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Problems based on Law of Conservation of Mass

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What is Matter? What is its Characteristics?

States of Matter

Law of Conservation of Mass

Problems based on Law of Definite Proportions / Constant Composition

Law of Definite Proportions / Constant Composition

Mole Concept – Importance and formulas


Problem : 1 

4 grams of hydrogen reacts with some oxygen to make 36 grams of water. Figure out how much oxygen must have been used by applying the law of conservation of mass?

Solution:
Word equation –
Hydrogen + Oxygen  →   water
Masses of each substance given –                                   4 g                ?                36 g

According to law of conservation of mass –           Mass of reactants  =  Mass of products
4 + ?       =           36
So,  32 grams of oxygen was used.


Problem : 2

In a chemical reaction 150 g Baking soda mixture containing sodium bicarbonate and vinegar on heating gives 87 g of carbon dioxide gas. What mass of solid residue will left in food?

Solution:
According to law of conservation of mass,
Total mass of reactants = Total mass of products.

Here, baking soda mixture (reactant) on heating gives solid residue and carbon dioxide ( products).Here baking soda mixture (reactant) on heating gives solid residue and carbon dioxide ( products).

MBaking soda = MSolid residue + MCarbon dioxide

Hence,  the mass of solid residue is 150g – 87g = 63g.


Problem : 3
How much oxygen will add with 36 g carbon to give 132 g carbon dioxide assuming complete combustion of carbon?

Solution:
According to law of conservation of mass,
Mass of carbon + Mass of oxygen = Mass of carbon dioxide
36g         +             x               =             132 g
Hence,  the mass of oxygen is 96g.


Problem : 4

A thin strip of iron with a mass of 15.5g is placed into a solution containing 21.0g of copper (II) sulfate and copper begins to form. After some times, the reactions stops because all the copper (II) sulfate has reacted. The iron strip is found to have a mass of 8.5g. The mass of copper formed is found to be 8.60g. What mass of iron (II) sulfate has been formed in the reaction?  

Solution:
Mass of iron = Initial mass – Final mass
=     15.5 – 8.5 = 7.0g
Mass of copper (II) sulfate =  21.0g
Mass of copper  = 8.60g
According to law of conservation of mass,
Mass of iron + Mass of copper (II) sulfate = Mass of copper + Mass of iron (II) sulfate
7.0g       +                  21.0g                      =         8.60g          + Mass of iron (II) sulfate

So,       Mass of iron (II) sulfate    =     19.40g


Problem : 5

Silicon dioxide, made up of elements silicon and oxygen, contains 46.7% by mass of silicon. With what mass of oxygen will 10 g of silicon combine?

Solution:
100g of silicon dioxide contains  46.7 g of silicon,  or  100 – 46.7 ( i.e. 53.3 g) of oxygen.

It means, 10g of silicon will contain (10/100) × 53.3 = 5.33 g of oxygen.


Problem : 6

When 0.0976g of magnesium was heated in air, 0.1618g of magnesium oxide (MgO) was produced. What is the mass of oxygen needed to produce 0.1618g MgO?

Solution:

According to law of conservation of mass,

Total mass reactants  =  Total mass products
mass of Mg + mass O = mass of MgO
0.0976g Mg + mass O = 0.1618g MgO

mass O = 0.1618g – 0.0976 =  0.0642g O

 


Quiz – 1


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