[Music] measurements are extremely important in science and the way we measure the amount of a substance or chemical is with a very specific measurement value called a mole a mole is a unit of measurements that is defined by avagadro's number the number is 6.02 * 10 23rd you can think of this value as a constant that is used to count the number of something in the same way that you would use 12 as a number to count a value of one dozen so if you had one dozen eggs it would mean you have 12 eggs and if you have one mole of eggs it would mean you have 6.02 * 10 23rd eggs and that's a lot of eggs as you can see with the largely positive exponent of 23 this value is very very large a mole is a large number by Design as it needs to efficiently count substances that we call Elementary entities which are atoms molecules ions electrons or any other very small group of particles so avagadro's number is a defined value of 6.2 * 10 23rd particles which is 62 sextilion particles which again equals 1 mole because avagadro's number relates the two units of measurement which are particles and moles moles we can use this value as a conversion factor between the two in this case we would take the number of particles 6.02 * 10 23rd and place it over 1 mole which can be used as a conversion because dividing this out equals 1 so let's say you had a glass of water that contained 13.3 moles of water how many actual water molecules would you find in that glass of water let's use our conversion factor we start with 13.3 moles of water and then multip mly by our conversion factor which we know that 1 mole of a substance equals 6.02 * 10 23rd molecules we multiply across 13.3 * 6.02 * 10 23rd equal about 8.66 * 10 24th for the numerator the denominator is 1 and the mole units cancel out so we can say that 13.3 moles of water has around 8 septian move the decimal 24 places to the right molecules within it as that is such a large number you can see how it is easier to just measure and talk about it in terms of moles this conversion factor will be used throughout your entire IB chemistry course so be sure to remember it and get used to using it now that we have discussed how we can use the mole and the amount of particles as units of measurement in chemistry we will discuss how we also use Mass to quantify various chemicals how can we find the mass of a particular atom or molecule let's luckily this information about relative atomic mass has been laid out for us by previous scientists on the periodic table typically a periodic table will have two numbers that accompany each element the smaller number being the atomic number and larger number being the relative atomic mass which means the mass of a singular atom the reason why it is called relative atomic mass is because these masses you see on the periodic table are all relative to the mass of 112th a carbon at Mass so if you take a look at the periodic table here you will notice that oxygen has a relative atomic mass of about 16 units which is saying an oxygen atom is 16 * the mass of 112 a carbon atom something else to note is that relative atomic mass does not have any units because these values are generated from a ratio or comparison to carbon 12 so the units used for the masses end up canceling out so what about relative formula mass this is the exact same concept as the relative atomic mass except it is for a compound or molecule with a more complex formula than a singular atom take this carbon dioxide molecule for example there is one carbon atom and two oxygen atoms while we do not have CO2 on the periodic table we can use the individual relative atomic masses of carbon and oxygen to calculate the overall relative formula mass of CO2 we will be adding up each Rel relative atomic mass included in this compound this includes the 12.01 units for carbon the 16 units for oxygen and another 16 units for the second oxygen be sure to pay close attention to the subscripts in the chemical formula to let you know how many of each element we must include in this calculation in this case since there are two oxygens we can just multiply our 16 units by two doing this will make your calculation easier especially when you have larger subscripts and more complex molecules at hand the relative formula mass of CO2 should come out to be 44.0 1 units determining the mass of a molecule is routine in chemistry so the moment you notice you need a Mass for a calculation you should automatically refer to your periodic table for help we just started using some values on the periodic table for the relative atomic masses and we are doing so again for the molar masses we can do this because the difference between relative atomic mass values and molar mass values is negligible now what exactly is molar mass molar mass is the amount of mass that is present per one mole of a specific element which is expressed in units of G per mole we can use these molar mass values shown by the periodic table as conversion factors to convert between mass and moles of a substance for example if you are carrying