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Analytical Chemistry
Learn the toughest concepts covered in your Analytical Chemistry class with step-by-step video tutorials and practice problems.
Chemical Equilibrium
The pH Scale
The pH scale is useful in converting very small concentration values into more manageable numbers.
pH and pOH
1
concept
pH and pOH
5m
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when it comes to most solutions, the concentrations of H plus and O h minus tend to be very small numbers in order to deal with these very small numbers. The ph scale was invented. So we're going to say here that under normal conditions and that means that our concentrations of H plus and O h minus are less than one Moeller. The ph scale will range between 0 to 14. So if you're a church plus concentration were greater than one Moeller we'd get a ph less than zero. If you're O h minus concentration were greater than one molar, then you would get a ph greater than 14. Okay, so the whole thing about the ph scale being from 0 to 14, that's only true. If these two ions are less than one Moeller. Now we're gonna say by taking the negative log of H plus an O H minus, you can get P H and p O H respectively. So ph is just negative log of H plus or H +30 plus because remember they're the same thing and P O H equals negative log of O H minus. So this is just telling us that P just means negative log. Now if we can recognize the relationship between H plus and O H minus, we can establish relationships between ph and P O. H as well. So for example, P H equals negative log of H plus If we take, if we divide both sides by negative one then we would get that ph negative P H equals log of H plus. We would take the anti log in order to get rid of that log And when I take the anti log of the other side it becomes 10 to that value. So it's 10 to the negative ph. So what this is saying is if I know what my H. Plus um if I know what my ph value is, I just do 10 to the negative ph and I'll have my H. Plus concentration in the same way. We can establish the relationship between P. O. H. And O. H minus. So again we divide both sides here by -1. So now we're gonna get negative P. O. H. Equals log of O. H minus. We would take the anti log of both sides again so tend to the negative P. O. H. Equals O. H minus now. So if you know if you know p O. H. O. H minus is. Now we're gonna say in general um as the ph value increases so our ph is getting higher there's gonna be a decrease in my H. Plus concentration and an increase in my O. H minus concentration. Because as you go higher and higher in terms of ph you become more and more basic. Now we're gonna say here in classifications of solutions we're gonna say species with a ph equal to seven is classified as neutral. Now if we're talking about a neutral solution that means that H. Plus and O. H minus are equal to one another. But more specifically they're equal to the number of 1.0 times 10 to the negative seven moller. At neutral ph that's because K. W equals H plus times O H minus. If they're both equal to each other, they're both equal to X, which is X squared K. W. Which is your your ion product constant for water is 1.0 times 10 to the negative 14 at 25 degrees Celsius, solving for X. That's where this number here comes into play. When you're neutral, all of them are equal to one another. Now if your ph is greater than seven, you're classified as a basic solution and if your basic that means you're O H minus concentration will be greater than your H. Plus concentration. And that's because O. H minus would be greater than this value of 1.0 times 10 to the negative seven moller. Which in turn would be greater than H. Plus. Finally, if your ph is less than seven, your acidic. So now H plus concentration will be greater than both of them. So would be greater than the value of 1.0 times 10 negative seven moller and which in turn is greater than O. H minus concentration. Finally, if you know ph you know P. O. H. If you know P. O. H. You know ph because together they're connected to P H plus P O. H equals 14. So these are some of the fundamental ideas behind the relationships between your K. W, which is your eye on product constant, your H plus and minus concentrations, as well as ph and P O H. As we delve deeper into the different types of acids and bases or learn different approaches in order to calculate their Ph and P O. H values, respectively. Now that we've seen all of that, look to see if you can attempt the example question left at the bottom of the page attempted on your own. But if you get stuck, just go to the next video and see how I approach that question. Okay.
