in this video we're gonna talk about how to solve some very basic acid based problems in chemistry like this one what is the pH of the solution if the hydronium ion concentration is 2.5 times 10 to the minus four moles per liter so I want you to be familiar with some common equations and how to use them the first formula you need to note is that the pH is equal to negative log of the hydronium ion concentration now before you use a calculator what do you think the pH is gonna be if you have to estimate arrange for it what would you say now the first number you should look at is the exponent negative for the pH is gonna be close to 4 now do you think it's going to be 3 to 4 or somewhere between 4 to 5 if this number here is greater than 1 choose the lower region if it's less than 1 let's say if it was like point four times ten to the minus four I would choose this value so we know it's somewhere between 3 to 4 now let's go ahead and plug it in and see what the exact value is so if we type in negative log 2.5 times 10 to minus 4 this will give us a pH value of 3.6 which is indeed between 3 to 4 so that's how you can get like a ballpark answer just by looking at this if you don't have access to a calculator now let's see what the pH of this solution will be so if we type a negative log point 4 times since the minus 4 that will give you a value of 4 point 4 which is between 4 & 5 number 2 the Poh of the solution is 4 point 5 calculate the pH of a solution so that's another simple question what formula do we need in order to calculate the pH given the Poh so here's the next equation that you want to write you should add it to your list of equations for acids and bases the pH plus the Poh of a solution must add to 14 especially at 25 degrees Celsius because kW the auto-ionization constant of water is 1 times 10 to the minus 14 so to calculate the pH it's simply going to be 14 minus the Poh and so that's going to be 14 minus 4.5 now 14 minus 4 is 10 and 10 minus 0.5 is 9.5 so this is the pH of the solution now here's a question for you at 25 degrees Celsius would you say the solution is acidic basic or neutral what would you say now it's important to understand that if the pH is less than 7 we have an acidic solution if the pH is equal to 7 at 25 degrees Celsius the solution is neutral but in our example because the pH is greater than 7 we have a basic solution so in a basic solution the amount of hydroxide is greater than the amount of hydronium ions in a solution if it's basic which it is so 9.5 is the answer for this problem number 3 if the Poh is 3.8 what is the hydroxide concentration now we know that the pH is equal to the negative log of the hydronium ion concentration so therefore it stands to reason that the Poh must be equal to a negative log of the hydroxide concentration so we need to solve for this so how can we rearrange this equation well let's do a basic review of logs let's say if we have a log base a of B which is equal to C you could change that equation from its logarithmic form to its exponential form by doing this a raise to the C is equal to B now the base of a log is 10 so first we need to move the negative sign to the other side so negative Poh is equal to log base 10 of the hydroxide concentration so therefore we can say that 10 raised to the negative Poh is equal to the hydroxide concentration and so this is the formula that you want to add to your list because that's what you're going to use to get the hydroxide concentration so it's 10 raised to the negative 3 point 8 and so that comes out to be one point five eight times 10 to the minus 4 moles per liter so that's how you can calculate the hydroxide concentration if you're given the Poh of the solution number four what is the Poh of the solution if the h3o plus concentration is four point two times 10 to the minus three so go ahead and pause the video and try this if you want to now the first thing I recommend doing is calculating the pH of the solution which is negative log of the hydronium ion concentration which we already have and that's 4 point 2 times 10 to the minus 3 so looking at this number we know that the pH is going to be somewhere between 2 & 3 so if you type that in this is going to give us a pH of 2 point 3 7 7 so now we need to calculate the Poh by looking at or using this formula rather the Poh is going to be 14 minus the pH of the solution so that's 14 minus 2 point 3 77 and that works out to be 11 point 6 2 3 so that's the Poh of the solution number 5 if the KA of an acid is 1.8 times 10 to the minus 5 calculate the pKa and pkb values of the acid so let's start with the pKa value it's a negative log of the KA value just as the pH is the negative log of the H+ or h3o plus value so this is going to be negative log of 1.8 times 10 to the minus 5 so looking at this number we know that the pKa has to be somewhere between 4 & 5 and so it turns out that the pKa for this acid is 4 point 7 4 5 and so that's the first answer for this problem now let's focus on a second answer what is the PKB value now PKA plus PK B adds up to 14 so to calculate the PK B value it's going to be 14 minus the pKa value so that's 14 minus 4 point 7 4 5 and so the pKa B value of this acid is nine point two five five and so that's the second answer for this problem number six if the hydronium ion concentration is seven point one times 10 so minus 2 what is the hydroxide concentration go ahead and calculate it so first in this example I'm going to calculate the pH it's a negative log of h3o plus or a negative log of this number so I should get an answer somewhere between 1 & 2 and so the pH is one point one four nine so now I'm going to calculate the Poh so that's 14 minus the pH so 14 minus one point one four nine is twelve point eight five one so once I have the Poh I can easily calculate the hydroxide concentration it's simply 10 raised to the negative Poh so that's going to be 10 raised to the negative twelve point eight five one and so that's one point four one times ten to the minus thirteen so that's the concentration of hydroxide in a solution now it turns out that there's another way to get the same answer and it's used in the autoionization constant for water kW which is 1 times 10 to minus 14 is equal to the product of the hydronium ion concentration and hydroxide at 25 degrees Celsius so the hydroxide concentration it's going to be kW which is this number divided by the h3o plus concentration so it's 1 times 10 to the negative 14 divided by seven point one times 10 to minus 2 and so you get the same answer in 1.41 times 10 to the negative 13 number 7 if the pKa of an acid is three point seven what is the KB value of the acid so first let's calculate P KB which is going to be 14 minus the pKa value so that's 14 minus 3.7 14 minus 3 is 11 and 11 minus point seven is 10 point three so now that we have the P KB value to calculate Kb it's simply 10 raised to the negative P KB value in this case that's 10 to the negative 10 point 3 and that comes out to be five point zero 1 times 10 to the minus 11 so that's a simple way to calculate KB from PKA now here's the last one calculate the pH of a solution if the hydroxide concentration is point zero 1 5 so the first thing I would calculate is the Poh it's a negative log of the hydroxide concentration which is point zero one five and so that's going to be one point eight two four and then just calculate the pH which is 14 minus the Poh so 14 minus one point 8 2 4 that's 12 point one 76 and so that's all the problems in this video hopefully this gave you a good understanding of how to calculate pH and Poh so let's review some of the basic formulas that you need to know so keep this in mind pH is equal to the negative log of the hydronium ion concentration or H+ concentration and the Poh is the negative log of the hydroxide concentration and then the pH plus the Poh has to add up to 14 now if you need to calculate H+ or h3o plus it's simply 10 to the negative pH likewise the hydroxide ion concentration is 10 to the negative Poh and h3o plus x OE h minus is equal to 1 times 10 to the negative 14 now in a similar way PKA is negative log of ka and PK B is negative log of KB so KB is the base dissociation constant ka is the acid dissociation constant now just as pH plus Poh adds up to 14 so PKA plus B KB is 14 now if it needs to calculate ka that's 10 to the negative PKA and KB is 10 to the negative peak a B and finally ka times KB is 1 times 10 to the minus 14 so those are some formulas that you want to write them and make sure you know if you're taking an acid base chemistry test