The pH Scale - Video Tutorials & Practice Problems
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1
concept
The pH Scale
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2m
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So here we're going to say that the pH scale is used to classify the acidity or the basicity of aqueous 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 temperatures around room temperature, which is approximately 25 degrees Celsius and a molarity of 1. If we go outside a molarity of 1, 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 7, we are classified as being neutral. If it's below 7 then it becomes acidic, and the lower we go in terms of a pH the more acidic we become. Above 7 we're basic, and the higher we go up the more basic we become. So here we're going to 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, 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+ 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 3. Now neutral, what we can think of here is purified water. Its pH is equal to 7. For it to be neutral, the hydronium ion concentration H+ and hydroxide ion concentration Oh Oh 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+ concentration. Here for this example, the pH of this ammonia cleaner is around 12. So remember, under normal conditions where temperature is around room temperature and the molarity is up to 1 molar, our pH scales from 0 to 14. Remember below 7 is acidic, above 7 is basic, at 7 is neutral.
2
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 point 2 molar, HCl is 0.025 molar, sulfuric acid is 0.27 molar, and then we have perfluoric acid at 0.019 molar. We want the least acidic, this would translate to the one with the lowest concentration of H+ ions. For the numbers that we see, the smallest concentration for H+ ions 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 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+ and or Oh- 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+ concentration. Here we could rework this in which we can determine the concentration of h+ from our known pH value. So here we would take the inverse log, we divide both sides by minus 1, to give me negative pH equals log of concentration of H+. You would take the inverse log of both sides. Doing that would help you realize that H+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+ 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- concentration. By dividing both sides by minus 1 and taking the inverse log, we can see that Oh- concentration is equal to 10 to the negative pOH. So again, if I know my Oh- concentration I can find pOH, or if I know my pOH value, I I can use it to find my Oh- concentration. So just keep in mind the relationship between pH and pOH with their concentrations of H+ and Oh-.
4
example
The pH Scale Example
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1m
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A solution is prepared by dissolving hydrocyanic acid in 2 liters of water. If the H plus concentration was found to be equal to 0.34 moles, calculate the pH of this hydrocyanic acid solution. Alright, 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+ion given to us, divided by the 2 liters. So that 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 going to give me 0.77 at the pH of this solution. Here the number's a little bit high because we're able to generate quite a bit of H+ ion. 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 1 molar to fall within that range. 0.17 is pretty high up there along the way to towards 1 molar. Again we figured out the concentration of H+ 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
6
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 at temperature of 25 degrees Celsius and up to a concentration of 1 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+ 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.
7
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 degrees Celsius. What will be the pOH and the concentration of hydronium ions of this solution? Alright, 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 hydronium ion concentration, remember hydronium ion is just H+orh3o+. We can determine this since we know the pH that's because H+ equals 10 to the negative pH. So take the pH that we found and plug it in, when we do that we get 5.01187 times 10 to the minus 3 molar for our hydronium ion concentration. Here we round to 2 sig figs, and that'll be your answer. Okay, so 5.0 times 10 to the minus 3. And actually we should round to 1 sig fig, 5 times 10 to the negative 3, concentration because we don't need all those extra digits. So here our pOH oops, our pOH not pH, our pOH is 11.7, and our pH is from that we can determine our h plus as being 5 times 10 to the minus 3 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
1.8 × 10−11 M
9
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