out a lab that requires you to measure out 0o .5 moles of NAC you likely don't have a measuring device that has units of moles but we do have electronic balances that measure in grams which is the unit for mass so we can find out how many grams we need to measure of NAC by using the given value of 0.5 moles to begin with and multiplying this by the molar mass of NAC the marolar mass can be found by checking the mass values for both sodium and chlorine on the periodic table if we have had 1 mole of sodium atoms it would have a mass of about 22.99 G and if we had 1 mole of chlorine atoms it would have a mass of about 35.45 G adding these two values together we get a molar mass for NAC which equals about 58.4 G per mole as you can see here we must multiply our starting value by the molar mass because this is how we can get moles to cancel and leave us in units of GRS after multiplying these values we determine that we need to measure out 29.2 G of NAC for our lab which is equal to 0.5 moles in a hypothetical problem where we were given a known value in units of grams instead and needed to convert said value to moles we would need to divide our starting value by the molar mass if you ever unsure of whether to multiply or divide just remember to show the units in your work to ensure they will leave you with your dis desired units after calculations like avagadro's number molar mass is another conversion factor you should get comfortable with using these two conversion factors also have the mole in common as avagadro's number takes us from particles to moles and molar mass from moles to G if you ever find that you need to convert from particles to mass or vice versa you will have to use both conversion factors in your calculation let's practice that with another example let's say a reaction we want to carry out requires 10.0 G of potassium bromide and we are curious of exactly how many particles of KBR we will be adding for this reaction before we start plugging numbers in for our calculation let's map out exactly what we need to do for this two-step conversion there is no conversion factor that Bridges mass of a substance to the number of particles of that same substance since both mass and particles can be converted to moles we will use this as our Bridge so our first step will be converting from the given mass of KBR to moles of KBR our second step will be converting from the moles of KBR to particles of KBR now that we know the structure of our calculation let's plug the numbers in our given amount of KBR is 10.0 G the conversion factor for gam to moles is molar mass the molar mass for potassium is 39.0948 .94 G per mole adding those together we get 1192 G per mole as the molar mass for KBR which we will place right here for our calculation notice that we desire to cancel grams out in this step to convert to moles of KBR because of this we will divide by 1192 G which leaves the one mole in the numerator and mole as the unit for our answer for this step you can either do the step separately or calculate steps 1 and two together let's first go through the calculation with the two steps separately before seeing what it would look like combined 10.0 G of KBR divided by 1192 G of KBR and multiplied by 1 mole of KBR is equal to 0.084 moles of KBR since our second step is converting from moles to particles we will use the 0.084 Z moles of KBR is our next starting value the conversion factor for moles to particles is that's right avagadro's number 6.02 * 10 23rd particles per mole this time our conversion factor has the unit we desire in the numerator of the unit particles over one mole so we will be multiplying our given value of 0.084 moles of KBR by the 6.02 * 10 23rd particles this way the one mole is on the bottom canceling the unit of moles out and leaving us with an answer that has units of particles so 0.084 moles of KBR multiplied by 6.02 * 10 23rd particles of KBR and divided by 1 mole of KBR is equal to about 5.06 * 10 202 particles of KBR it makes sense that our answer is smaller than avagadro's number the amount of particles in one mole because 0.084 moles is less than one mole if we were to have calculated these two steps together we would have written it like this notice that all the values and units are in the same places an easy way to calculate a multi-step conversion like this is to a multiply all the values at the top to find the numerator 10.0 G * 1 mole * 6.02 * 10 23rd particles = 6.02 * 10 24 G moles particles this is a pretty funky unit right now but that's because we haven't canceled anything out yet B multiply all the values at the bottom to find the denominator 19.2 G * 1 mole = 1192 G moles lastly C divide the resulting numerator by the resulting denominator 6 6.02 * 10 24 / 11 19.2 = 5.06 * 10 22nd particles you can see how the gam and moles canceled in this step and left us with the value we calculated using the separated method either way we'll get you the same answer as long as it is calculated correctly but combining the steps May save you some time [Music] [Music]