CBSE ADDA

Solved problems:Class 9 _Atoms and Molecules  1. Q. 5 g of calcium combine with 2 g of oxygen to form a compound. Find the molecular formula of the compound. (Atomic mass of Ca = 40 u; O = 16 u)  Ans. Number of moles in 5g of calcium = mass / molar mass    =5 /40    =  0.125  Number of moles in 2g of oxygen = mass / molar mass         =  (2 / 16)  =  0.125 Now we will calculate the simplest ratio of the element by dividing the number of moles of each element by the smallest value.  Since number of moles of each element is 0.125, therefore calcium and oxygen are present in a ratio of 1 : 1. Thus the empirical formula of the compound is CaO. For calculating the molecular formula, we need the molecular mass of the compound. However, because a compound with the formula CaO is known, Therefore the molecular formula of the compound is CaO. . Q. (i) Name the body which approves the nomenclature of elements and compounds.

Fully Solved Questions from previous year board Paper Question: 1. What causes buoyant force (or up thrust) on a boat? Answer: Buoyant force is due to volume of boat and density of water. Question: 2. what is the cause of buoyant force? Answer: : The pressure difference on lower part and upper part of a body cause of buoyant force . Question: 3. An Object of volume V is immersed in a liquid of density P. Calculate the magnitude of buoyant force acting on the object due to the liquid Answer: Vd g Question: 4. why does a block of wood held under water rise to the surface when released? Answer: This happens because as upward force is greater than the weight. Question: 5. What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid. When does the buoyant force become maximum? Answer: The buoyant force exerted by a fluid is equal to the weight of fluid displaced. Therefore the buoyant force increases as more and more vol