[Music] hi there in our previous lesson we talked about the general classes and uses of organic compounds in this video we will talk about a mole concept to express the mass of substances so let's get started have you ever tried counting how many grains of rice can fit in one bowl i'm pretty sure that would take a very long time in the same sense counting atoms molecules or ions is not that simple because of their very small size therefore scientists have devised a different scale of counting atoms molecules or ions [Music] a more convenient way to handle calculations in chemistry is by defining a specific number of particles just like when we say a dozen eggs refer to 12 eggs or a ream of paper is 500 pieces of paper this specific number of particles is the mole a mole is defined as the number of particles may it be an atom molecule or iron present in exactly 12 grams of carbon-12 the number 6.02 times 10 raised to the 23rd power is popularly known as the avogadro constant or avogadro's number and is often denoted by the symbol n sub a the elementary entities that can be represented in moles can be atoms molecules monoatomic or polyatomic ions and other particles such as electrons any measurement can be broken down into two parts the numerical magnitude and the units that the magnitude is expressed in for example when the mass of a ball is measured as 2 kilograms the magnitude is 2 and the unit is kilogram when dealing with particles at an atomic or molecular level even one gram of a pure element is known to contain a huge number of atoms this is where the mole concept is widely used it primarily focuses on the unit known as a mole which is a count of a very large number of particles one mole refers to the amount of substance of a system that contains as many elementary entities as there are atoms in 12 grams of carbon-12 the number of moles of a substance in a given pure sample can be represented by the following formula n equals n over n sub a where the small n is the number of moles of the substance or elementary entity to capitalize n is the total number of elementary entities in the sample and n sub a is the avogadro constant the number of moles of a molecule may not always be equal to the number of moles of its constituent elements for example a mole of water contains n sub a number of h2o molecules however each water molecule contains two hydrogen atoms and one oxygen atom therefore one mole of h2o contains two moles of hydrogen and one mole of oxygen [Music] all right so here's a sample problem how many molecules are there in four moles of sodium chloride here are some steps to follow to easily calculate what is asked step one identify what is given the known number of moles is four moles of sodium chloride step 2 identify what is asked the unknown is the number of molecules of sodium chloride the desired conversion is mole to molecules we need a conversion factor that gives molecules or mole we know that 6.02 times 10 raised to the 23rd power molecules of sodium chloride is 1 mole of sodium chloride step 3 now we can start calculating 4 moles of sodium chloride multiplied by 6.02 times 10 raised to the 23rd power molecules of sodium chloride over 1 mole of sodium chloride we can cancel out common units this equals to 24.08 times 10 raised to 23rd power molecules of sodium chloride or if we move one decimal point to the left 2.41 times 10 raised to 24 power molecules of sodium chloride here's another sample problem how many moles of magnesium is 3.01 times 10 raised to 22nd power atoms of magnesium the known number is 3.01 times 10 raised to the 22nd power atoms of magnesium the unknown is the number of moles the desired conversion is atoms to moles the expression relating the unit is one mole equals 6.02 times 10 raised to the 23rd power atoms of magnesium the solution will be 3.01 times 10 raised to the 22nd power atoms of magnesium multiplied by one mole of magnesium over 6.02 times 10 raised to the 23rd power atoms of magnesium we can still cancel common units this equals 0.5 times 10 raised to the negative 1 power moles of magnesium or 5 times 10 raised to the negative 2 power moles of magnesium here's the last sample problem how many fluoride ions are in 1.46 moles of aluminum fluoride the known number is 1.46 moles of aluminum fluoride the unknown is the number of fluoride ions the desired conversion is moles to formula units to ions we know that one mole is equal to 6.02 times 10 raised to the 23rd power formula units the first conversion factor will be formula units over moles to convert formula units to ions we need to know how many fluoride ions are part of each formula unit thus we need to write the formula for aluminum fluoride we find that this compound has three fluoride ions per formula unit the second conversion factor will be three fluoride ions per one formula unit of aluminum fluoride now let's start calculating 1.46 moles of aluminum fluoride multiplied by 6.02 times 10 raised to the 23rd power formula units of aluminum fluoride over one mole of aluminum fluoride multiplied by three fluoride ions per one formula unit of aluminum fluoride take note that we can cancel common units this equals to 26.37 times 10 raised to the 23rd power of fluoride ions or 2.64 times 10 raised to 24 power of fluoride ions all right that's all for the calculations let's proceed to another concept the atomic mass of an element is the mass of one atom of the element it is expressed in atomic mass units for example the atomic mass of hydrogen is 1.00794 atomic mass units and that of oxygen is 15.9994 atomic mass units on the other hand the molecular mass of an element is the sum of the atomic masses of all its constituent elements this quantity is also represented in terms of atomic mass units therefore the molecular mass of water is equal to the sum of the atomic masses of its constituents hydrogen and oxygen water molecules contain two hydrogen atoms and only one oxygen atom the atomic mass of hydrogen is 1.00 atomic mass units and that of oxygen is 15.9994 atomic mass units to get the molecular mass we multiply the hydrogen twice and add oxygen thus the molecular mass of h2o or water is 18.0154 atomic mass units lastly the molar mass of a substance is defined as the total mass of one mole of the substance it is often represented in terms of grams per mole however the international system unit of this quantity is kilograms per mole molar mass can be represented by the following formula molar mass of a substance equals mass of the substance in grams over number of moles for example the molar mass of sodium chloride can be calculated for finding the atomic mass of sodium and atomic mass of chlorine and combining them the molar mass of sodium chloride is 58.44 grams per mole now here's a quick recap the mole is represented by avogadro's number which is 6.02 times 10 raised to the 23rd power atoms or molecules per mole by understanding the relationship between moles and avogadro's number scientists can convert between number of moles and number of atoms the atomic mass of an element is the mass of one atom of the element the molecular mass of an element is the sum of the atomic masses of all its constituent elements the molar mass of a particular substance is the mass of one mole of that substance that's all for now we will be discussing about percentage composition of a compound in our next video so stay tuned see you on our next video and don't forget to keep your minds busy if you like this video please subscribe to our channel and hit the notification icon for more videos like this