The Law of Conservation of Mass states the following;
Law of Conservation of Mass was introduced by Antoine Lavoisier in around 1789. He had conducted many experiments especially of combustion reactions in closed containers to validate the rule.
In Stoichiometry we use the Law of conservation of mass to calculate various quantities, be it the amount of product expected or the amount of reactant needed to be added to a reaction or the percent yield of a reaction.
Let us take an example;
CaCO3 on thermal decomposition produces CaO and CO2.
CaCO3 → CaO + CO2
"Mass can neither be created nor destroyed"In simple words the weight of the reactants are always equal to the weight of the products if all reactants are consumed in the reaction.
Law of Conservation of Mass was introduced by Antoine Lavoisier in around 1789. He had conducted many experiments especially of combustion reactions in closed containers to validate the rule.
In Stoichiometry we use the Law of conservation of mass to calculate various quantities, be it the amount of product expected or the amount of reactant needed to be added to a reaction or the percent yield of a reaction.
Let us take an example;
CaCO3 on thermal decomposition produces CaO and CO2.
CaCO3 → CaO + CO2
On complete decomposition of 100g of CaCO3 produced 56g of CaO and 44g of CO2 The total weight of the reactants is equal to the total weight of the products.
As in the previous case, if we start with 50g of CaCO3 and allow it to decompose completely we should get 28g of CaO and 22g of CO2.
Try to solve the following question in the fastest time possible.
If 70g of CaCO3 on complete decomposition produces 39.2g of CaO, how many grams of CO2 will be produced?
How much time did you take? How did you arrive at the answer?
According to the Law of Conservation of mass, the weight of the products (CaO and CO2 formed should be equal to the weight of the reactant (CaCO3). So, the total weight of CaO and CO2 should be 70g. 39.2g of CaO is formed and hence (70 - 39.2)g = 30.8g of CO2 will be produced.
A 100g sample of CaCO3 on thermal decomposition gives 40g of CaO. What can we infer from this?
We can infer either of the following two things.
1. The reaction is incomplete and
2. The sample is impure.
Let us say that the reaction is complete. This leaves us with the only option that the sample must have been impure. What must be the percentage purity of the sample? Let us calculate.
We know that, 100g of pure CaCO3 on complete decomposition should produce 56g of CaO. Or in other words we can say that 56g of CaO should have been produced from 100g of pure CaCO3. So, 40g of CaO must have been produced from [(100/56) * 40]g = 71.43g of CaCO3. Hence the sample must be 71.43% pure.
What if the reactants did not completely react? How will we be able to calculate the amount of products formed and also the amount of reactants reacted?
The above questions can be answered by using the concept of "Limiting Reagent". We shall discuss about it next.
A 100g sample of CaCO3 on thermal decomposition gives 40g of CaO. What can we infer from this?
We can infer either of the following two things.
1. The reaction is incomplete and
2. The sample is impure.
Let us say that the reaction is complete. This leaves us with the only option that the sample must have been impure. What must be the percentage purity of the sample? Let us calculate.
We know that, 100g of pure CaCO3 on complete decomposition should produce 56g of CaO. Or in other words we can say that 56g of CaO should have been produced from 100g of pure CaCO3. So, 40g of CaO must have been produced from [(100/56) * 40]g = 71.43g of CaCO3. Hence the sample must be 71.43% pure.
What if the reactants did not completely react? How will we be able to calculate the amount of products formed and also the amount of reactants reacted?
The above questions can be answered by using the concept of "Limiting Reagent". We shall discuss about it next.
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