scale from 0 to 14, we can go up to one molar to fall within that range. 0.17 is pretty high up there along the way to towards one molar. Again, we figured out the concentration of H plus ions here, plugged it into the formula and we're able to deduce what our ph is at 0.77.
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Problem
Problem
A solution of NaOH was prepared in a chemistry lab and the pOH was determined to be 9.3. What is the concentration of OH− ions of this basic solution?
A
5 × 10−11 M
B
5 × 10−10 M
C
2 × 109 M
D
2 × 108 M
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concept
Additional pH and pOH Calculations
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58s
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Now recall that we say the Ph scale can go up to 14, a temperature of 25 °C and up to a concentration of one molar. Because of this, we can calculate Ph from POH or vice versa. Using the following equation. That equation is that Ph plus POH equals 14 itself. In addition to this, we have on the right side, some equations that we've seen before where Ph equals negative log of H plus and therefore H plus concentration equals 10 to the negative Ph POH equals negative log of oh minus and therefore the concentration of oh minus equals 10 to the negative POH. So we're starting quite a collection of different formulas that are connecting Ph and POH to one another. Just remember the addition between Ph and POH equals 14.
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example
The pH Scale Example
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1m
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You prepare a solution of hydrochloric acid with a Ph of 2.3 at 25 °C. What will be the POH and the concentration of hydro ions of the solution? All right. So remember that Ph plus POH equals 14. We know the Ph is 2.3. So subtract 2.3 from both sides. So Ph equals 11.7. Now for hydro ion concentration, remember hydro ion is just H plus or H +30 plus, we can determine this since we know the Ph that's because H plus equals 10 to the negative Ph. So take the PHL we found and plug it in when we do that, we get 5.01187 times 10 to the minus three molar or our hydro ion concentration. Here we round a 26 F and that'll be your answer. OK. So 5.0 times 10 to the minus three. And actually we should round to one sig fig 55 times 10 in negative three concentration because we don't need all those extra digits. So here our po eight oops our POH not ph our POH is 11.7. And our PH is from that, we can determine our H plus as being five times 10 to the minus three molar.
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Problem
Problem
Calculate [OH−] of a lemon juice solution at 25°C with a [H+] = 5.7 × 10−4 M.
A
5.7 × 1010 M
B
4.7 × 10−11 M
C
2.7 × 10−18 M
D
1.7 × 10−11 M
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Problem
Problem
A 345 mL bottle of antacid (Mg(OH)2) contains 1.45 × 10−2 moles of hydroxide ions. Determine pH and pOH of the antacid.
A
pH = 12.384 pOH = 1.616
B
pH = 12.623 pOH = 1.377
C
pH = 12.161 pOH = 1.839
D
pH = 12.338 pOH = 1.662
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Problem
Problem
Which of the following statement(s) on aqueous solutions is/are correct?
a) aqueous solutions have a pH of 7
b) as concentration of hydronium ion increases, concentration of hydroxide ion decreases
c) solutions of weaker acids generally have a higher pOH then solutions of stronger acids
d) pH of pure water equals to 7 at 35º C.
A
aqueous solutions have a pH of 7
B
as concentration of hydronium ion increases, concentration of hydroxide ion decreases
C
solutions of weaker acids generally have a higher pOH then solutions of stronger acids