Atoms and molecules class 9 solved question

QUESTIONS FROM Exam PAPERS

Questions: 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)

Solution: 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.

Questions: When 3.0 g of carbon is burnt in 8.0 g of oxygen, 11.0 g of carbon dioxide is produced.
What mass of carbon dioxide will be formed when 3.0 g of carbon is burnt in 50.00 g of oxygen ?
Which law of chemical combination will govern your answer ? State the law.

Solution: The mass of the carbon dioxide formed will be only 11 grams. The remaining oxygen is not used up.

This indicates the law of definite proportions which says that in compounds, the combining elements are present in definite proportions by mass.

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