in this video we will learn about how the mass of reactants is conserved as they change into products we will discuss how molecules atoms or ions react in specific ratios which is the reason while we balance equations we will discuss how to use these balanced equations to determine the amount of a product formed in a reaction in the late 18th century French chemist antoan la guier proposed the law of conservation of mass this law states that in a closed system the mass of a system remains constant over time regardless of the processes occurring within that system this means that the total mass of the substances involved in a chemical reaction or physical process before the reactional process equals the total mass of the substances after the reaction or process as we learned about in chemical reactivity the law of conservation of mass is the basis for quantifying and tracking mass changes involved in a chemical reaction if a log is burned the mass of the carbon hydrogen and oxygen atoms present in the chemical structure of wood when combined with the mass of the oxygen atoms in molecular oxygen from the air must equal the mass of the carbon dioxide water vapor and Ashes that are produced post combustion and understanding how to apply the law of conservation of mass allows us to balance chemical equations and track the amounts of reactants and products through stochiometry these skills are the focus of this video atoms datomic elements ionic compounds and molecular compounds can be represented through the general rules of formula writing these chemical representations are critical in understanding reactions in chemistry because the correct substance representations allow us to track how reactants undergo chemical change to become products accurately for example when methane CH4 is combusted we know that it combines with molecular oxygen O2 and produces carbon dioxide CO2 and water vapor H2O in a chemical equation the reactants or initial substances present are shown on the left side of the arrow and products formed are shown on the right side the arrow generally represents the change that is occurring in the process if we track the atoms of each element present on the reac reactant and product side and the total mass of the atoms on each side we can immediately see that the law of conservation of mass has been violated we can see that neither the atoms of each element nor the system's mass is conserved a simple fix exists because atoms and molecules do not necessarily react with one another in a one as to one ratio the solution to this problem is to use coefficients coefficients in front of the correct formula representation indicate how many units of each substance react in a particular chemical reaction for example the mass and atom imbalance can be balanced by understanding that one molecule of methane will react with two molecules of oxygen to produce one molecule of carbon dioxide and two two molecules of water note that by adding coefficients of two in front of oxygen and water the number of atoms of each element Remains the Same as the substances undergo a chemical change and move from reactants to products because the number of atoms in each type of element has been conserved the overall mass is also conserved and the law of conservation of mass has been obeyed as we will learn there are many types of chemical equations representing different types of chemical reactions but all must be balanced with coefficients so that the chemical change represented obeys the law of conservation of mass here are some examples of some chemical equations that are a bit more involved when balancing aluminium oxide can be purified from borite and then the aluminium oxide Mater material under goes electrolysis to extract pure aluminium calcium carbonate or Limestone can Aid in the neutralization of phosphoric acid aluminium metal can replace copper in a solution of copper 2 chloride taking the balanced equation we used previously for the combustion of methane the equation could be interpreted in the following way one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor working with individual atoms and molecules is Impractical for a chemist it is much more practical to work with moles of atoms and molecules if all the molecules within the balanced equation are multiplied by avagadro's constant which is 6.02 * 10 23 then the balanced equation can also be interpreted as one mole of methane reacts with 2 moles of oxygen to produce one mole of carbon dioxide and 2 moles of water vapor working with moles and interpreting the coefficients from a balanced equation as a mole ratio allows a chemist to easily quantify and track amounts of reactants consumed and amounts of products produced it is important to remember that this ratio does not refer to masses but relative moles only we can compare any two substances from the balanced equation and create mole ratios for example one mole of methane will produce 2 moles of water or when two moles of water vapor are produced only one mole of carbon dioxide will be produced in a laboratory the amount of a substance is determined by its mass and measured in gr although the SI unit for the amount of a substance is moles substances cannot be measured this way because the number of moles of a particular substance is dependent upon the molar mass of the substance and this is dependent upon its chemical composition a laboratory scale will only discern Mass so the chemist is left to calculate the number of moles in a sample using the mole ratios derived from a balanced equation enables a variety of problems to be solved for example allowing the tracking of the amount of a reactant consumed or the amount of a product produced calculate ations involving mole ratios are called stochiometric problems or stochiometry let's use a balanced equation from a previous slide in a stochiometric problem if 3.50 G of aluminium metal is added to a solution of copper 2 chloride that is in excess what mass of copper metal will be produced first we need a balanced chemical equation then the amount of aluminium metal is is given in grams and must be