[Music] hi I'm Jared Heyman an assistant professor of chemistry at Elon University today I'll be taking you through the appropriate technique and procedure for preparing a chemical solution one of the most important abilities at all levels of chemistry is the preparing of a solution of a specific concentration first we'll Define some of the terms used in this process a solution is a homogeneous mixture of two or more chemicals for the simplest solution a smaller amount of one chemical called the solute is placed in a larger amount of a second chemical known as the solvent today we will be preparing a solution of sodium chloride using a small amount of sodium chloride as our solute added to a larger volume of water our solvent before beginning any experiment it's important to observe appropriate lab safety procedure and know the hazards of any chemicals you're working with in any chemical reaction it's important to keep track of the exact amount of reactants present and for stochiometric purposes we use the term mole to define the amount of a substance a mole is a counting term similar to a dozen eggs where one mole is equal to 6.022 * 10 23rd things as you can see this is a really large number the periodic table uses molar counting as a means of converting to a more experimental term of mass for example 1 mole of carbon atoms has a mass of 12.01 G in a compound such as sodium chloride it's made up of 1 mole of sodium atoms with a mass of 22.98 G per mole along with 1 mole of chlorine atoms with a mass of 30 5.45 G per mole giving us a total of 58.4 3 G per mole of sodium chloride this means of using mass to determine the amount of a substance is useful for Pure chemicals but in the case of solutions we now have a mixture of multiple different substances therefore we express amount as a concentration the most common unit for concentration in chemistry is marity abbreviated capital M which indicates the number of moles of solute dissolved in a certain volume of the total solution this gives marity derived units of moles per liter it is very important to note that this is a volume of solution not the volume of solvent today we will be preparing 500 mL of a 0.25 molar sodium chloride solution before beginning it's important to map out your procedure for preparing this solution here we'll look at the amount of sodium iium chloride we need in this solution we know that we want to prepare a 0.25 mol solution which means that there is 0.25 moles per liter of solution we also know that we only want 500 M of this solution or a/ lit if we multiply these two terms together we can see that we will need 0.125 moles of sodium chloride from earlier we can remember that there are 58.4 3 G per mole of sodium chloride we can determine that the exact mass that must be measured out is 7.34 G of sodium chloride in lab obtain some sodium chloride and a clean lab scoop and using a balance with some weighing paper measure out as close to 7304 G as possible mark down your exact Mass note that if you go over the desired Mass it's never appropriate laboratory procedure to put the excess reagent back into the original container always use an additional container to collect the excess once you've measured out your solid and recorded the exact Mass transfer all of the solid to the appropriate sized volumetric flask volumetric flasks are made specifically for this purpose and should be used over other less accurate labware such as as beakers Erin Meer flasks or graduated cylinders here we just transferred our solute to the 500ml volumetric flask if any solute remains on the weighing paper use a spatula or a small amount of water to transfer the remaining salt next add enough water so that the bulb area at the bottom of the flask is approximately half full this will give you enough room to swirl the liquid allowing the solute to completely dissolve again it's critical that all of the solute is dissolved prior to filling to the appropriate volume as different solutes may take up more volume in solution than their undissolved salts now that the sodium chloride is completely dissolved we can fill the rest of the bulb area and some of the neck with water you should now carefully and slowly add water or solvent as it begins to fill the neck once you approach the etched or printed line on the neck of the flask you should add solvent dropwise monitoring the location of the iscus water as a polar solvent tends to cling to the glass walls of a flask this phenomenon creates what is called a meniscus the bottom of this curved meniscus should sit directly level with the line on the flask at this point there is exactly 500 mL of solution with an air of 0.2 ml different volume flasks have different air values however the air on a volumetric flask is much lower than any other type of glassware of similar volume for example the volume markings on a beaker or an early Meer flask are only accurate to plus orus 5% meaning if the meniscus were on the 500ml line it could be anywhere between 475 ML and 525 ML on the other hand using a volumetric flask we know that the volume is exactly somewhere between 499.00 2 ml meaning it's much more accurate we now must determine the exact concentration of our Solution by taking the mass that we measured out and dividing by formula mass for sodium chloride 58.4 G per mole which gives us a total number of moles in our solution of 01253 moles we can then take this number and divide by our volume of 0.5 l to give us a total exact concentration of this solution of 0.251 molar sodium chloride this value and the contents of the solution should be clearly labeled on the chosen storage container for laboratory safety protocol you've now learned how to make a solution from a solid everything you need to do this demonstration in your lab is available from Carolina we have experts who can assist you with any of your science demonstration needs visit us at at carolina.com to see our 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