In this video we're going to go over some Malawi practice problems we're going to focus on how to calculate molarity of a solution given Graham's milliliters or if we're given molarity density or mass percent and vice versa so these are some formulas that you need to know molarity of a solution is equal to the moles of solute divided by liters of solution molality is another form of concentration and it equals the moles of solute divided by the kilograms of solvent and the mass percent is the mass of the solute divided by the total mass and another formula that you need to be worth which will be useful in this section is the density of a solution it's equal to the mass of the solution divided by the volume so you need to know that as well and so we're going to do is we're going to go over a few problems and so you could put this information in practice so let's go ahead and begin so here's the first one 30 2.5 grams of sodium fluoride is dissolve in 425 grams of water calculate the molarity of the solution so Malawi is moles of solute divided by kilograms of solvent the solute is the sodium fluoride the solvent is the water sodium fluoride is dissolved in water so we need to convert the grams of sodium fluoride into moles so first you need the molar mass of sodium fluoride and we can forget that information from the periodic table the molar mass of NA is about twenty-two point nine nine so we're going to say twenty-three and for fluorine is 19 so the molar mass is 42 grams per one mole so let's start with 32 point five grams of sodium fluoride and let's convert that into moles so one mole of sodium fluoride has a mass of 42 grams so notice that the units grams of any F cancel now the next thing we need to do is we need to divide by the kilograms to solve it right now we have moles moles over kilograms it's going to give us the nini Malala scene now notice that we have 425 grams water to convert that to kilograms we simply need to divide by thousand one kilogram is equal to one thousand grams so the unit's grams cancel so 425 divided by thousand is going for two five kilograms so this is going to be run over point four to five kilograms so notice that we have moles of solute on top kilograms of solvent on the bottom and that's kilograms of water moles of a kilograms is basically equal to the molarity so now at this point let's go ahead and calculate the molarity of the solution so thirty two point five divided by forty two that's point seven seven three eight moles and then take that divided by point four two five and you'll see that the molality is about one point eight two lowercase M is for morality so that's how you can calculate it number two 32 grams of sodium hydroxide is dissolved in enough water to make a 325 milliliter solution calculate the molarity of the solution so we know molarity is defined as the moles of solute divided by the liters of solution so we need to convert grams of sodium hydroxide which is the solute into moles so let's find the molar mass first sodium has the molar mass of 23 according to the periodic table and for opt 16:4 hydrogen it's about one so sodium hydroxide has a molar mass of 40 grams per mole so if we start with 32 grams of NaOH we can convert it to moles by dividing by 40 so one mole has a mass of 40 grams so notice that the units grams of sodium hydroxide cancel now we need to convert milliliters into liters of solution and it turns out that one liter is equal to a thousand milliliters so by dividing 325 by thousand we can get the liters which is point three two five so if we divide the moles by liters we can get the molarity of the solution so 32 divided by 40 is about point eight moles and then if you divide that by point three to five liters you'll have the molarity which is two point four six capital M stands for molarity low case M is usually associated with molality number three seventy four point five grams of calcium chloride is dissolve in 560 grams of water the density of the solution is 1.15 grams per milliliter calculate the molarity of the solution to find the molarity we need the moles of solute which we can get by using the grams of calcium chloride and we need to divide that by the liters of solution we don't have the milliliters so we got to find it so let's focus on the solution part now we have the mass of the solute which is calcium chloride and we have the mass of the solvent which is water the solute plus the solvent is equal to the solution the solute and the solvent combined makes up the solution so if we have seventy four point five grams of solute and 560 grams of the solvent we therefore have have these two numbers 630 4.5 grams of solution so we can use that and we can convert it to milliliters using the density so let's do that 630 4.5 grams of solution over 1 and the density is equal to 1 point 1 5 grams per milliliter that means that there are 1 milliliter of solution for every one point 1 5 grams of solution so notice that the units grams of solution cancel and we need to take it to one step further let's convert milliliters into liters now for every thousand milliliters there's 1 litre of solution so 634 point 5