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## Saturday, October 20, 2012

### 9th Atoms and molecules Study Notes, Q & A

NCERT Solution, MCQs, Study Notes, Q & A
Q1: Name the international organization who approves names of elements.
Ans IUPAC (International Union of Pure and Applied Chemistry)
Q2: How do we know the presence of atoms if they do not exist independently for most of the elements?
Answer: Atom join in different way to form matter(neutral molecules or ion) that we are able to touch, feel and see.
Q3: Give an example to show Law of conservation of mass applies to physical change also.
Ans: When ice melts into water, is a physical change. Take a piece of ice in small flask, cork it and weight it (say Wice gm). Heat the flask gently and ice (solid) slowly melts into water (liquid). Weigh the flask again (Wwater gm). It is found there is no change in the weight i.e. Wice= Wwater.
Heat(Δ)
Ice  ----------   Water
This shows law of conservation of mass holds true for physical changes.
Q4: Explain with example that law of conservation of mass is valid for chemical reactions.
Ans: According to this law, total mass of the products (chemicals produced) is equal to the total mass of the reactants (chemicals take part in reaction). e.g. Carbon combines with Sulphur to form Carbon disulphide. The mass of reactants i.e. carbon and sulphur is same mass of products (carbon disulphide).
Carbon + Sulphur -----   Carbon DiSulphide
C    +   S     -----     C2S
1g   +  5.34g    =       6.34 g
LHS          =     RHS
Q5: Is there any exception to law of conservation of mass?
Ans: Later after atoms were discovered, it was found during nuclear reactions this law does not hold good. In a nuclear reaction, some of the mass gets converted into energy, as given by famous Einstein's mass-energy relationship (E = mC2). The law was rechristened as Law of conservation of mass and energy.
Q6.In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
sodium carbonate + ethanoic acid  → sodium ethanoate + carbon dioxide + water
Ans:  Mass of  sodium carbonate (Na2CO3) = 5.3 g  Mass of ethanoic acid (CH3COOH) = 6g
Mass of carbon dioxide (CO2) = 2.2g  Mass of Water (H2O) = 0.9 g
Mass of sodium ethanoate (CH3COONa) = 8.2 g
Total mass of reactants = 5.3g + 6g = 11.3g
Total mass of products = 2.2g + 0.9 g + 8.2g = 11.3g
Since, Total mass of reactants = Total mass of products, the observations in this reaction are in agreement with the law of conservation of mass.
Q7 If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?
Ans: Given, the reaction is carbon + oxygen → carbon dioxide
Mass of carbon = 12g       Mass of oxygen = 32g
Mass of carbon dioxide (CO2) = xg
Since the reaction is in agreement with law of conservation of mass, then
Total mass of reactants = Total mass of products
i.e. 12g + 32g = x        i.e. x = 44g
mass of carbon dioxide (CO2) formed = 44g
Q.8.  A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Ans: Given, mass of boron = 0.096 g      mass of the oxygen = 0.144g
mass of the sample compound (boron+oxygen) = 0.24 g
According to Law of definite proportions, boron and oxygen are in fixed ratio in the given compound.= 0.096 : 0.0144  = 2:3
% of boron = mass of boron × 100 / mass of compound = 0.096 × 100/0.24 = 40%and % of oxygen = mass oxygen  × 100 / mass of compound = 0.144 × 100/0.24 = 60%
Q.9. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Carbon + Oxygen Carbon Dioxide
3g   +  8g    =   11g
Total mass of reactants = Total mass of products
Law of conservation of mass is obeyed.
It also shows that carbon dioxide (CO2) contains carbon and oxygen in fixed ratio i.e. 3:8 which follows Law of Constant proportion.
3g of carbon (C) will react with 8g of oxygen (O) to give 11g of carbon dioxide (CO2).
Remaining oxygen (50 -8 = 42g) will not participate in the reaction.
Q10 : Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium?
Answer: Magnesium (Mg) and Oxygen (O) combine in the ratio = 3:2 to form compound magnesium oxide (MgO). It follows Law of Constant proportions.
3g of Mg combines with = 2g of O.
∴ 12g of Mg combines with = 2 × 12/3 = 8g of O.

Q.11.Why are Dalton's symbol not used in chemistry?
Answer:Dalton was the first scientist to use the symbol for the name of the elements a specific sense but it was difficult to memorize and in uses so Dalton's symbol are not used in chemistry
Related post to IX(9th)  Atoms and Molecules