2
example
pH and pOH
2m
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So here it asks what is the hydroxide ion and hydrogen ion concentration of a solution with a ph equal to 5.88. Alright so we need to determine what R. O. H minus concentration is and what are H. Plus or H. Three plus concentration is recall up above. I said that if you know ph you know what your H. Plus concentration is because H. Plus is equal to 10 to the negative ph So it's 10 to the negative 5.88. When you punch that into your calculator that will give you the concentration of your hydro knee um ion. So here that gives me 1.32 Times 10 to the -6 Moller. As my answer. Now we need to figure out what O. H minus is. We can find it in two different ways. So one method we can use is realize that ph plus P. O. H. Equals 14. So P. O. H would equal 14 minus my ph which gives me 8.12. If we know what the P. O. H. Is then we know what O. H minus is because O. H minus equals 10 to the negative P. O. H. Plug that in. So when we do that We get an answer of 7.59 times 10 to the -9 moller as the concentration for hydroxide ion. Another method we could have used to figure out my hydroxide ion concentration is to realize that K. W. Equals H plus times O. H minus K. W. Since they don't give us a temperature, we assume it's room temperature, so 25 degrees Celsius. So that's 1.0 times 10 to 1914 for my K. W. Value. We just found out what H plus is earlier. So plug that in and all we have to do now is isolate O H minus. So you divide both sides here by the value we got for H plus. Remember any time you have a number of times 10 to any power we should put them in parentheses to avoid any computer, any calculator errors that could happen. So if you do this correctly you would get the same exact value here for O H minus. So two different approaches to find your hydroxide ion concentration. Use the method that you're most comfortable with to get your final answer. So remember there are connections that exist between H plus and O. H minus and their relationships to P H. And P O H. Respectively remembering them is key to getting your final answer. When doing any types of these calculations will continue with our discussion of ph and P O. H. As we delve deeper and deeper into discussions of different types of acids and bases
pH and pOH Calculations
3
example
pH and pOH Calculations 1
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here it states of the following options, a solution with which ph would have the greatest concentration of hydro ni um ions On the previous page we said that if we increased our ph concentration that would lead to a drop in the amount of H plus ions available here. We want the opposite, we want the greatest amount of H plus ions available. So we would look for the lowest ph so the lowest ph would translate into the highest amount of H plus ions and the lowest amount of hydroxide ions. So for this question it's simply option four. Now remember we're dealing with log function here. So if you're comparing a ph of four to a ph of five, just realize here that increasing ph by one unit means a tenfold increase for O H minus concentration. And it would mean a tenfold decrease. Mhm. For H plus concentration in the opposite way, If you went from PH equals four, two P. H equals let's say two. So every one unit um that we move up and down the ph scale, that's a tenfold increase. Or decrease for H plus or oh minus concentration here we're going to ph of two. So we're becoming more acidic. So that means we're increasing the amount of H plus concentration. But here we're moving it moving down by two units, every ph unit we go down we increase H plus tenfold since we're moving here down by two units that be 10 times 10. So that would be 100 fold increase for H plus concentration and 100 fold decrease for O H minus concentration. So this question itself was simple here. I added a couple more things to it. Just remember when you're going on the ph scale, moving up and down the ph scale by units of one that that translates to a 10 fold increase or decrease to either your H. Plus concentration or your O. H minus concentration. Use the two examples I gave here as basically a road map to see what happens to their concentration as we go up and down the ph scale. Now that we've attempted this one, try to do example to hear HBR is a strong acid. So we have to understand that in order to find its true mass at the end. So attempted on your own. But if you get stuck, come back and see how I approach that same exact question.
4
example
pH and pOH Calculations 1
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So here it says what mass of HBR should a student mix into 250 mls of water to make a solution with a ph of 3.850. Alright, so they're asking us to find mass. So we're looking for the grams of HBR. Now realize here that because we know what ph is and we're dealing with an acid, we can figure out what our H plus concentration is. So with this information, I'll be able to find the polarity of H plus realize that we have more clarity and we also have volume involved. Polarity equals moles over leaders. I can figure out my polarity from the ph I can change milliliters to leaders with those two pieces of information. I can isolate moles because moles equals leaders times more clarity. So what I'm gonna do first is remember that H plus equals 10 to the negative ph so that's 10 to the negative 3.850. When we plug that in, that gives me 1.41254 times 10 to the negative four molar of H Plus here, try not to round into the very end to avoid any types of rounding errors. Here we have 250 mls. So what I'm gonna do here is I'm gonna convert mls into liters. So one milli is 10 to the negative three liters. You could also say that one leader is 1000 ml. So use whichever convention is like more familiar to you Moeller charity means moles over liters. So this polarity that we found really means 1.41254 times 10 to the negative four moles of H plus over one leader. So anytime we have polarity it's that number of moles over one leader leaders cancel out. Now I have moles of H B R. Because we're dealing with a strong acid in the form of HBR. Remember here that strong acids and strong basis dissociate completely winning solution. This will give me a church plus and B. R minus and I'll form 100% of these. Because of that, we can say that there is a 1 to 1 relationship between the acid and these ions. So we're gonna say here that for every one mole of H B R, there's exactly one mole of H plus within that compound. So it's this connection that allows us to go from H plus two HBR. Finally, because we have the moles of H B R, we can figure out our grams of H B R for the end. So HBR has in it one hydrogen and one bro mean if we look at the periodic table will see the atomic masses of hydrogen and B are respectively. So H on the periodic table is 1.794 g And BR weighs 79.904g. Yeah, one mole of HBR here add those two numbers for the mass of hydrogen and br together it comes out as 80.9119 g of HBR. When I add these two numbers together so most of HBR cancel out. So I'll have grams at the end that comes out to 0.2857 g of H. B. R. And when we round that, that simply gives us option A as the correct choice. So just remember here they're asking for the mass of the acid. We need to pick up on cues being given to us within the question we had volume given to us. If they give us ph we can find concentration. So remember concentration which is more clarity. Times leaders gives me moles by knowing moles. I can figure out what my grams will be at the end. Now that we've attempted this question. Try to do the practice question left at the bottom of the page attempted on your own. We've done examples that are similar to it. Once you've done that, come back and take a look at my video and see if your answer matches mine. Yeah.
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Problem
What is the hydronium ion concentration in a solution having a pOH of 3.62?
A
3.8 × 10−5 M
B
4.2 × 10−11 M
C
3.8 × 10−4 M
D
2.6 × 10−11 M
E
5.1 × 10−10 M
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