00:00The amount of lead-2-fluoride is dissolved in 250 mL of water until the solution is exactly saturated.
00:08The question is, what is the mass of PbF2, if the solubility product constant of PbF2 is 2.7 x 10-8?
00:19OK, the first step is identification.
00:22The volume of water, V is equal to 0.25 liters.
00:27Ksp of PbF2 is equal to 2.7 x 10-8.
00:34The relative molecular mass of lead-2-fluoride, 207.2 plus 2 x 19.
00:43The result is 245.2 g per mole.
00:49The next step is to analyze the ion equilibrium.
00:56In water, a molecule of lead-2-fluoride ionizes into a Pb2 plus ion and 2 F minus ions.
01:04The Ksp value in saturated and supersaturated solutions is exactly the same.
01:10The following illustration is a supersaturated condition because there is already a precipitate in the solution.
01:17Assume that the number of Pb2 plus ions is S mole.
01:22So the number of F minus ions is 2 S mole.
01:27And the volume of the solution is 1 liter.
01:33So the molarity of Pb2 plus ions is S molar.
01:37And the molarity of F minus ions is 2 S molar.
01:42The next step is to write the chemical equilibrium equation.
01:47A molecule of PbF2 will ionize into a lead ion, 2 fluoride ions.
01:54Now, we can calculate the value of S from the Ksp equation.
01:59Ksp is equal to the concentration of Pb2 plus ions, multiplied by the concentration of F minus ions to the power of 2.
02:09Plugging in the values of ion concentration and Ksp.
02:15For S to the power of 3 is equal to 2.7 times 10 to the power of negative 8.
02:23We can write this equation as S to the power of 3.
02:29Using a calculator, S is about 1.89 times 10 to the power of negative 3.
02:37This value of S is also the value of moles of PbF2 dissolved in 1 liter of water in a perfectly saturated solution.
02:49So, the molarity of PbF2 is 1.89 times 10 to the power of negative 3 molar.
02:58The fifth step is to calculate the mass of PbF2.
03:02Using the concept of molarity, big M equals N over V.
03:08Putting both values in.
03:11N equals 4.75 times 10 to the power of negative 4 moles.
03:17This is the mole value of PbF2 in a 250 milliliter solution.
03:23The mass of PbF2 dissolved is N times the relative molecular mass.
03:28Putting both values in.
03:31The result is equal to 0.115857 grams or 115.9 milligrams.
03:41This is the mass of PbF2 dissolved in 250 milliliters of water so that the solution is exactly saturated.
03:51Yup, hopefully useful.
03:53And, don't forget to follow this channel.
Comments