Science - Class 9

Q71. Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g mol–1 and 32 g mol–1 respectively.

 

Q72. Calculate the molar mass of the following substances.

(a) Ethyne, C₂H₂

(b) Sulphur molecule, S₈

(c) Phosphorus molecule, P₄ (Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO₃

 

Q73. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.

 

Q74. Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

 

Q75. Which has more number of atoms?

100g of N₂ or 100 g of NH₃

 

Q76. What are the postulates of Dalton's atomic theory?

 

Q77. State the law of constant proportions. Give one example to illustrate this law.

Or

Explain giving a suitable example: Law of constant proportion.

Or

How can you say the law of constant proportion is a valid justify with an example?

 

Q78. 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

 

Q79. How do elements get their names and symbols?

 

Q80. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆,

C₂H₄, NH₃, CH₃OH.