converted to moles the moles of aluminium consumed can be converted to moles of copper metal produced using the appropriate mole ratio from the balanced equation finally the moles of copper can be converted back to GRS if required or we can combine these steps into a single expression notice that the mole ratio from the balanced equation allows a quantity of one substance to be converted to the corresponding quantity of another substance most of the time in a chemical reaction the reactants present will not be present in the precise amounts that are needed usually one of the reactants is present in excess meaning there is more than enough of that reactant to completely use up the other reactant for example here only one unit of a is required re ired for every one unit of B and so since two units of a are supplied but only one unit of B we can conclude that a is in excess it is the excess reactant then at the end of the reaction some of a is left over or is unreacted B is considered to be limiting when B is used up the reaction can no longer proceed and the excess quantity of a remains in the previous problem it was noted that the copper 2 chloride solution was in excess this means that there is more than enough copper 2 chloride to react with all of the 3.50 G of aluminium and some copper 2 chloride will remain when the aluminium is exhausted and the reaction therefore stops so in this problem copper 2 chloride is called the excess reactant or excess reagent all of of the 3.50 G of aluminium is used up since there is more than enough copper 2 chloride and so aluminium is called the limiting reactant the limiting reactant is always the reactant which is used up first since aluminium is the limiting reagent its amount 3.50 G will determine how much aluminium chloride and copper is produced so the amount of limiting reactant is used to determine the theoretical yield of each product for copper this value is 12.4 G the theoretical yield in a chemical reaction refers to the maximum amount of product that can be produced from a given amount of reactants assuming that the reaction proceeds to completion with perfect efficiency and that all reactants are converted into products according to the balanced equation the theoretic yield is a calculated value not the mass of product actually weighed out in the lab the mass of product or yield weighed out after the reaction usually differs to the calculated maximum theoretical yield slightly due to factors such as impurities side reactions and incomplete reactions often it is not clearly stated which reactant is limiting as in the previous problem most of the time amounts of each of the reactants are given and it is up to you to determine which reactant is limiting and will therefore determine the theoretical yield let's use the same reaction as in our previous problem but note the difference in the wording here if 3.50 G of aluminium metal is added to 550 0 cm cubed solution of copper 2 chloride that has a concentration of 0 . 375 m per decim cubed what mass of copper metal will be produced note that it is no longer clear that aluminium is the limiting reactant amounts of both aluminium and copper 2 chloride are given which will be used up first and is therefore the limiting reactant there are several approaches to determining the limiting reactant and we will examine a few momentarily nonetheless the first two steps of the problem remain the same first Write the balanced chemical equation then determine the moles of aluminium as before then determine the moles of the other reactant from the given data it is very important to note that the limiting reactant is not necessarily the reactant with the fewest number of moles the limiting reactant must now be determined using the mole ratios from the balanced equation one of the quickest methods of determining the limiting reactant is to divide the number of moles of a substance by its coefficient from the balanced equation this means that 0.065 moles of aluminium are available per mole of copper 2 chloride and 0.687 mol of copper 2 chloride are available per 1 mole of aluminum aluminium is the limiting reactant because there is less available to react per mole required so the available aluminium will determine the theoretical yield of both products namely the yield of copper and the yield of aluminium chloride present in solution there are other ways to determine the limiting reactant here are a few options in alternative method one choose one product in this example let's choose copper and calculate its theoretical yield from each reactant amount we calculate how much copper would be produced if aluminium was the limiting reactant and how much would be produced if copper to Chloride were the limiting reactant since aluminium produces a smaller amount of copper it is the limiting reactant note that the limiting reactant can also be determined by calculating only the number of moles of copper aluminium would produce fewer moles of copper a third myth method or alternative method two would be to use the moles of one of the reactants and the mole ratio to determine if enough of the other reactant is available let's choose to use the moles of copper 2 chloride available here we would calculate that 0.137 moles of aluminium is required to react with or use up all of the copper 2 chloride since the 0.137 moles of aluminum needed to use up all of the copper 2 chloride is not available only 0.130 is available or present aluminium must be the limiting reactant and copper 2 chloride is the excess reactant now let's summarize what we have learned we learned that mass is conserved during a reaction and that the total mass of reactants equals the total mass of products the stochiometric coefficients or mole ratio in a balanced equation represent the relative number of reacting or produced atoms ions molecules or formula units or their relative number of moles the reactant used up first is the limiting reactant the reactant not completely used up is the excess reactant and lastly we learned that the mold moles of the limiting reactant determines the moles of product which can form